Study of the charged current interactions of the CNGS neutrino beam in the OPERA detector target

Transcription

1 Università degli Studi di Padova DIPARTIMENTO DI FISICA E ASTRONOMIA GALILEO GALILEI Corso di Laurea Magistrale in Fisica Tesi di laurea Magistrale Study of the charged current interactions of the CNGS neutrino beam in the OPERA detector target Laureanda: Virna Bordignon Relatore: Dott.ssa Chiara Sirignano Correlatore: Dott. Alessandro Bertolin Anno Accademico

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3 Abstract In this thesis the charged current interactions of the CNGS neutrino beam in the OPERA detector target are studied. The analized sample covers the whole data taking period, from 28 to 212. The PoT number for each year is calculated and corrected for the DAQ inefficiencies. The track reconstruction efficiency is investigated, performing a comparison between an algorithm not relaying on tracks for charged current identification and an algorithm looking for muon tracks. The muon track finding efficiency is evaluated and the detector stability is checked, testing whether the number of charged current events and muon track events is constant during the data taking period, once corrected for the number of recorded PoT and for target mass variations. For the charged current sample, the charge momentum distribution is studied and the µ + to µ ratio is evaluated. The momentum spectra are examined and the Bjorken y distribution is investigated in detail.

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5 Contents 1 Introduction 1 2 Neutrino physics Theory of neutrino oscillations Two neutrino mixing Neutrino interactions Experimental results on neutrino oscillation Solar neutrino oscillation Atmospheric neutrino oscillation Reactor neutrino oscillation Accelerator neutrino oscillation Current estimates of the neutrino mixing parameters The OPERA experiment The CNGS beam Target and electronic detectors Data Acquisition and Software Synchronization Reconstruction of tracks OpCarac algorithm for the event classification Monte Carlo simulations ECC Bricks and Nuclear Emulsions τ detection Number of PoT evaluation Cosmic rays rate distribution Detailed gaps investigation TT event rate distribution OnTime event rate distribution µ track reconstruction efficiency Comparison between two methods to evaluate x, y and z coordinates of a muon track CC events without a muon µ to CC events ratio

6 6 Detector stability Total event rate Number of CC events during data taking Number of µ events during data taking Studies on the CC sample qxp for µ events µ + to µ ratio Momentum spectrum and MC comparison Bjorken y Conclusions 79

7 Chapter 1 Introduction Neutrino oscillation is a interesting phenomenon proposed in the late 195s by Pontecorvo. It was succesfully tested to explain the solar neutrino problem arised by the Homestake experiment results, which were the first evidence of the deficit of electron neutrinos coming from the Sun, later explained as an oscillation result thanks to the detailed measurements of the SNO experiment. Oscillations between different neutrino flavours are possible only if neutrinos are massive and a flavour mixing is present. From the solution to the solar neutrino problem until nowadays, several experiments with solar, atmospheric, reactor and accelerator neutrino have provided compelling evidences for oscillations. The data collected by these experiments has allowed to determine the relevant parameters of the mixing, even if some of them are still missing as the absolute neutrino mass values. The OPERA 1 experiment is designed to study the neutrino oscillation with an accelerator produced neutrino beam. The peculiarity of OPERA is that it is an appearance experiment that aims at detecting the direct appearance of ν τ in a almost pure beam of ν µ. The ν τ is detected through its charged current interaction in the target that produces a τ lepton whose decay vertex can be reconstructed. To fulfill this task a very high spatial resolution is needed and hence the emulsion film technique was used combined with electronic detectors. In this thesis the charged current interactions of the CNGS neutrino beam in the OPERA detector target will be studied. The data taking of the OPERA electronic detectors ended with the 212 run, so the analized sample covers the whole data taking period. Chapter 2 will be dedicated to a review of the neutrino physics, for what concerns the theory of flavor oscillations and the most important experimental results obtained with solar, atmospheric and artificially produced neutrinos. The final section of this chapter summarizes the current estimates of the neutrino mixing parameters. In Chapter 3 we will give a detailed description of the CNGS neutrino beam and of the OPERA experiment. The target and the electronic detectors are presented. We will also discuss the Data Acquisition, Software and Monte Carlo simulations. The process that allows the τ detection will be explained, discussing the connection of tracks from the electronic detectors prediction to the interaction vertex in the ECC brick. The τ decay peculiarities and the kinematical cuts applied to the tracks will be briefly presented. The three τ candidates 1 Oscillation Project with Emulsion-tRacking Apparatus. 1

8 identified so far will be shown. The following chapters describe the results obtained from the data. Chapter 4 will deal with the number of PoT evaluation. The recorded number of PoT will be corrected for the DAQ inefficiencies, in particular studying the cosmic rays rate and the OnTime event rate, where the OnTime flag is set when an event time can be correlated to the time of a CNGS beam spill. The effective PoT number will be calculated using the OnTime event rate. This numbers will be used for normalization purposes. In Chapter 5 the muon track reconstruction efficiency will be investigated, performing a comparison between an algorithm not relaying on tracks for charged current identification and an algorithm looking for muon tracks. The muon track finding efficiency is evaluated. After discussing the factors that can influence the normalization, the detector stability will be tested and the results will be presented in Chapter 6. We will study the total events, the contained CC events and the bending muon events yield, checking if they remain constant during the data taking period. Finally, in Chapter 7 the charged current data sample will be studied in detail, to evaluate some significant quantities and also to test the compatibility with the Monte Carlo predictions. In particular, the charge momentum distribution will be studied, the µ + to µ ratio evaluated, the momentum spectra examined and the Bjorken y distribution investigated in detail. 2

9 Chapter 2 Neutrino physics Neutrinos are neutral and weakly interacting particles that are assumed to be massless Dirac fermions in the Standard Model (SM). The number of neutrino types has been set to be 3 by LEP experiments. The combined result from the four LEP experiments is [1]: ν = ±.8. (2.1) These three types are called flavours and are respectively: electron, muon and tauon, each of them is linked to the particle that is produced in association with the neutrino. For example, ν e is the neutrino which is produced with the e + in semileptonic hadron decays and ν e produces an e in Charged Current (CC) weak interaction processes. Neutrinos can also undergo NC (Neutral Current) processes with a coupling to a Z boson. Neutrinos (antineutrinos), are always produced in weak interaction processes in a state that is left-handed (LH) (right-handed (RH)). This evidence is confirmed by all the existing data from neutrino experiments and this is why neutrinos (antineutrinos) are described in the SM by a chiral LH (RH) flavour neutrino field. An alternative to the SM neutrino is the Majorana neutrino that can have a non-zero mass. Majorana s theory implies the hypothesis that neutrino and antineutrino are the same particle [2]. This theory is currently under investigation: a validation will come only if a neutrinoless double-β decay will be observed. The absolute neutrino mass scale is still not known, the neutrino mass m ν is compelled to be much smaller than the masses of charged leptons and quarks from currently available data of neutrino experiments and cosmological argumentations. The remarkable smallness of neutrino mass is supposed to be related to the existence of a new fundamental mass scale in particle physics and thus to new physics beyond the SM. An extension of the Standard Model, the see-saw mechanism [3], points out that the RH neutrinos can play a crucial role in the generation of neutrino masses and in the explanation of the observed matter-antimatter asymmetry of the Universe. If RH neutrinos exist, their interaction with matter should be much weaker than the weak interaction of the LH neutrinos, so they are usually referred as sterile neutrinos. 3

