SAPIENZA Università di Roma Facoltà di Scienze Matematiche Fisiche e Naturali Corso di Laurea Magistrale in Fisica Dipartimento di Fisica CERN-THESIS-13-94 19/1/13 Study of the decay B χ c1 K + π and search of exotic resonances at LHCb Candidato: Francesco Sbordone Matr. 1348865 Relatore: Dr.ssa Roberta Santacesaria Correlatore: Dr. Antonio Augusto Alves Jr Anno accademico 1/13
Abstract In 8 the Belle Collaboration reported the observation of two charged resonancelike structures in the χ c1 π mass spectrum produced in the decay B χ c1 K + π. These were labelled as Z 1 (45) and Z (45). Alternatively, a single wider resonance hypothesis was also pursued by Belle, and labelled as Z(415). The fact that these are charged states would be a clear sample, if they really exist, of four quark bound systems; for this reason this observation has given rise to a great deal of interest. In 1 the BABAR Collaboration has searched for these resonances in the channels B,+ χ c1 K +, π and did not find any evidence of them. In this thesis a search for these claimed exotic charmonium-like states Z 1 (45) and Z (45) is presented, in the decay B χ c1 K + π, where χ c1 J/ψγ and J/ψ µ + µ. Charged conjugate are implied throughout the whole thesis. The analysis is performed using data collected with the LHCb detector at the p p Large Hadron Collider operating at a center-of-mass energy of 7 and 8 T ev. The data sample consists of the LHCb full statistics from years 11-1 data taking, corresponding to an integrated luminosity of 3 fb 1. LHCb has more than twice the Belle and BABAR cumulative B χ c1 Kπ signal events. Hence it is the right environment to clarify the existence of exotic states in the χ c1 π system. The analysis strategy adopted in this thesis is inspired to what BABAR did in []. The BABAR approach, unlike the Belle one, is model independent and its philosophy could be shortly summarized saying that any exotic explanation should not be given as long as a reasonable explanation of the data can be found in terms of traditional effects, namely if the data can be explained only in terms of the relatively well known resonances in the K + π system. In the BABAR analysis, first a representation is given of the K + π mass and angular distribution structures dominating the final state in terms of their expected angular-momentum intensity contributions. Then the reflection of each K + π component into the χ c1 π mass distribution is investigated in order to establish if any additional signal is needed or not. BABAR conclusion is that, in the limit of their statistical significance, no additional states are required in the χ c1 π spectrum to explain the data. The reflections of the known resonances in the system K + π are sufficient, and there are no needs for introducing any exotic Z. 3
Contents Contents 1 Introduction 7 Exotic charmonia experimental and theoretical status 9.1 Overview.................................... 9. Theoretical Models.............................. 1..1 Charged charmonium-like states................... 13.3 Experimental status............................. 14.3.1 X(387) quantum numbers..................... 14.3. Z(443) exotic............................ 16.4 Z 1 (45) and Z (45) Belle observation................. 17.5 Z1 and Z search at BABAR.........................6 Z1 and Z search in LHCb......................... 3 The LHCb experiment 3 3.1 LHC...................................... 3 3. The LHCb detector.............................. 5 3..1 Tracking detectors.......................... 7 3..1.1 Magnet........................... 8 3..1. Vertex Locator....................... 8 3..1.3 Tracking system...................... 31 3.. Particle identification (PID)..................... 34 3...1 RICH system........................ 34 3... Calorimeter system..................... 35 3...3 Muon system........................ 36 3.3 LHCb analysis workflow........................... 37 3.3.1 Trigger................................. 39 3.3. Monte Carlo.............................. 4 4 Data Analysis 41 4.1 Data sample.................................. 41 4.1.1 Stripping line cuts.......................... 4 4.1. Decay chain reconstruction...................... 43 5
Contents 4.1.3 Cut-based selection.......................... 44 4.1.4 Multivariate selection......................... 5 4.1.4.1 Training phase....................... 53 4.1.4. Application phase and MV results............ 61 4. B mass fit and background subtraction strategy............. 64 4..1 Background subtraction....................... 67 5 Analysis Method 71 5.1 Dalitz Plots and unidimensional projections................ 71 5. Resolution effects............................... 74 5.3 Efficiency estimates and parametrisation.................. 77 5.4 Model independent approach......................... 84 5.4.1 K + π angular distribution representation............. 84 5.5 Toy MC prediction.............................. 9 5.6 Fits to data χ c 1π mass distribution.................... 98 6 Conclusions 13 A splot technique 15 References 19 Ringraziamenti 113 6