10 2.1 Theory of neutrino oscillations The phenomenon of neutrino oscillation implies that if a neutrino of a given flavour is produced the probability (called oscillation probability) to find a neutrino of a different flavour at a sufficiently large distance from the source is not zero. This phenomenon was first described in 1957 by Bruno Pontecorvo [4] and later formalized by Z. Maki, M. Nakagawa and S. Sakata in 1962 [5]. The mechanism that underlies neutrino flavour oscillation is the mixing of the states with definite flavour (ν e, ν µ, ν τ ), interaction eigenstates, and states with definite mass, eigenstates of the Hamiltonian operator, (ν 1, ν 2, ν 3 ). The weak interaction eigenstates are therefore expressed as combinations of mass eigenstates that propagate with slightly different frequencies due to their different masses and this fact produces a change or oscillation in the neutrino flavour. The oscillation mechanism requires two conditions that are not predicted by the SM: neutrinos have to be massive and there must be a flavour violating interaction (an interaction that does not conserve leptonic flavour number). We can write the flavour eigenstates (ν e, ν µ, ν τ ) as a linear combination of the mass eigenstates (ν 1, ν 2, ν 3 ): ν e U e1 U e2 U e3 ν µ = U µ1 U µ2 U µ3 ν τ U τ1 U τ2 U τ3 ν 1 ν 2 ν 3 (2.2) A convenient parametrization for the unitary mixing matrix consists in writing it as the product of three real rotation matrices, considering three angles θ 12, θ 23 and θ 13 and one complex phase factor δ. Defining c ij cos θ ij and s ij sin θ ij, the mixing matrix becomes: 1 c 13 s 13 e iδ c 12 s 12 U = c 23 s 23 1 s 12 c 12 (2.3) s 23 c 23 s 13 e iδ c 13 1 which is known as Pontecorvo Maki Nakagawa Sakata (PMNS) matrix. Let us suppose that a neutrino with flavour α is produced at time t =. The initial state can be expanded into a linear combination of mass eigenstates, with coefficients given by the PMNS matrix: 3 ν α > = U αk ν k > α = e, µ, τ (2.4) k=1 If we assume that neutrino propagates in vacuum as a plane wave of energy E k, at time t > each mass eigenstate ν k > will have evolved into: The temporal evolution of (2.4) is given by: ν k (t) > = e i(e kt) ν k > k = 1, 2, 3 (2.5) ν α (t) > = 3 e i(ekt) U αk ν k > (2.6) k=1 4

11 or, expanding it in series of the flavour eigenstates: 3 ν α (t) > = e i(ekt) U αk Uβj ν β > (2.7) k=1 β=e,µ,τ The transition amplitude of a flavour oscillation ν α ν β is then given by: A να ν β (E, t) = k e i(e kt) U αk U βk (2.8) Since neutrinos are always ultra-relativistic, we can write E k p + m2 k 2E and the relative phase factor is: E kh = E k E h = m2 kh 2E Taking the square of the transition amplitude, the oscillation probability is: P να νβ (E, t) = δ αβ 4 ( Re[Uαh U βh U αk Uβk ] m 2 kh t ) sin2 4E k<h + 2 ( Im[Uαh U βh U αk Uβk ] sin mkh 2 t ) 2E k<h (2.9) (2.1) In the case of the survival probability, ν α ν α, expression (2.1) simplifies to: P να να (E, t) = 1 4 ( U αk 2 U αh 2 sin 2 m 2 kh t ) 4E k<h (2.11) There are two possible kinds of experiments to detect neutrino mixing: disappearance experiments, in which the survival probability P να να is determined, or appearance experiments, in which P να νβ, with α β is measured. In both cases, the interesting quantity is: r = m kh 2 t (2.12) 2E If r 1 the oscillation effects are too small to be detected. If r 1 the experiment is sensitive only to the average oscillation probability. The neutrino mixing can be studied only if r O(1). The mass splittings m 2 kh and the rotation angles θ kh that are present in (2.3) are physical constants. The parameter that can be controlled is the ratio t/e, usually expressed as L/E, where L is the distance between source and detector and E is the beam energy. The experiment must be designed appropriately in order to be sensitive to a certain range of m kh 2 values. Short-baseline experiments, which can take place near reactors or accelerators, are designed with L from a few metres up to 1 km. The energy spectrum spans the interval 1 MeV 1 GeV, so the sensitivity to the m 2 parameter is limited to m 2.1 ev 2. Long-baseline experiments are designed with L in the interval 1 km 1 4 km, with an energy spectrum of 1 MeV 1 GeV, so the sensitivity can reach values up to m ev 2. All the experiments using ν µ beams from proton accelerators (as OPERA, MINOS or K2K) and also experiments related to the atmospheric neutrinos are long-baseline. Finally, very-long-baseline experiments can probe m 2 values up to 1 12 ev 2, such as solar neutrino experiments. 5

12 Figure 2.1: ν e survival probability as a function of the neutrino energy for L = 18 km, m 2 = ev 2 and sin 2 2θ =.84 [6] Two neutrino mixing In the case of only two neutrino flavours the treatment is simplified. If ν e and ν µ are the two flavour eigenstates, they can be expressed as a linear combination of two mass eigenstates, ν 1 and ν 2, as given by the unitary transformation involving only one mixing angle θ: ( ) ( ) ( ) νe cos θ sin θ ν1 = (2.13) sin θ cos θ ν µ ν 2 The oscillation probability becomes ( k m P (ν µ ν e ) = sin 2 2θ sin 2 2 ) L E (2.14) where the numerical factor k has the value 1.27 if L is expressed in metres, E in MeV and m 2 in ev 2. In Fig. 2.1 an example of the ν e survival probability as a function of the neutrino energy is shown. 2.2 Neutrino interactions Neutrinos can interact with matter by neutrino-electron interactions and neutrino-nucleons scattering. These processes are described with accuracy by the Standard Model. Neutrinos of each flavour can undergo charged current (CC) or neutral current (NC) interactions. In a CC process, via W exchange, the corresponding charged lepton appears in the final state. In a NC process, via Z exchange, the incoming neutrino flavour is preserved in the final state. 6

13 Low-energy neutrinos, with E ν greater than some MeV, interact with electrons through the elastic scattering process ν α + e ν α + e. (2.15) This process is used, for example, in water Cherenkov solar neutrino detectors (see sec ). The elastic scattering process does not have a threshold, since the final state is the same as the initial state. The only effect of an elastic scattering process is a redistribution of the total energy and momentum between the two participating particles. However, an energy threshold of some MeV s is needed in order to have a signal above the background [7]. Muon neutrinos with energy above the µ production threshold, E ν m µ, can interact with electrons through the quasi-elastic CC process ν µ + e ν e + µ. (2.16) In a CC interaction between a neutrino and a nucleon we can distinguish different cases. When the incident neutrino energy is quite low but above the threshold for the respective lepton production, a quasi-elastic (QE) process can take place. In this case only the final state charged lepton is observable in the detector. ν l + n p + l, (2.17) ν l + p n + l +. (2.18) Such a process is used in detectors of electron antineutrinos produced in reactors (for example, CHOOZ and KamLAND, see sec ). When the hardness of the interaction between the neutrino and the struck nucleon increases, the process is labeled as resonant (RES). In this case a few hadrons may be observed in the detector together with the charged lepton. At high neutrino energies, CC neutrino-nucleon interactions are dominated by the deep inelastic scattering regime (DIS) and the chance of observing additional hadrons in the final state is high. ν l + N l + X, (2.19) ν l + N l + + X. (2.2) Figs. 2.2(a) and 2.2(b) show the Feynman diagrams for a CC-DIS and a CC-QE process with a nucleon respectively. In a NC process, via Z exchange, the incoming neutrino is preserved in the final state. Fig. 2.3 shows the Feynman diagram for a NC interaction with a nucleon. The values of the cross sections of neutrinos on nucleons depends linearly on the incident neutrino energy and many processes contribute to them. The cross sections has been measured by many experiment, as shown in Fig. 2.4, where ν µ and ν µ CC inclusive scattering cross sections divided by neutrino energy are plotted as a function of neutrino energy [8]. The charged current total cross section for the muon neutrino and anti-neutrino are σν CC µ 38 cm2 = (.677 ±.14) 1 E ν GeV σ CC ν µ E ν = (.334 ±.8) cm2 GeV (2.21) (2.22)

14 (a) (b) Figure 2.2: The Feynman diagrams for a CC-DIS (a) and a CC-QE (b) neutrino-nucleon interaction, where x stands for the flavour. Figure 2.3: The Feynman diagram for a NC interaction with a nucleon in the DIS regime. Figure 2.4: Measurements of ν µ and ν µ CC inclusive scattering cross sections divided by neutrino energy as a function of neutrino energy. Note the transition between logarithmic and linear scales occuring at 1 GeV. 8

15 Data are taken from [9], where the cross section values are obtained from an average of results from dedicated experiments. The NC to CC ratio on isoscalar target is predicted to be.31 by using the procedure outlined in reference [9]: R = σnc νn σνn CC = g 2 L + g 2 R σcc νn σ CC νn (2.23) where gl 2 and g2 R are expressed as a function of the Weinberg angle and the ratio of ν and ν charged current cross sections is taken to be.54 ±.3 from the previously cited values. 2.3 Experimental results on neutrino oscillation Solar neutrino oscillation The first experimental evidence of the existence of neutrino mixing came in the late 196s with observations that contradicted the predictions of Solar Standard Model (SSM) about the emission rate of electron neutrinos from the Sun, the issue known as solar neutrino problem. The SSM explains the energy production in the Sun core using thermonuclear fusion reactions. The solar neutrinos are produced by some reactions in the proton-proton (pp) chain and also in the CNO cycle. The most important reactions that produces neutrinos in the SSM are: p + p d + e + + ν e (pp neutrinos), p + e + p d + ν e (pep neutrinos), 3 He + p 4 He + e + + ν e (hep neutrinos), 7 Be + e 7 Li + ν e ( 7 Be neutrinos), 8 B 8 Be + e + + ν e ( 8 B neutrinos). The combined effect of all the thermonuclear reactions can be written as 4p + 2e 4 He + 2ν e MeV E ν (2.24) where MeV is the thermal Q-value. The average neutrino energy is E ν.6 MeV. The energy spectrum of solar neutrinos is shown in Fig The main components of the spectrum and their fluxes are listed in table 2.1. At energies above 5 MeV, solar neutrino oscillation takes place in the Sun through a resonance known as the Mikhaev-Smirnov-Wolfenstein (MSW) effect that takes into account the matter effect in the oscillations. While all flavours of neutrino undergo scattering from electrons via Z exchange (NC), in the MeV energy range only ν e and ν e can scatter via W ± exchange (CC), since ν µ and ν τ have insufficient energy to generate the corresponding charged leptons. So the ν e experiences an extra potential, which underlies the resonance phenomenon. It happens that a ν e starts out in the solar core, predominantly in what in a vacuum would be termed as the ν 1 eigenstates of mass m 1. The extra potential increases the effective mass of the ν e to the mass value m 2, which is the ν µ flavour eigenstate in vacuum. 9

16 Figure 2.5: The predicted solar neutrino energy spectrum for the main reactions of the proton proton chain, taken from [1]. For continuous sources, the neutrino fluxes are given in cm 2 s 1 MeV 1 at the Earth s surface. For the monoenergetic lines the units are cm 2 s 1. ν Flux (cm 2 s 1 ) pp 5.97(1 ±.6) 1 1 pep 1.41(1 ±.11) 1 8 hep 7.9(1 ±.15) Be 5.7(1 ±.6) B 5.94(1 ±.11) 1 6 Table 2.1: The neutrino fluxes for the pp-chain reactions in the Sun, predicted by [11] 1

17 So, at the end the result is that the m 2 state emerges from the Sun and a ν e ν µ conversion is enhanced. The first experiment that showed evidence of the solar neutrino problem was carried out by R. Davis et al. at the Homestake mine in South Dakota, USA [12] in the 196s. In this experiment, the target used for detecting solar ν e was a huge tank located 148 m underground in the mine, filled with tetrachloroethylene (C 2 Cl 4 ). Neutrinos were detected in the reaction: ν e + 37 Cl 37 Ar + e (2.25) which has a threshold of 814 kev. Then the 37 Ar can be extracted and counted with the electron-capture reaction: 37 Ar + e 37 Cl + ν e + E (2.26) Due to the energy threshold, the Homestake experiment was sensitive mainly to 8 B and 7 Be neutrinos, as shown in Fig The measured rate, 2.56±.23 SNU, was about one third of that predicted by the SSM, with a discrepancy of more than 3σ [12]. Another important disappearance experiment was GALLEX, later renamed GNO [13], which was also a radiochemical experiment, using 71 Ga instead of chlorine as the target for neutrino interactions. It confirmed the Homestake results in a wider spectrum range: it had an energy threshold of 233 kev so it was sensitive also to the pp neutrinos [13]. Another class of disappearance experiments are the water-cherenkov ones. They use a water mass (of the order of kton or tens of ktons) as the target and, at the same time, as a Cherenkov detector to reveal the electrons scattered in the elastic interactions with neutrinos. Since the outgoing e has the same direction of the incoming ν e it is possible also an angular distribution measurement. The elastic scattering reaction makes the detector sensitive to all neutrino flavours. However, the sensitivity to ν µ and ν τ is much smaller than the sensitivity to ν e since σ(ν µ,τ ).16 σ(ν e ). (2.27) Kamiokande and Super-Kamiokande (located in the Kamioka mine in Japan) [14] and SNO (the Sudbury Neutrino Observatory, located in the Craighton mine, Ontario, Canada) [15] belong to this category of experiments. SNO has been able to confirm the two-flavor oscillation of the solar ν e and the measurement of the survival probability, P νe νe 1/3, with a 7σ significance [15]. This measurement has been made using 1 kton of D 2 O in which could occur the CC process: ν e + d e + p + p (2.28) the NC process: and the elastic scattering process (ES): ν x + d p + n + ν x (2.29) ν x + e ν x + e (2.3) The peculiarity is that the last two processes (NC and ES) are equally sensitive to the three neutrino flavors, not only electronic ones, and this could verify the SSM predictions independently of the oscillation hypothesis. 11

18 Figure 2.6: Allowed regions in the (tan 2 θ 12, m 12 2 ) plane, for solar and KamLAND (see sec ) data from the three-flavor oscillation analysis, where θ 13 is a free parameter. The side panels show the χ 2 profiles projected onto the tan 2 θ 12 and m 12 2 axes [18]. Another real time solar neutrino experiment is Borexino, located in the underground facility at Laboratori Nazionali del Gran Sasso (LNGS). This experiment succesfully observed solar neutrinos via ν e scattering in 3 tons of ultra-pure liquid scintillator. The detection threshold was 25 kev, so the flux of 7 Be neutrinos has been directly observed for the first time [16]. Exploring a low energy range in the spectrum of solar neutrinos, E < 1 MeV, is crucial for the understanding of the oscillation mechanism, especially in relation to the Mikheyev-Smirnov-Wolfenstein (MSW) effect. Borexino also measured the flux of pep solar neutrinos and an upper limit of the CNO solar neutrino flux has been also determined by assuming the MSW large mixing angle solution [16]. All the experiments described are compatible with the hypothesis of a two-flavor oscillation in the (ν 1, ν 2 ) sector, ruled by the parameters m 12 2 and θ 12. Some constraints on the 12

19 mixing parameters can be obtained from a global best-fit of experimental data. An up-to-date version of such constraints includes the global results from the solar neutrinos experiments and from KamLAND experiment, in the assumption of MSW oscillations (see [17] and also section 2.3.3). The constraints deriving from the fit are displayed in Fig They indicate as best-fit solution the so-called large mixing angle (LMA) solution: m ev 2 and θ Atmospheric neutrino oscillation Atmospheric neutrinos are generated by the interaction of primary cosmic rays (mainly protons) in the upper atmosphere, through the reactions: π + µ + + ν µ, µ + e + + ν e + ν µ (2.31) π µ + ν µ, µ e + ν e + ν µ (2.32) In the low energy region, E ν < 1GeV, all the muons decay in flight and the flavor ratio R = (ν µ + ν µ )/(ν e + ν e ) 2 (2.33) is predicted at the level of a few percent in the range.1 1 GeV [19]. Moreover, the neutrino flux is expected to be up/down symmetric with respect to the horizon, given the spherical symmetry both of the cosmic radiation and of the atmosphere. Atmospheric neutrinos have been studied by several underground experiments, in particular by Super-Kamiokande (SK) [14]. The SK detector is simultaneously sensitive to the downgoing neutrinos coming from above, which have travelled a distance L 2 km, and to the upgoing neutrinos, which are detected after crossing the Earth, so their covered lenght can be estimated as Earth s radius: L 1 km. SK detects ν µ and ν e through neutrino-nucleon collisions: ν l + N l + X (2.34) which produce, respectively, muons and electrons. These can be distinguished thanks to their different Cherenkov light pattern through the water bulk, allowing the distinction between µ-like and e-like events. However, a water Cherenkov detector cannot measure the charge of the final-state leptons, therefore neutrino and antineutrino induced events cannot be discriminated. The measurement of R in SK showed a deficit if compared to its expected value. In addition, the ν µ flux proved to be dependent on the zenith angle, with the rate of downgoing µ events predominating over the upgoing ones. On the other hand, the e-like events didn t exhibit any up/down asymmetry [14]. This characteristic feature may be interpreted that muon neutrinos coming from the opposite side of the Earth s atmosphere oscillate into other neutrinos and disappeared, while oscillations still do not take place for muon neutrinos coming from above the detector. Disappeared muon neutrinos may have oscillated into tau neutrinos because there is no indication of electron neutrino appearance. SK data can be interpreted as vacuum two-flavour oscillations regulated by the m 2 23 and θ 23 parameters. Fig. 2.7 shows the results of the global fit on SK data [2] for two-flavor ν µ ν τ oscillations as a function of L/E. The oscillation is confirmed by the characteristic sinusoidal behavior of the conversion probability. The position of the dip fixes m ev 2, while its depth fixes sin 2 2θ