1 Introduction In this thesis, an analysis for the decay channel B χ c 1K + π with the LHCb detector at the Large Hadron Collider, at the countries boundary between France and Switzerland, is presented. This channel has been previously studied by the Belle Collaboration, at the KEKB e + e collider in Tsukuba, Japan, and the observation of two new states Z1, χ c 1π was reported. In the perspective of the charmonia spectrum, a charged state s minimal quark content is necessarily exotic, namely c cdū. The BABAR Collaboration also studied this decay, at the PEP-II e + e storage ring in Stanford, USA, and did not find evidence for the existence of these states. In particle physics, a well accepted rule is to await at least two experiments to observe or to neglect a new state, before considering each single result as established. In fact, Belle and BABAR find different results in analysing the peaking features of the χ c 1π mass spectrum. The LHCb experiment offers the possibility to resolve the controversy. The thesis is divided into the following main sections. In Section, after a brief introduction concerning possible type of bound states in the Standard Model of particle physics, a short summary of the heavy quarkonium physics framework is presented, bringing out the importance of new states evidences and discoveries. Some consequent theoretical interpretation are also presented, showing that the topic of charged exotics gave rise to a considerable interest, and these are well discussed among theorists. Greater focus is set on the description of the Z resonances, and the different approaches of Belle and BABAR, in analysing data showing approximately the same features. In Section 3 a description the LHCb detector is given, emphasising its features of importance for a correct reconstruction of the decay channel under study. Some focus is spent on the LHCb offline processing, explaining the LHCb Stripping process, which is a particular LHCb trademark and it is not used in other LHC experiments. The main components of the LHCb software framework, named the GAUDI Framework, are also described; this is used to elaborate data, perform reconstructions and produce simulations. In Section 4 a description of the full decay chain reconstruction and signal selection for B χ c 1K + π is presented. Its starting point is the semi-inclusive decay p + p B J/ψ + anything, where J/ψ candidates are high transverse momentum di-µ with a detached vertex from the reconstructed p p primary vertex. Signal selection is 7
1 Introduction achieved by means of two steps: a fiducial cut-based selection and a multivariate analysis. Background subtraction is obtained using the splot technique. Section 5 describes in detail the analysis method. First the resolution effects and the efficiencies correction procedure are presented. Then a description of the analysis strategy, and, finally, the results, are shown. Section 6 briefly summarises the results and points of discussion. Some future needed steps are also presented. Appendix A gives, finally, a detailed description of the splot technique. 8
Exotic charmonia experimental and theoretical status.1 Overview The Standard Model (SM) of elementary particle physics provides an amazing description of the three fundamental interactions pertaining with today s furthest microscopical universe. Strong interactions are responsible for the existence of the observed heavy particles such as nucleons, pions, kaons, etc. Quantum chromodynamics (QCD) is the theory which describes these interactions; its constituents are quarks and gluons. Such observed hadrons, i.e. strong interacting particles, are bound states of quark and gluon quantum fields. However, the hadronic properties of these states are fairly well described by the quark model, which mainly emphasises the quark content of the wave function. A naive explanation resides in the fact that gluons carry no intrinsic quantum numbers apart from color charge, which has never been directly observed. Today s established hadronic bound states of quarks are canonical q q mesons, such as pions or kaons, qqq baryons, such as nucleons or Σ s, and weakly bound (qqq)(qqq) baryonic molecules, such as deuterons. All these states are indeed combinations of quarks that allow the resulting color charge for being unobservable. For some reason, (q q)(q q) mesonic molecules are far less settled. Nevertheless, other combinations are in principle allowed for being observable in nature: tetraquarks (q qq q), pentaquarks (qqqq q) and other bound states involving gluons (gg and q qg). Since none of them have been clearly observed for tens of years from the theory s first formulation, which dates back to the 6 s of the past century, these states are called exotics. Great effort has been spent, both theoretically and experimentally, on light meson spectroscopy, without reaching any solid conclusion. On the one hand calculating, through models or straight lattice QCD, bound states multiplets composed of four light quarks gives rise to large errors on mass values, which makes it hard to export precise predictions; on the other hand those states are wide and overlapped one to each other (and to other non-exotic states), which makes harder the possibility of a straightforward observation and requires great statistics data samples as well as complex analysis. Heavy spectroscopy, especially in the quarkonium sector, represents instead both a broad QCD-probing system and a rich source of newly observed phenomena concerning 9
Exotic charmonia experimental and theoretical status exotics. In particular, since the discovery of the J/ψ, the charmonium system has been extensively studied, mainly revealing the surprising power of theoretical predictions. Indeed, with heavier flavor quarks the non-relativistic regime helps the treatment of the potential models, although relativistic corrections are in any case required. Starting with the discovery of X(387) resonance in 3 [4], several neutral charmoniumlike states, inconsistent with the acknowledged predictions on the charmonium spectrum, have been observed. These states are thought to be bound states with additional quarks or gluons. Further hints come from the recent observation of three new structures by Belle: Z(443) in 7, Z 1 (45) and Z (45) in 8; these are (still not confirmed) charged states apparently decaying into charmonium states. This makes these mesons extremely interesting, since their minimal quark content is, independently of the applicability of the predictions assumptions, necessarily exotic, namely c cdū.. Theoretical Models The bound state of two charm quarks, usually named charmonium, is the atomic system analogue for strong interactions, color charges being the corresponding analogue of electrical charge. Since the discovery of the J/ψ, in 1974, the charmonia have been extensively explored. The energy levels are basically found by solving a non-relativistic Schrödinger equation with the phenomenological potential: V (r) = κ r + ar, (.1) where the linear term accounts for quark confinement. Each state can be described in terms of (n, L, S, J): n is the radial quantum number, L is the relative angular momentum between the charm and the anti-charm, S is their combined spin, while J = L + S is the total angular momentum of the state. The orbital levels are labelled by S, P, D,..., corresponding to L =, 1,,.... Fermions c and c couple to give the total spin singlet S =, or the total spin triplet S = 1. The parity is P = ( 1) L+1 and the C-parity is C = ( 1) L+S. Degeneration between equal-n states is removed by spin-dependent interactions, that produce relativistic corrections. This gives rise to splitting within multiplets, as shown in Figure.1; this shows the settled charmonium levels arranged by quantum numbers 1
. Theoretical Models S+1 L J. In addition to the mass values prediction, QCD models provide the partial Figure.1: Charmonium conventional mesons diagram. widths, for the predicted bound state, in its electromagnetic (e.g. χ c 1 J/ψγ), hadronic (e.g. ψ(s) J/ψππ) and open-charm (e.g. ψ(377) D D) decays. These represent an important benchmark to test a conventional charmonium interpretation as opposed to an exotic one. The theoretical description of the observed QCD bound states, which relies on a nonrelativistic treatment due to the high mass of the quark charm, has been significantly reliable until the X(387) discovery in 3. All charmonium states below the D D opencharm mass threshold, i.e. the kinematic threshold allowing the strong decay (c c) D D, had been observed. The X(387) is a charmonium-like system, since it decays into J/ψππ. One would thus expect the X(387) to be a charmonium excitation, but its width is surprisingly narrow Γ X 1. MeV, although its mass is above the opencharm threshold. Furthermore, the X was found to have equal decay rate in its decays X J/ψρ J/ψπ + π and X J/ψω J/ψπ + π π, where the final states have different isospin quantum numbers. These X(387) features literaly opened a debate about its nature; these are summarised in [3]. The X(387) gave rise to three main theoretical interpretations: Hadronic Molecules. Since the X(387) is very close in mass, to the D D mass threshold, hence the first thought has been [9, 1] to identify this state as a molecule 11
Exotic charmonia experimental and theoretical status Figure.: Belle observed peak in J/ψππ mass spectrum in the B + (from the original paper). This was labelled as X(387). K + π + π J/ψ of open charm mesons. However, its left binding energy would require this state to be very large. Tetraquark. Since the molecular hypothesis seemed to have some problems in explaining some of the observed X(387) features, a di-quark anti di-quark model, [cq][ c q ], was proposed [11]. However, according to this model, not only the X(387) is predicted to be made of two components, but also a charged partner X ± is expected to show up in the B weak decays. This was not observed so far. Hybrids. The QCD lagrangian contains also the gluons; these can not only partecipate as strong interaction mediators, but could also act as dynamical degrees of freedom, so becoming valence gluons for the wave function. The X(387) might then be c cg bound state [8]. These hadronic states have been studied using different approaches, but no solid conclusions have been achieved, in order to explain the X(387) properties. In one way or another, a big deal of new effective potentials has been discussed, as an alternative to the conventional picture [1, 3]. 1