20 Figure 2.7: Ratio of the data to MC events (points) as a function of the reconstructed L/E value in SK, where the MC prediction is set to the hypothesis of no oscillations. The error bars are statistical only. The best-fit for two-flavor ν µ ν τ oscillations is also shown (solid line) Reactor neutrino oscillation Reactor beams are low energy ones, extending to E ν 1 MeV only, with the maximum event rate for the detection process ν e + p n + e + (2.35) at 5 MeV. Thus such beams can only be used for disappearance experiments, since even if the transformation ν e ν µ occurs the energy is below the threshold to produce a muon. Reactor ν e disappearance experiments with L 1km and E ν 3MeV are sensitive to E/L ev 2. At this baseline the angle θ 13 can be directly measured. The reactor neutrino oscillation experiment at the CHOOZ nuclear power station in France [21] was the first experiment of this kind. It found no evidence for ν e disappearance within the experimental sensitivity, but it could provide a limit on the oscillation probability expressed by an esclusion plot in the m 2 versus sin 2 2θ plane. In March 212, the reactor neutrino experiment Daya Bay succeeded in measuring θ 13. Its results on reactor ν e disappearance measured with near and far detectors [22] yielded sin 2 2θ 13 =.92 ±.16 ±.5. (2.36) Another reactor experiment is KamLAND, a 1-kton ultra-pure liquid scintillator detector located at the old Kamiokandes site in Japan, with a sensitive m 2 range down to 1 5 ev 2 [23]. The primary goal of this experiment was a long-baseline neutrino oscillation studies using ν e emitted from nuclear power reactors. KamLAND obtained clear evidence of an event deficit, expected from neutrino oscillations. A combined global solar + KamLAND analysis showed that the Large Mixing Angle is a unique solution to the solar neutrino problem with a confidence level > 5σ [24]. 14

21 2.3.4 Accelerator neutrino oscillation The m ev 2 region can be explored by accelerator-based long-baseline experiments with typically E 1 GeV and L 1 km. The aim is to reproduce the observations on ν µ ν τ oscillations made with the atmospheric neutrinos. With a fixed baseline distance and a known neutrino spectrum, the value of m 2 and also the mixing angle probed by atmospheric neutrinos experiments can be studied. The K2K (KEK-to-Kamioka) long-baseline neutrino oscillation experiment [25] is the first accelerator-based experiment with a neutrino path length extending hundreds of kilometers. K2K aimed at confirmation of the neutrino oscillation in ν µ disappearance in the m ev 2 region. A wide-band muon neutrino beam having L = 25 km and E ν 1.3 GeV was produced. A near detector provides informations about the unoscillated beam spectrum and composition, while a far detector studies the ν µ oscillations in a region where the L/E ratio is close to the first oscillation minimum. The neutrino energy was measured using the quasi-elastic channel: ν µ + n µ + p (2.37) The experiment observed a distortion and a suppression of the energy spectrum. In a twoflavor oscillation scenario, the allowed m 2 region at sin 2 2θ 23 1 is between 1.9 and ev 2 at 9 % C.L., with a best-fit value of ev 2, results which are consistent with the atmospheric neutrinos experiments [25]. MINOS is the second long-baseline neutrino oscillation experiment with near and far detectors. Both K2K and MINOS are disappearance experiments because the energy is under the τ production threshold. MINOS uses a high intensity neutrino beam, with an average energy of 4 GeV, produced at FERMILAB and directed to the far detector located in the Soudan mine in Minnesota, about 73 km away. MINOS also observes distortions of the ν µ energy spectrum [26]. Fig. 2.8 shows the constraints on m 23 2 and θ 23 deriving from a global fit to SK and MI- NOS results [26]. Appearance experiments, where a second flavour of neutrino is detected in an initially single-flavour (or almost so) beam, can probe to much smaller mixing angles. For such experiments the detection depends on the observation of the corresponding charged lepton and thus requires a beam energy above threshold for its production. This the case of the OPERA experiment, which will be described in detail in the next section. Another neutrino oscillation experiment with an accelerator-produced beam is T2K, which is an off-axis long-baseline experiment. The baseline distance is 295 km between the J-PARC in Tokai, Japan and Super- Kamiokande. A narrow-band ν µ beam is directed 2.5 off-axis to SK. With this configuration, the ν µ beam is tuned to the first oscillation maximum. In June of 211, the T2K Collaboration reported indication of ν µ ν e oscillations, i.e., appearance of ν e in a beam of ν µ with six candidate ν e events. This observation implies a non-zero θ 13 with a 2.5 σ significance [27]. 15

22 Figure 2.8: Likelihood contours of 9 % C.L. around the best fit values for the mass splitting and mixing angle obtained by SK and MINOS experiments [26]. 2.4 Current estimates of the neutrino mixing parameters Our present knowledge about neutrino flavor oscillation comes mostly from best-fit analysis of data coming from different experiments, which fixes constraints on the possible values of m kh 2 and θ kh. m 21 2 can be identified with the smaller of the two neutrino mass squared differences, which, as it follows from the data, is responsible for the solar ν e oscillations and the reactor ν e oscillations observed by KamLAND. The larger neutrino mass square difference m 31 2 or m 32 2, can be associated with the experimentally observed oscillations of the atmospheric ν µ and ν µ and accelerator ν µ. It follows from the more recent data of the Daya Bay experiment [22] that the element U e3 = sin θ 13 of the neutrino mixing matrix is small. Hence it is possible to identify the angles θ 12 and θ 23 as the neutrino mixing angles associated with the solar ν e and the atmospheric ν µ (and ν µ ) oscillations, respectively. As a consequence m 21 2 and m 31 2 are often referred to as the solar and atmospheric neutrino mass squared differences [8]. The best-fit values of the 3-neutrino oscillation parameters are listed in table 2.2. Not all the results and analysis available are in agreement with the best-fit values. Recent experiments have found anomalies in the reactor ν e disappearance data providing some support for the existence of a fourth non-standard neutrino state that could drive neutrino oscillations at short distances, i.e. a sterile neutrino. Experiments at reactordetector distances L < 1 m leads to a ratio of observed event rate to predicted rate of.943 ±.23, corresponding to a percentage of antineutrinos missing. Reactor data lead to a solution for a new neutrino oscillation, such that m 2 new > 1.5 ev 2 (95% C.L.) and 16

23 Parameter Best-fit m ev 2 m ev 2 sin 2 θ sin 2 θ sin 2 θ ±.34 Table 2.2: The best-fit values and allowed ranges of the 3-neutrino oscillation parameters, derived from a global fit of the current neutrino oscillation data. For more details see [8]. sin 2 (2θ new ) =.14 ±.8 (95% C.L.) [28]. The sterile neutrino hypotesis is in good agreement also with the so-called Gallium Anomaly that refers to the discrepancies in the ratio of measured to observed events in gallium solar neutrino experiments. At least one more light neutrino with mass in the 1 ev range is needed also to understand the LSND and the MiniBooNE results. The short-baseline accelerator experiment LSND [29] observed a possible indication of ν µ ν e oscillations. Also the MiniBooNE Collaboration reported a 1.5 excess of ν e events [3], which is marginally consistent with the LSND data. These hypotheses should be checked with future studies and experiments. 17

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25 Chapter 3 The OPERA experiment OPERA [31] is a long-baseline hybrid experiment dedicated to the direct observation of ν µ into ν τ oscillations through τ appearance measurements, using the CNGS beam [32]. The detector is a massive lead/nuclear emulsion target complemented by electronic trackers and spectrometers. Nuclear emulsions are used as high resolution tracking devices, for the observation of the τ leptons decay that may be produced in neutrino charged current interactions. Electronic detectors are needed to have a time trigger, to determine the event position, to identify muons and reconstruct their charge and momentum. The experiment is placed in the Hall C of the underground LNGS laboratories. About 14 m of rock provide excellent shielding from cosmic rays, corresponding to about 38 meters of water equivalent, so that the flux of cosmic muons is reduced by a factor 1 6. The residual muon flux is 1 m 2 h 1 (θ <.4 rad). The physics runs started in 28 and a first ν τ candidate was observed in 21 [33]. The observation of a second tau neutrino interaction was announced by the OPERA Collaboration in 212. On March 213, a third ν τ candidate was presented. 3.1 The CNGS beam Figure 3.1: Main components of the CNGS beam line. Two focusing magnets, called horn and reflector focus positive π and K within a wide range of momenta and angles in the LNGS direction. A hadron stopper absorbs the π and K that didn t decay in the 1 km long decay tube, as well as the protons of the primary beam which didn t interact in the target. 19