. Theoretical Models..1 Charged charmonium-like states Since charged states decaying into charmonia necessarily imply an exotic quark composition, immediately after their discovery, the charged resonance-like structures Z 1 (45) and Z (45) were tried to be explained as two-meson molecules [13], but QCD sum rules were not able to reproduce precisely the observed masses. However, the prediction of the Z (45) mass improves, if one introduces a phenomenological width, by forcing a Breit-Wigner function in the pole term. This is done in [14], where the Z 1 (45) is instead interpreted as a D D threshold effect, at the very close value 4 MeV. Neither the boson exchange potential for the D D system seem to favour the Z 1 (45) as a hidden-charm molecule [15, 16]. It has been recently pointed out [6] that the charm quark is not heavy enough to be considered as non-relativistic, especially for excited states. Therefore, a reliable treatment of above-threshold charmonia might require a completely relativistic treatment of the charm quark, without the usual non-relativistic expansion. Such a model is reported in [6], and it seems to be in agreement with the non-relativistic model for the well established 1 states of the charmonium spectrum (see Figure.4), which are thought to be conventional c c states. Furthermore, the application of the full relativistic treatment to tetraquarks [cq][ c q ], might be able to describe the observed Z (45) as [cd] S= [ cū ] S=. However, with this approach, no tetraquark candidates for the Z 1 (45) are found. It has been also reported [7] that another interpretation of peaking structures in the χ c 1π mass spectrum in B χ c 1Kπ (and the ψ(s)π mass spectrum in B ψ(s)kπ ), could be result of a rescattering effect, due to the D sj + resonance. The hypothesised process diagram for the ψ(s) is shown in Figure.3, for the charged conjugate process. Figure.3: Rescattering diagram for the decay B ψ(s)k π + ; this may create bumps in the ψ(s)π mass spectrum. 13
Exotic charmonia experimental and theoretical status.3 Experimental status In the past ten years numerous charmonium like states have been discovered with lots of them not fitting into the conventional charmonium spectroscopy. Among these, the most discussed are Y (414) seen by CDF and the X(3915) seen by Belle. A thorough review concerning the XYZ states, that appear to lie outside the conventional quark model, can be find in [17] and [18]. In Figure.4 a summarising picture [4] concerning charmonium-like exotics, together with the established charmonium states. Figure.4: The charmonium spectrum including newly observed exotics (red dots). The solid line indicates the quark model predictions, the shaded lines are the observed conventional charmonia, while the blue dashed lines represent various open-charm thresholds..3.1 X(387) quantum numbers The CDF Collaboration studied [1] the angular distributions and correlations of the J/ψππ final state, finding that only a restriction to J P C = 1 ++, + explained their 14
.3 Experimental status measurements adequately. The BABAR Collaboration found evidence for the decay X(387) J/ψω [], in the B J/ψπ + π π K channel, where the + (P-wave final state) option is favoured with respect to 1 ++ (S-wave final state). Their resulting π + π π mass is compared to the simulated distribution, for both these hypothesis, in Figure.5. Figure.5: The πππ mass distribution for events in the X(387) region of the J/ψω mass spectrum. Black dots indicate data, while the red solid (blue dashed) histogram is the simulation histogram for P-wave (S-wave) events. The red dashed line indicates the ω nominal mass. LHCb has recently proved [3] that the X(387) quantum numbers are J P C = 1 ++ (Figure.6). This analysis is based on angular correlations in the decay B + X(387)K +, where X(387) J/ψπ + π and J/ψ µ + µ. 15
Exotic charmonia experimental and theoretical status Figure.6: Distribution of cos θ X for the data (black dots), compared to the simulated distributions for J P C = 1 ++ (red solid histogram) and J P C = + (blue dashed histogram). θ X is the X(387) helicity angle..3. Z(443) exotic At the beginning of 8, the Belle Collaboration observed [5] a charged resonancelike structure, in the ψ(s)π invariant mass of the decay B ψ(s)k + π. This was labelled as Z(443) and its first observation is shown in Figure.7. Belle fitted the ψ(s)π mass distribution with a relativistic S-wave Breit-Wigner, to model the peak, and a smooth phase-space-like function. The mass value from the fit was m Z = 4433 MeV/c, and a width of about 45 MeV. The significance of the peak was estimated to be of 6.5σ. In 9 Belle presented [6] a more accurate analysis of the same channel, based on a 16