26 nu_mu flux n/cm2/gev/1^19 pot hnumuflux Entries 8 Mean RMS n/cm2/gev/1^19 pot anti_nu_mu flux hantinumuflux Entries 4 Mean RMS E (GeV) (a) E (GeV) (b) n/cm2/gev/1^19 pot nu_e flux hnueflux Entries 2 Mean 26.2 RMS E (GeV) (c) anti_nu_e flux n/cm2/gev/1^19 pot 2 hantinueflux Entries 2 Mean RMS E (GeV) (d) Figure 3.2: The four neutrino components of the CNGS beam; data are taken from [34]. The incoming ν µ beam is produced at the CNGS facility at CERN [32]. A sketch is shown in Fig A 4 GeV proton beam extracted from the Super Proton Synchrotron (SPS) is directed onto a graphite target, producing several outgoing particles, mainly kaons and pions. Only the positively charged mesons are selected, using two focusing magnets, since their decays give µ + ν µ pairs. A helium bag is placed after each magnet, in order to minimize the probability of secondary interactions. The mesons decay in flight in a 1 km long vacuum tube (decay pipe). The muons are absorbed within some hundred meters of rock, while neutrinos can travel underground for about 73 km before reaching the target at the LNGS. At CERN the muons are monitored in order to provide indirect measurements of the neutrino beam intensity and energy spectrum. The CNGS beam has a small contamination of ν µ (2.1 %), ν e (.8 %), ν e (.7 %) and a negligible ν τ content. The fluxes of the four components of the beam are displayed in Fig The mean energy of the ν µ beam is 18 GeV, a value which is optimized for the study of ν µ ν τ oscillations. Fig. 3.3 shows the overlap of the CNGS beam ν µ flux with the function P osc (E) σ τ (E). For the OPERA experiment purpose the ν µ spectrum should not extend much beyond 3 GeV [35]. 2

27 Figure 3.3: ν τ CC cross section times oscillation probability for small m 2 and full mixing, compared to the ν µ fluence [35]. 3.2 Target and electronic detectors As shown in Fig. 3.4 the OPERA detector [36] is composed by two identical super modules (SM). It is 2 m long in the beam direction, which is referred as the z-axis, and has a transverse section of about 1 1 m 2. Each of the two SM consists of about 73 lead/emulsion bricks arranged in 31 vertical target planes, also called walls. Each target wall is followed by a pair of orthogonal scintillator strip arrays with an effective granularity of cm 2. Strips are connected to multi-anode photomultiplier tubes. The signal amplitude is read out in terms of number of recorded photo-electrons. The horizontal strips read the y coordinate, while the vertical strips read the x coordinate, see Fig The scintillator active part of each target is called Target Tracker (TT). A muon spectrometer downstream of each SM allows to measure the muon charge and momentum. Each muon spectrometer consists of a dipolar magnet made of two iron arms, with a measured magnetic field intensity of 1.52 T. The iron plates of the magnets are interleaved with planes of resistive plates chambers (RPC), 11 planes in each of the two arms of the magnet. Each RPC plane is equipped with two orthogonal sets of readout strips cm 2 in the bending plane with a digital readout. These planes provide a coarse tracking, range measurement of the stopping particles and a calorimetric analysis of the hadrons escaping the target along the incoming neutrino direction. In Fig. 3.6 a picture of an OPERA spectrometer is displayed, while Fig. 3.7 shows the transverse section of a RPC module. Each spectrometer is also equipped with 6 stations of vertical drift-tube planes, called High Precision Trackers (HPT), for the precise measurement of the bending of the muon track, see Fig This allows accurate momentum and charge determination. The spatial resolution of a single HPT tube is measured to be less than 3 µm. Two planes of resistive plates chambers (XPC) are also placed after each target section, with the readout strips inclined by ±42.6 with respect to the horizontal, in order to resolve the left/right ambiguities in the reconstruction of the tracks. 21

28 Figure 3.4: View of the OPERA detector; the neutrino beam enters from the left. The upper horizontal lines indicate the position of the two identical super modules (SM1 and SM2). The target area is made of walls filled with lead/emulsion bricks interleaved with 31 planes of plastic scintillators (TT) per SM. Arrows show the position of the 2 VETO planes, the high precision tracker planes (HPT), the XPC planes (there is a pair of them at the end of each target area), the magnets and the 44 RPC planes (22 in each SM) installed between the magnet iron slabs. The Brick Manipulator System (BMS) is also visible. See [36] for more details. A large size anti-coincidence detector placed upstream of the first SM, the VETO, allows to exclude (or tag) interactions occurring in the material and in the rock upstream of the target. It consists of two planes of glass RPCs. The ensemble of these detectors is named Electronic Detectors (EDs). Although the EDs are not conceived to perform calorimetric measurements, they can be used for this purpose with a coarse resolution once calibrated. Their final and more important purpose is to select the brick in which the interaction took place through complex algorithms of tracks reconstruction, wall finding and brick finding. An example of a ν µ CC event, as seen by the OPERA EDs, is shown in the upper part of Fig. 3.9, where the long tail of hits easily identifies a high momentum muon track. The lower part shows instead a NC event. 22

29 Figure 3.5: Scheme of the target. Each target plane is followed by two planes of scintillators, one with horizontal strips, which read the y coordinate, and one with vertical strips, which read the x coordinate. ECC bricks will be described in 3.5 Figure 3.6: Picture of an OPERA spectrometer. In the oval frame, a schematic lateral view of one of the 2 arms: RPC modules are interleaved with the iron slabs of the magnet. Being the magnetic field orientated along the y axis, the bending of the tracks is on the horizontal plane. 23

30 Figure 3.7: Schematic picture of a section of one RPC module. The 2 bakelite electrodes are separated by a 2 mm gap, filled with a gas mixture composed of Argon, Tetrafluoroethane, Isobutane and Sulphur Hexafluoride (SF6). The RPCs are operated in streamer mode. The x and y strips for the readout of the induced electric signal are also visible (in red). Figure 3.8: Schematic layout of each spectrometer. Interleaved with the iron slabs (in the picture, the brown strips) are the RPC planes, 11 for each arm (the white strips). The 6 HPT planes are denoted by x 1 - x 6. 24

31 Figure 3.9: Charged current event (upper plot) and neutral current event (lower plot) as seen in the OPERA electronic detectors. The long tail of hits in the upper plot easily identifies the muon track. 25

32 3.3 Data Acquisition and Software The aim of the DAQ is to create a permanent database of the detected events. OPERA has no hardware trigger and the DAQ is based on a software selection of meaningful events to separe them from noise. Taking advantage of the low rate, data from all the electronic channels are kept until their online analysis is completed [31]. The online filter algorithm is based on requirements on the observed activity in the TT and RPC detectors and on a cut in the overall number of recorded hits [37] Synchronization The events recorded by the OPERA DAQ can be correlated in time with the CNGS beam by comparing their coordinated universal time (UTC timestamp) with the one of the proton extractions from the SPS [36]. The CNGS timing structure is such that, for each CNGS cycle, protons are extracted from the SPS in two spills lasting 1.5 µs each and separated by 5 ms. This is illustrated in the left part of Fig. 3.1: a two peak structure, with peaks 5 ms apart, can be clearly seen. In the right part of Fig. 3.1 the 1.5 µs time width of each peak is displayed. The events of the peaks can be correlated in time with the CNGS beam and hence marked by the reconstruction program with the OnTimeWithCNGS flag. The offline program correlating the events with the beam selects events where the difference of the OPERA and CNGS time stamps is within a window of 2 µs. The events outside the two peaks are cosmic events. Taking into account all time calibrations, the time synchronization accuracy is of about 1 ns. This accuracy is largely sufficient to correlate genuine events with the beam Reconstruction of tracks The events recorded by the OPERA DAQ are processed with an offline reconstruction program, named OpRec. At first hits are reconstructed quasi ontime in the electronic detectors. As a result of the pattern recognition phase, hits are merged in segments in the x-z and y-z plane. Hits that do not seem to belong to any segment are left alone. Segments in the x-z and y-z plane are then merged together to obtain three dimensional tracks. Two algorithms have been developed to perform the muon identification [38]. The first algorithm can be applied to all events. The criterion to classify CC and NC events is based on the total number of ED planes containing hits. The TT walls and the RPC planes are equally treated in this calculation. In order to meet the requirement of the OPERA detector proposal of a CC tagging efficiency greater than 95%, the lower cut on the number of ED planes must be set to 14 [36]. The second algorithm is based on a precise reconstruction and identification of the muon track. It can therefore be applied only to events where a track exists. The 3D track lenght multiplied by the density along the track path is computed and used to identify a generic track as a muon track. Requiring a muon identification at the level of 95% implies a cut at 66 g cm 2. For each reconstructed track, the algorithm also provides an estimate of the momentum and 26

33 Figure 3.1: Beam timing structure as measured by the OPERA experiment. charge. If the track stops in the target or leaves the target along the beam direction but does not fully cross at least one spectrometer arm, the measurement of the momentum can be inferred from the range of the particle, but the charge measurement is not available. The condition under which the charge of the particle is measured is called bending condition. This condition corresponds to a particle passing through both RPC arms of one SM and hence being bent by the magnetic field OpCarac algorithm for the event classification The OpCarac event classification algorithm [39] is run after the offline reconstruction. As a first step, the algorithm performs a selection based on the OnTimeWithCNGS flag. All cosmic events recorded by OPERA are hence discarded after this first filter. Among the selected events the algorithm identifies the following classes: FRONTMUON: events with at least one muon track that has a high probability of being produced upstream of the target. SIDEMUON: events with at least one muon track. The z coordinate of the track start is within the target boundaries but the transverse position of the track start lies within some border areas, at the edge of the active target. SPECTRO: events produced by a neutrino interaction in the iron of the first or second spectrometer of the OPERA target. CONTAINED: any event that does not belong to the already mentioned classes, i.e. events with the primary vertex in the target. 27

34 3.4 Monte Carlo simulations In order to get a prediction for the number of expected neutrino interactions in OPERA, ν µ CC quasi elastic (QE), resonant (RES) and deep inelastic scattering (DIS), ν µ CC DIS and ν µ NC DIS interactions have been simulated and studied. The final states for the different processes have been generated using the NEG MC [4] package, developed in the framework of the NOMAD experiment [41]. Once primary vertices are generated, the outgoing particles are tracked and the interaction in the material is performed by using the Geant3 [42] simulation package. All MC events are then processed with the same version of reconstruction package used for the data, OpRec, and the event classification algorithm, OpCarac. Neutrino interactions do not occur only in the target, so primary vertices have been generated not only in the lead/emulsion volumes and in the volumes of the TT scintillator bars. Other sources of neutrino interactions that can actually project particles in the target are: detector structures surrounding the target, any material present in the experimental hall, the rocks surrounding and upstream of the detector with respect to the incoming neutrino direction and also the BOREXINO detector and its related facilities located in Hall C. Especially muons, produced by CC interactions in these volumes, can easily reach the OPERA target: all these events are called external events and have to be carefully taken into account in the simulations. In order to obtain a prediction for the number of expected neutrino interactions in OPERA, the differential neutrino cross sections, dσ/de, are convoluted with the CNGS neutrino flux. The following cross sections on isoscalar target are assumed: σ CCDIS = cm 2 /GeV < E > where < E > is the average ν µ beam energy set to 18.5 GeV. for the QE interaction, ν µ + n µ + p σ QE = cm 2 for the three reactions building up the RES contribution: ν µ + n µ + p + π, σ RES n,π = cm 2 ν µ + n µ + n + π +, σ RES n,π + = cm 2 ν µ + p µ + p + π +, σ RES p = cm 2 The total cross section on isoscalar target becomes then: where σ RES n = σ RES n,π + σ RES n,π +. σ tot iso = σ DIS + σqe σres n + σ RES p 2

35 The CC-DIS, CC-QE and CC-RES fractions are corrected for the non-isoscalarity effects of the materials used in the OPERA detector. The NC to CC ratio on isoscalar target is assumed to be.3 using the procedure outlined in [9]. Finally the number of expected events for each process and volume is calculated using the mass of the material. The ν µ event rates are normalised to 1 kton 1 19 PoT. The ν µ disappearance oscillation probability, P (ν µ ), has also been applied to the MC CC samples. The formula used was where: k = GeV/(km ev 2 ), L is the CNGS baseline: km, P = 1 sin 2 (k m 2 L/E) (3.1) m 2 is the mass difference in the two mass eigenstates scenario. The central value is taken to be ev 2, E is the MC event neutrino energy. The generated events can then be mixed according to the computed fractions. Further details are given in [43]. 3.5 ECC Bricks and Nuclear Emulsions A brick (or ECC, Emulsion Cloud Chamber) is a mechanical unit which contains 57 emulsion layers interleaved with 56 lead sheets 1 mm thick. The transverse size of the brick is cm 2. Each emulsion layer has two 44 µm thick emulsion films (called top and bottom respectevely) paused on a 212 µm thick plastic base, see Fig Downstream of each ECC brick there is an emulsion films doublet, called Changeable Sheets Doublet (CSD), as shown in Fig The CSD plays two major roles. The first one is to confirm that the ECC brick which contains the neutrino interaction vertex is the one pointed to by the TT reconstruction. The second one is to provide the neutrino-related tracks for the ECC brick analysis. Nuclear emulsions consist of AgBr crystals scattered in a gelatine binder. After the passage of a charged particle, electron-hole pairs are created in the AgBr crystal. The excited electrons are trapped in the lattice defects on the surface of the crystal and Ag metal atoms are created, which act as latent image centers. During a chemical-physical process known as development, the reducer in the developer gives electrons to the crystal through the latent image center and creates silver metal filaments using silver atoms from the crystal. This process multiplies the number of metal silver atoms by several orders of magnitude and the grains of silver atoms, of about.6 mm diameter, become visible with an optical microscope. After the development, automated microscopes, available at several OPERA institutions in Japan and Europe, allow the emulsion-based data taking. This is a complex, multi-step task consisting of the location of a neutrino interaction vertex by scanning backwards (scanback) along the path of the selected tracks, the vertex reconstruction by scanning a volume of 29

36 Figure 3.11: Side view of an ECC brick. The light yellow bands represent the emulsion films, while the light blue band in between is the plastic support. emulsion around the presumed end points of the tracks, the identification and the validation of any τ decay topology and further optional measurements for efficiency study and kinematical analysis [44]. At the LNGS laboratories Scanning Station, ten microscopes work 24 hours per day and 7 days per week to scan only CS. They have a 2 cm 2 /h scanning speed, a.3 µm spatial resolution, a 2 mrad angular resolution and a 95% base-track detection efficiency. A picture of one of these microscopes is shown in Fig The connection of tracks from the TT to the ECC bricks and to the parent vertex are critical issues, because we have to pick-up a small unit in a massive apparatus and make a link between the approximately 1 cm transverse resolution of the TT to the 1 µm 3D spatial resolution of nuclear emulsions. CSD are used as interfaces between each TT plane and the corresponding ECC brick. When a brick is tagged by the electronic detectors as the one which most probably contains the interaction vertex, it is extracted from the detector and, before proceeding to the development of the 57 emulsion layers, the CSD is checked. The validation of the selected brick is achieved if at least one track compatible with hits reconstructed in the electronic detectors is detected in the CSD films. Fig shows the accuracy distributions of the electronic predictions for the position and the slope, respectively. When a brick is wrongly identified by the TT, a new CS doublet is placed on that brick and it is put back in the target. The brick which contains neutrino interaction is brought up to the surface laboratory, exposed for 12 hours to the flux of cosmic rays for the purpose of film-to-film alignment and disassembled for the development. A schematic illustration of the OPERA scan-back method for event location is shown in Fig

37 Figure 3.12: Picture of a real brick that will be inserted in the OPERA walls; CS is the box containing the two Changeable Sheets [36]. Figure 3.13: Picture of an OPERA European Scanning System τ detection The signal of the occurrence of a ν µ ν τ oscillations is the CC interaction of a ν τ in the detector target with the production of a τ lepton. The reaction is identified by the detection of the τ lepton in the final state through the decay topologies. The branching ratio for the decay in the leptonic channel, τ l + ν l + ν τ, is about 36%. Other decay modes and the respective branching ratios are listed in table 3.1. The expected signal and backgrounds are also shown. All track measured in the CS are sought in the most downstream films of the brick and followed back until they are not found in three consecutive films. The stopping point is considered as the signature either for a primary or a secondary vertex. The vertex is then confirmed by scanning a volume with a transverse size of 1 cm 2 in 15 films in total, 5 upstream and 1 downstream of the stopping point [33]. 31