.4 Z 1 (45) and Z (45) Belle observation Figure.7: ψ(s)π mass distribution. The solid histogram represents data. The solid curve indicates the result of a Breit-Wigner on a smooth phase space. (m K + π, m ψ(s)π ) Dalitz plot fit including the known resonances of the K+ π system. The overall significance of the peak in this analysis was found to be of 6.4σ, while both the fitted mass and width were found to be a bit higher, namely m Z = 4443 MeV/c and Γ Z = 17 MeV. The BABAR Collaboration also searched for this Z(443) resonance [5] in both B ψ(s)kπ and B J/ψKπ. In the former channel, an excess of events at m ψ(s)π 447 MeV/c with only.7σ significance, which reduces to.1σ fixing mass and width to the Belle observed values. No evidence of peaks in the m J/ψπ was found in the B J/ψKπ channel. Very recently the Belle Collaboration has performed a full amplitude analysis of these decays [7] to constrain the quantum numbers of this exotic; their result favours the J P = 1 + hypothesis..4 Z 1 (45) and Z (45) Belle observation In 8 the Belle Collaboration reported [1] the results for their B χ c 1K + π decay study, with a signal yield of 16 ± 56 ± 4 events. Belle observed a horizontal band in the (m K + π, m χ c ) scatter plot, at m 1 π χ c 17 GeV /c 4, thus giving hints for the 1 π 17
Exotic charmonia experimental and theoretical status presence of a resonance in the χ c 1π system. This Dalitz plot is shown in Figure.8. Figure.8: Belle observed Dalitz plot for the decay B χ c 1K + π. The claimed resonances area is the horizontal band at m χ c 1 π 17 GeV /c 4 This feature motivated Belle for a Dalitz plot amplitude analysis. The fit model included the K known resonance involved in the K + π system, plus an exotic resonance Z in the χ c 1π system. Their results from the fit are m Z = (415 +31 15) MeV/c and Γ Z = (35 +99 43) MeV. The fit fraction was found to be about 3%, with an overall peak significance, calculated comparing fits with and without a Z, of 1.7σ. Since the unidimensional χ c 1π mass projection, in some K + π mass slices, presented a doubly peaked shape, Belle also performed the Dalitz plot amplitude fit including two exotics, labelled as Z1 and Z, in the χ c 1π system. This second fit was found to be favored over the first at 5.7σ level; the fit results are summarised in Table.1, where the fit parameters for the single Z(415) hypothesis are also repeated. From the fit fractions, the resulting branching ratios were found to be comparable to those of Z(443) and other charmonium-like states. Both the hypothesis of spin J = and J = 1 have been pursued, but the fit quality is not sensitive to this quantum number. A χ c 1π mass projection observed by Belle is shown in Figure.9, which also shows the 18
.4 Z 1 (45) and Z (45) Belle observation Exotic m (MeV/c ) Γ (MeV ) Fit fraction (%) CL Z 1 451 ± 14 + 41 8 +1+47 17 8. +3.8+9.5. 4. 4 % Z 448 +44+18 9 35 177 +54+316 39 61 1.4 +6.1+51.5.3.7 Z 415 +31 15 35 +99 43 33.1 +8.7 5.8.5 % Table.1: Belle fit parameter values for both the single Z and the two Z 1 (45) and Z (45) resonances hypothesis. f is the resulting fit fraction in %. fitted PDF for the Z 1 and Z hypothesis. These Z 1 resonances are labelled in the PDG as X(45) ± and X(45) ±. 19
Exotic charmonia experimental and theoretical status Figure.9: Belle χ c 1π projected mass distribution (black dots) for the K + π mass slice 1. < m K + π < 1.75 GeV /c 4. The red solid line is the projected fit result including Z 1 (45) and Z (45). The blue dashed line is the result of the fit without any exotic. The green and magenta dashed lines represent respectively the Z1 and Z fitted peaks..5 Z 1 and Z search at BABAR The BABAR collaboration has also searched for the Z1 and Z resonances, claimed by Belle, in the channels B χ c1 K + π and B χ c1 K S π. At the beginning of last year (1) they reported [] their results showing they don t see any exotic decaying in χ c 1π. The BABAR analysis method is different from Belle s, and it follows the same procedure adopted for the Z(443) search [5]. Their selection ends up with 491 signal events, for the two channels combined data sample. Their Dalitz plot is similar to the one observed by Belle, and the wide horizontal band shows there actually is activity in χ c 1π mass distribution. This is shown in Figure.1. However, the BABAR strategy is to explore also the Kπ angular distribution, by decomposing it in different angular momentum orders. Since the Dalitz plot and the angular distribution for the Kπ system show a great deal of structures, they investigated the extent to which the reflections from this system can explain the χ c 1π mass spectrum. That is, instead of a Dalitz plot
.5 Z 1 and Z search at BABAR (a) B χ c1 K + π (b) B χ c1 K S π Figure.1: BABAR Dalitz plot for signal events in the decay B χ c 1Kπ. amplitudes fit, they provided a model independent approach with an accurate study of the Kπ system invariant mass and angular structure. They first perform a fit to Kπ mass distribution, including the known K resonances involved in the studied channel; then they incorporate the studied features of the Kπ system angular structure in a high statistics toy Monte Carlo, generated according to the Kπ mass fitted PDF. The resulting comparison between this weighted Monte Carlo and data is shown in Figure.11. They conclude that the features observed in the Kπ system, are sufficient to explain χ c 1π mass spectrum, and there is no need to introduce in it exotic resonances. 1
Exotic charmonia experimental and theoretical status Figure.11: BABAR χ c 1π mass distribution. The red solid represents the weighted Monte Carlo including the K + π system invariant mass angular structures.6 Z 1 and Z search in LHCb A deeper explanation of the BABAR approach will be presented in Section 5, since this is the same approach adopted for the present analysis. The LHCb search for these Z 1, states has been invoked in theoretical literature, such as in [19, ]. A large number of events is in fact collected at LHCb. It will be shown, at the end of Section 4, that LHCb has twice statistics, for the B χ c 1K + π channel, of the cumulative events from Belle and BABAR. The same analysis strategy as the BABAR one, is here adopted. A full amplitude analysis is out of the scope of this thesis.
3 The LHCb experiment The LHCb is a dedicated heavy flavor physics experiment situated at the LHC collider. The primary purpose of this experiment is searching for new physics in CP violation and the rare decays of hadrons containing beauty and charm quarks. This chapter gives a brief overview of the LHCb detector, describing its sub-detectors and their performance. More detailed information and references on LHCb design and operation can be found in [9]. First the properties of the LHC accelerator are presented, followed by an overview of the LHCb detector. Then the outline of sub-detectors used for tracking and particle identification is given, followed by the description of trigger system that is an important part for selecting the most interesting events while reducing the event rate. Finally, the software used at the LHCb is described. 3.1 LHC The Large Hadron Collider (LHC) is a circular proton-proton collider located at the European Organization for Nuclear Research (CERN), on the French-Swiss border, near Geneva. Before the injection of the proton bunches into the main LHC ring, protons pass through series of low-energy pre-accelerators, as shown in Figure 3.1. The initial linear accelerator (LINAC) accelerates protons energy to 5 MeV, then they are fed through the Proton Synchrotron Booster (BOOSTER) which accelerates them to 6 GeV, and finally protons are injected into the LHC complex, after being accelerated in the Super Proton Synchrotron (SPS), at an energy of 45 GeV. The four main LHC experiments situated at the beam crossing points shown in Figure 3.1: ATLAS, ALICE, CMS, LHCb. ALICE dedicated to heavy ion physics. ATLAS and CMS are general purpose detectors, which primary goal is to discover production of new particles. More details on the LHCb experiment, from which the data analysed in this thesis are collected, are given in the next section. The new particles are expected to have large masses and their production processes have small cross sections, so the LHC machine is designed with both a center-of-mass energy and a luminosity as large as possible. The operation of the LHC can be shown as follows: two bunch of protons move in opposite direction in an orbit along the 7 km circumference of the accelerator deviated 3
3 The LHCb experiment Figure 3.1: The LHC Accelerator System. by the magnetic field of superconducting magnets. A temperature K is preserved for magnets coils to generate a maximum magnetic field of 8 T. This field allows to produce the design center-of-mass energy of s = 14 T ev. Finally the bunches are designed to collide with a frequency of 4 MHz at the interaction points to achieve a design luminosity of 1 34 cm s 1. The main LHC design parameters are shown in Table 3.1. 4
3. The LHCb detector Circumference Center-of-mass energy Injection energy Field at 45 GeV Field at 7 T ev Helium temperature Luminosity Bunch spacing Luminosity lifetime Time between fills 7 km 14 T ev 45 GeV.535 T 8 T K 1 34 cm s 1 5 ns 1 h 7 h Table 3.1: The main LHC design parameters 3. The LHCb detector LHCb is an experiment dedicated to precision measurements of CP violation and rare decays of hadrons containing beauty(b) and charmed(c) quarks as an indirect search of new physics beyond the Standard Model. The LHCb experiment is thought to work at an instant luminosity of 5 1 3 cm s 1, smaller than LHC nominal luminosity. This is achieved using less focusing of the beams than for other LHC experiments. This allows a better identification of the point where the p-p collision took place (the primary vertex) and a point where a short-lived but flying particle decayed (the decay vertex). The precise reconstruction of these points is essential for the physics in the experiment. The LHCb detector is a forward single-arm spectrometer with forward angular coverage from 1 mrad to 3 mrad in the bending plane and 1 mrad to 5 mrad in the nonbending plane. The choice of the unique LHCb geometry is justified by the fact that, at high energies, both the b- and b-hadrons are predominantly produced in the same forward or backward directions cones; the angular distribution of b and b quarks in p-p collisions at LHC are shown in Figure 3.. LHCb allows the full reconstruction of exclusive decays of the b- and c-hadrons in a variety of leptonic, semi-leptonic and purely hadronic final states. In order to achieve this 5