38 Figure 3.14: Accuracy distributions of the electronic predictions for the position (left) and the slope (right). τ decay channel BR (%) Signal Background τ µ τ e τ h τ 3h ALL Table 3.1: τ decay modes, respective branching ratios, expected signal in OPERA and backgrounds. Signal and background are expressed as a number of events. When a vertex is located, a decay search procedure is applied to detect possible decay or interaction topologies on tracks attached to the primary vertex. If also secondary vertices are found in the event, a kinematical analysis is performed, using particle angles and momenta measured in the emulsion films. The detection of decay topologies is triggered by the observation of a track with a large impact parameter with respect to the primary vertex. If the daughter lepton is a µ, typically it produces a long track in the electronic detectors, which is easy to recognize, and the τ decay can be identified from the kink formed by the µ track with respect to the τ track. In general, for decays of the τ to a single charged hadron it is required that: the kink angle is larger than 2 mrad; the secondary vertex is within the two lead plates downstream of the primary vertex; the momentum of the charged secondary particles is larger than 2 GeV/c; the total transverse momentum (P T ) of the decay products is larger than.6 GeV/c if there are no photons emitted at the decay vertex,.3 GeV/c otherwise. At the primary vertex, the selection criteria are: absence of tracks compatible with a muon or an electron; 32

39 Figure 3.15: Schematic view of a ν τ charged current interaction and the decay of the final state τ lepton as it would appear in an OPERA brick, in the interface emulsion films (CSD) and in the Target Trackers. missing transverse momentum smaller than 1 GeV/c; angle in the transverse plane between the τ candidate track and the hadronic shower direction larger than π/2. All ν µ CC interactions are a potential source of background for the τ search, since a large scattering angle of the outgoing muon can mimic the τ µ kink. Also charmed particles have similar lifetimes as τ leptons and, if charged, share the same decay topologies. Another source of background is due to the hadronic interactions that can mimic the τ hadronic decays. The background can be reduced by applying cuts on the kink angle and on the transverse muon momentum at the decay vertex [31]. The first τ candidate observed in OPERA [33] fulfilled the selection criteria mentioned above and was identified in the hadronic channel as τ ρ + ν τ π + π + ν τ. The reconstructed event is shown in Fig The π decays in two photons that create two electromagnetic showers. The photons point to the secondary vertex. The second ν τ candidate event is shown in Fig The τ decays in three prongs that have been identified as hadrons. The third ν τ candidate event was identified in the τ µ decay channel and is shown in Fig The corresponding event display picture showing the muon track is Fig

40 Figure 3.16: Reconstruction of the first τ candidate event. The track labelled with number 4 is the τ lepton, which decays producing a ρ. The ρ subsequently decays to π π. The π is identified with the number 8 in the picture [45]. Figure 3.17: Reconstruction of the second τ candidate event. The τ decay in three prongs is visible [45]. 34

41 Figure 3.18: Reconstruction of the third τ candidate event. The track labelled with number 4 is the τ lepton, which decays into a muon that is identified with number 1 [45]. Figure 3.19: Electronic detector event display of the third τ candidate event. The red track represents the µ coming from the τ decay. 35

42

43 Chapter 4 Number of PoT evaluation The number of PoT (Proton on Target) is the amount of protons extracted from the SPS that hit the graphite target at CERN, as explained in section 3.1. The aim of this study is the monitoring of the OPERA DAQ performances and, in particular, the correction of the number of PoT provided by CERN for the inefficiencies of the OPERA DAQ. These numbers are extremely important to estimate the expected number of events since the neutrino flux is obviously dependent on the primary proton flux that produces it. To achieve this we made a comparison between the PoT as a function of time and the activity of the OPERA DAQ: cosmic ray rate in sec. 4.1, TT event rate in sec. 4.2 and OnTime event rate in sec The PoT as a function of time were obtained from the TIMBER data base [46]. 4.1 Cosmic rays rate distribution At first cosmic ray events have been taken into account, since we expected a flat distribution as a function of time. Cosmic rays rate and the PoT rate are expressed as a number of events recorded in 3 minutes in Fig The presence of gaps in the cosmic ray rate distribution, as displayed in the zoomed picture, Fig. 4.2, shows that the OPERA DAQ was not always running during the PoT delivery. This was a sign that the real number of PoT had to be computed carefully and, in the time intervals when the DAQ was inactive, the incoming neutrino flux had to be removed from the total. In order to have the effective number of PoT we decided to exclude the PoT delivered while a gap in the event rate distribution is present. A gap is defined as a bin with a number of events under a defined threshold. We decided to set as a threshold for the cosmic ray distribution a rate of 2 events in 3 minutes. Other choices for the threshold have been attempted, but they did not lead to any significant change, because, when a gap is present, the rate is really low, if not exactly zero. With this threshold selection, we could estimate the PoT number for each data taking year and gather gaps related to the DAQ inactivity. 37

44 Cosmic rays rate Events/3 min /3/2 11/5/2 11/7/2 11/9/1 11/11/1 UTC time Events/3 min PoT rate /3/2 11/5/2 11/7/2 11/9/1 11/11/1 UTC time Figure 4.1: The upper plot shows the cosmic rate for 211 run. The lower plot represents the PoT rate distribution. Rates are expressed as a number of events recorded in 3 minutes. The binning is left on purpose as fine as possible to see all the gaps and still have a good statistics. Events/3 min Cosmic rays rate Events/3 min PoT rate /4/25 11/4/26 11/4/27 11/4/28 11/4/29 11/4/3 11/5/1 11/5/2 UTC time 2 11/4/25 11/4/26 11/4/27 11/4/28 11/4/29 11/4/3 11/5/1 11/5/2 UTC time Figure 4.2: Zoom of the two plots of Fig The upper plot shows a gap in the cosmic ray rate distribution for 211. The lower plot represents the corresponding PoT rate distribution. Rates are expressed as a number of events recorded in 3 minutes. 38

45 Year Gap Lenght Number (hours) of gaps wrong UTC dates missing data in Extraction empty extractions missing data in Extractions missing data in Extraction missing data in Extraction missing data in Extractions missing data in Extraction 91 missing data in Extraction missing data in Extraction 964 missing data in Extractions missing data in Extraction missing data in Extraction 968 missing data in Extraction missing data in Extractions missing data in Extractions missing data in Extractions wrong UTC dates in Extraction 884 empty Extraction empty Extraction missing data in Extraction 92 Table 4.1: Gaps in the cosmic ray rate distribution Detailed gaps investigation A detailed gaps investigation has been performed for gaps longer than 5 hours, finding out the different causes for the lack of data. For the bigger gap in 211 there were wrong UTC dates in 5 extractions, which were dated back in These were good data that could be analized thanks to a dedicated recovery. Other problems are shown in table TT event rate distribution To have a more data-focused estimate of the PoT number, we decided to compare the PoT rate to the TT event rate, that is the rate of events that produced a signal in the TT. The possible signal in the RPCs has not been taken into account in this evaluation, because the effective number of PoT is related to the possibility to reconstruct an interaction in a brick, not merely to have a signal of any kind. The strategy selected was to introduce a cut on the TT event rate in both SM and SM1. When the rate was lower than 1 events in 3 minutes in one SM at least, we decided not to sum the number of PoT. 39

46 For years , the TT event rate and the cosmics event rates are presented one after the other from figure 4.3 till 4.8. The TT event rate is divided into the two SM. For 28, the cosmic ray distribution collected before the end of August is not easily accessible due to technical problems, so the comparison has been carried out only for the TT event rate, see Fig. 4.3 and Fig The PoT evaluation using the TT rate is compared to the evaluation obtained from the cosmic ray rate in table 4.2. The tabel entries are δ = reduced initial initial (4.1) where initial is the total PoT integral and reduced is the new integral calculated by skipping the gaps. Year Cosmic ray rate > 2 TTRate SM > 1 AND TTRate SM1 > 1 28 no data 4.42% % 1.79% 21.72%.87% % 1.7% %.76% Table 4.2: Corrections for the PoT number. For 211 data, we did a test changing the binning from 3 minutes to 1 hour. This gave similar results for δ. 4