3 The LHCb experiment Figure 3.: The angular distribution of quarks b and b in p-p collisions at LHC, for s = 7 T ev. goal and extract the physics of interest, specialized sub-detectors perform the following major tasks: Precision vertexing: a sufficient separation between primary and secondary vertices is required to efficiently select b-hadron candidates and allow time dependent analyses to be performed. Such measurements are performed by the VErtex LOcator (VELO). Invariant mass determination: a finest invariant mass resolution is required in order to maximize the significance of signal with respect to background. Therefore, precision energy and momentum estimates of reconstructed tracks must be performed. This is achieved by LHCb tracking and calorimeter systems. Particle identification: hadronic decays of b- and c-hadrons, having identical topology but different flavour content in the final state, may peak at a common invariant mass; additional information is required to distinguish them from one another. Discrimination between charged hadrons (particularly pions and kaons) is 6
3. The LHCb detector Figure 3.3: Schematic layout of the LHCb detector [9]. The interaction point where the protons collide is on the left of the figure, and sub-detectors are labeled. achieved with a high performance Ring Imaging CHerenkov (RICH) system, whilst electrons, photons and muons are identified via the Calorimeter and Muon systems, respectively. Flexible and robust trigger and data acquisition: this is required in order to cope with rapid changes in running conditions and physics interests. A dedicated multi-stage trigger, capable of selecting many different final states in an hadronic environment, reduces the data rate from the initial 4 MHz of visible interactions to 3 khz which is suitable for offline storage and analysis. Figure 3.3 presents the layout of the detector sub-systems within the LHCb detector. More details on each sub-detector will be given in the next sections. 3..1 Tracking detectors The tracking system is an important part of the LHCb detector that collects such information about charged particles as vertexing (determining the distance between the production and the decay vertex of the B ) and momentum reconstruction. These two together are used for reconstruction of the mass, that is important for signal selection and background suppression during the offline analysis of B χ c 1K + π. Besides this, momentum and decay distance information are used in the trigger. 7
3 The LHCb experiment The LHCb tracking system is composed of a dipole magnet, the VELO and four planar tracking systems: the Tracker Turicensis (TT) upstream of the dipole magnet and three tracking stations T1, T and T3 downstream of the magnet. The latter three stations cover the entire geometrical acceptance of the spectrometer. To achieve the excellent tracking performance and also due to track multiplicity considerations, these three stations are composed of two distinct parts called the Inner Tracker (IT) and Outer Tracker (OT). The VELO, TT and IT use silicon strip technology while straw tubes are employed in the OT. Since the TT and IT share a common technology, they are called collectively the Silicon Tracker (ST). They have a very similar layout sharing the same silicon microstrip technology with a strip pitch of µm. Each of the four ST stations is composed of four detector layers with the strip directions arranged in a so called x-u-v-x layout: the first and fourth layers have vertical readout strips, while second (u) and third (v) layers have the strips rotated by a stereo angle of +5 and 5 respectively. This layout is designed to have the best hit resolution in the x direction (in the bending plane), without losing the stereo measurement of the tracks. 3..1.1 Magnet To provide a good momentum resolution, the LHCb experiment utilizes a (dipole) magnet (see Fig. 3.4), which bends the tracks of charged particles. The not superconducting magnet consists of two saddle-shaped coils. These are placed mirror-symmetrically, such that the gap left open by the magnet is slightly larger than the LHCb acceptance, and the principal field component is vertical throughout the detector acceptance. The quantity important for momentum resolutions, and hence for the analysis of a B -decaying channel, is the integrated magnetic field the magnet delivers. For tracks passing through the entire tracking system this is [9]: Bdl = 4T m making it possible to measure charged particles with momenta up to GeV/c within.5 % uncertainty. 3..1. Vertex Locator To provide precise measurements of track coordinates close to the interaction region, the Vertex Locator (VELO), consisting of a series of silicon modules, is arranged along the 8