47 Cosmic ray rate Events/3 min /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time TT Rate sm Events/3 min /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time TT Rate sm1 Events/3 min /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time Figure 4.3: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 28 data. 41

48 TT Rate sm Events/3 min /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time TT Rate sm1 Events/3 min /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time Events/3 min PoT rate /1/1 8/3/2 8/5/2 8/7/1 8/8/31 8/1/31 UTC time Figure 4.4: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 28 data. The event rate in both SM before the start of PoT are not related to the neutrino flux. 42

49 Events/3 min Cosmic ray rate /3/2 9/5/2 9/7/2 9/9/1 9/1/31 UTC time Events/3 min TT Rate sm /3/2 9/5/2 9/7/2 9/9/1 9/1/31 UTC time Events/3 min TT Rate sm /3/2 9/5/2 9/7/2 9/9/1 9/1/31 UTC time Figure 4.5: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 29 data. 43

50 Cosmic ray rate Events/3 min /3/2 1/5/2 1/7/2 1/9/1 1/11/1 UTC time TT Rate sm Events/3 min /3/2 1/5/2 1/7/2 1/9/1 1/11/1 UTC time TT Rate sm1 Events/3 min /3/2 1/5/2 1/7/2 1/9/1 1/11/1 UTC time Figure 4.6: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 21 data. 44

51 Cosmic ray rate Events/3 min /3/2 11/5/2 11/7/2 11/9/1 11/11/1 UTC time Events/3 min TT Rate sm /3/2 11/5/2 11/7/2 11/9/1 11/11/1 UTC time TT Rate sm1 Events/3 min /3/2 11/5/2 11/7/2 11/9/1 11/11/1 UTC time Figure 4.7: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 211 data. 45

52 Events/3 min Cosmic ray rate /1/1 12/3/2 12/5/2 12/7/1 12/8/31 12/1/31 UTC time Events/3 min TT Rate sm /1/1 12/3/2 12/5/2 12/7/1 12/8/31 12/1/31 UTC time Events/3 min TT Rate sm /1/1 12/3/2 12/5/2 12/7/1 12/8/31 12/1/31 UTC time Figure 4.8: From top to bottom: cosmics event rate, TT event rate in SM, TT event rate in SM1 for 212 data. 46

53 4.3 OnTime event rate distribution A further investigation has been performed looking at the gaps in the OnTime event rate distribution. The OnTime flag is put when an event time can be correlated to the CNGS beam time (at Cern), as explained in section It is in this event category that good interactions are searched for. This gap finding operation has the purpose not only to evaluate the effective PoT number, but also, even more importantly, to add new good events to the experiment database. The method is similar to the previous one: the OnTime event rate distribution is compared to the PoT rate distribution, see Fig. 4.9, where 29 data are displayed as an example. When OnTime event rate Events/8 hours /3/2 9/5/2 9/7/2 9/9/1 9/1/31 UTC time Events/8 hours PoT rate /3/2 9/5/2 9/7/2 9/9/1 9/1/31 UTC time Figure 4.9: In the upper plot the OnTime event rate is shown, while the lower one shows the PoT rate. Both histograms have 8-hours bins. a gap in the OnTime event distribution is present we do not sum the corresponding number of PoT. We are sure that these are different gaps compared to those in table 4.1 because we just put a more restrictive condition, i.e. OnTime rate > 5 events in 8 hours, keeping the requirement of a correct TT rate in both SM and SM1. To compute the PoT sum we had to make a comparison between histograms with different binning, because the OnTime event rate was too poor for the 3 minutes binning. We chose to have 8 hours bins for the OnTime distribution and 1 hour bins for the TT rates. 47

54 The updated gaps are listed in table 4.3 and the recalculated PoT are listed in table 4.4. Year Gap Lenght Number (hours) of gaps Extractions Extractions Extractions Extractions Extractions Extraction Extraction Extraction Extraction Extraction Extraction Extraction Extraction Extraction 812 Extraction Extractions Table 4.3: Gaps in the OnTime event rate distribution. Number of PoT for Y ear TTRate SM > 1 events/hour AND δ TTRate SM1 > 1 events/hour AND OnTime rate > 5 events/8 hours % % % % % Table 4.4: Final effective PoT numbers and related corrections. For the extractions with a wrong UTC time, the recovery has been successfully performed, because genuine neutrino interaction events with a wrong time label can be distinguished from cosmics using the overall event topology. So these events have been added to the database where to look for τ production, for example. This affects the effective number of PoT that has to be used in the decay search study. For 211, the extra PoT for the extractions with a wrong UTC time are PoT. These correspond to 36 recovered events. Also extractions with a correct UTC time but without the OnTime flag have been recovered, resulting in a total number of regained events equal to 143. For the electronic detectors data analysis of this thesis, the PoT number used are the ones 48

55 listed in table 4.4, because the manually recovered events could not be easily added to the regular ones. In any case, the statistical increase for the CC sample due to the manually recovered events would correspond to approximately 1 % and so it is rather marginal. For 29 data we could make a check of consistency with CERN official data. As obtained from the CERN web site, the date of the official start of the 29 run was Monday June 1st at 8:. During the CNGS commissioning period, i.e. the period before the official start in which there was already neutrino flux, the CERN web page states that the PoT sum was PoT. Moreover CERN counted PoT from Monday June 1st at 8: till the end of the 29 run. We estimated for the whole year PoT number, while for the PoT delivered till Monday June 1st at 8: (after conversion to UTC). So = , which is essentially identical to the number officially provided by CERN. This supports our analysis and confirms that is correct to count all the PoT from the beginning of the run, including commisioning time, since we identified good events also in that period. 49

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57 Chapter 5 µ track reconstruction efficiency The NC/CC event classification is achieved thanks to two main algorithms and some additional conditions on the data. A comparison between the results of the two algorithms allows a test on the µ track reconstruction efficiency. As previously explained in section 3.3.2, a first algorithm, CC-ID in the following, can be used to recognize CC events. The algorithm states that an event is labelled as CC when the number of TT walls and RPC planes that are fired in the event exceeds 14, otherwise the event is considered NC. The second algorithm, mu-id in the following, reconstructs good tracks and identifies them as muons 1. The mu-id gives charge and momentum of the particle, while the CC-ID does not give any information about them. The CC-ID has the advantage of being available for every kind of event, while the track reconstruction procedure may fail. The µ CC event cathegory contains events that are identified with the CC-ID but do not have a reconstructed track. They are studied in detail in sec The additional bending condition is set when a track passes through both RPC arms of a single super module and so it is bent by the magnetic field. This condition can be applied also to the CC events, because it simply requires a minimum number of crossed planes in each arm of the RPCs, and it does not request the track to be reconstructed. 5.1 Comparison between two methods to evaluate x, y and z coordinates of a muon track We performed a first comparison between the results of two different methods that can be used to determine the x, y and z coordinates of a muon event. The mu-id gives directly the track start coordinates, as it is seen by the electronic detectors 2, while another method, CC-pos in the following, has been developed to calculate x, y and z from a weighted average of the released photoelectrons in the TT walls. 1 The mu-id requirement is defined by a cut on the 3D track lenght multiplied by the density along the track path. If the track lenght density is greater than 66 g cm 2 the muon ID flag is set. 2 It must be pointed out that what is really returned by these methods is the first RPC plane or TT wall after the production of a muon, i.e. the first plane that gives a signal. These methods can not determine the interaction vertex inside a brick, but they can find the brick that most probably contains it. 51

58 Figure 5.1: Event display of a CC event with a long muon track. The number of left and right photoelectrons is available because the readout is made on both sides of the scintillator strip. An additional condition is put to avoid crosstalk. The wall with the highest number of photoelectrons is labelled as max and the average x is calculated as x = max+2 max 2 max+2 max 2 x p.e. p.e. (5.1) The x coordinate is obtained only with the vertically oriented TT walls, likewise the y coordinate uses the horizontally oriented TT walls. The CC-pos method does not require the presence of a long track, i.e. of a muon, so it can be used for every event. To make a preliminary comparison with the mu-id algorithm, only events with a reconstructed muon track, both positive and negative, have been taken into account in the study of the differences. Fig. 5.1 shows, as an example, a CC event with a long muon track that can be reconstructed by the mu-id and that can be studied also with the CC-pos. The distributions of the x, y and z differences for the two methods are shown in Fig. 5.2, 5.3, 5.4. The 211 data sample is used for this study. The binning is chosen commensurate with the brick dimensions 3. The distributions are well compatible with zero for the x and y coordi- 3 The brick dimensions are cm 3 52