3. The LHCb detector Figure 3.4: The LHCb dipole magnet. The proton-proton interaction region lies behind the magnet. beam direction. It is used to identify the detached secondary vertices typical for b-hadron decays and makes it possible to meet the requirement to reconstruct B χ c 1K + π decays. To provide accurate measurements of the position of the vertices, the silicon modules of the VELO are placed closed to the beam axis, namely at 8 mm. In order to detect the majority of the tracks originating from the beam spot (σ = 5.3 cm), the detector is designed such that tracks emerging up to z = 1.6 cm downstream from the nominal interaction point cross at least 3 VELO stations, for a polar angular window between 15 and 3 mrad, as shown in Figure 3.5. To enable fast reconstruction of tracks and vertices in the LHCb trigger, a cylindrical geometry with silicon strips measuring rφ coordinates is chosen for the modules. The strips of the r sensor are concentric semi-circles, the φ sensors measure a coordinate almost orthogonal to the r-sensor. The geometry is shown in Figure 3.6. A D reconstruction in the r-z plane alone allows to detect tracks originating from close to the beam line in the high-level trigger. These measurements are used to compute the impact parameter of tracks with respect to the production vertex, which is used in the trigger to discriminate between signal and background. 9
3 The LHCb experiment Figure 3.5: The setup of the VELO silicon modules along the beam direction. The left two pairs form the pile-up system. Indicated are the average crossing angle for minimum bias events (6 mrad), and the minimal (15 mrad) and maximal (39 mrad) angle for which at least 3 VELO stations are crossed. 39 mrad is the opening angle of a circle that encloses a rectangular opening angle of 5 x 3 mrad. The setup of the VELO is as follows. The half disc sensors are arranged in pairs of r and φ sensors and are mounted back-to-back. The sensors are 3 µm thick, radiation tolerant, n-implants in n-bulk technology [35]. The minimal pitch of both the r and the φ sensors is 3 µm, linearly increasing towards the outer radius at 41.9 mm. To reduce the strip occupancy and pitch at the outer edge of the φ-sensors, the φ-sensor is divided in two parts. The outer region starts at a radius of 17.5 mm and has approximately twice the number of strips as the inner region. The strips in both regions make a 5 stereo angle with respect to the radial to improve pattern recognition, and adjacent stations are placed with opposite angles with respect to the radial. In order to fully cover the azimuthal angle with respect to the beam axis, the two detector halves overlap, as is shown in Fig. 3.6. During beam injection the two halves of the VELO are retracted 3 cm away from the 3
3. The LHCb detector Figure 3.6: A VELO module and its dimensions. nominal beam position. The RF-foil is designed to minimize interactions. 3..1.3 Tracking system To perform accurate momentum estimates, important for mass and momentum resolutions in the reconstruction of the B χ c 1K + π channel, hit information downstream of the magnet is required, which is provided by three tracking stations. Since the magnet bends particles in the horizontal direction perpendicular to the beam pipe, the track density is largest in an elliptically shaped region around the beam pipe. In order to have similar occupancies over the plane, a detector with finer detector granularity is required in this region. Therefore, the Inner Tracker (IT), 1 cm wide and 4 cm high, as shown in Figure 3.7, is located in the center of the three tracking stations. Due to the high track density near the beam pipe, silicon strip detectors are used. The total active detector area covers 4. m, consisting of 194 readout strips of either 11 cm or cm in length. To improve track reconstruction, the four detector layers are arranged in an x-u-v-x geometry, in which the strips are vertical in the first and in the last layer, whereas the other two (u, v) layers are rotated by stereo angles of ±5, providing the sensitivity in the vertical direction. The pitch of the single-sided p + -on-n strips is 198 µm. In order to have similar perform- 31
3 The LHCb experiment Figure 3.7: Layout of the IT. ance in terms of signal-to-noise, the thickness of the sensors is 3 µm for the single-sensor ladders below and above the beam pipe, and 41 µm for the double sensors at the sides of the beam pipe. The four layers are housed in 4 boxes, which are placed such that they overlap. These overlaps avoid gaps in the detector and, more importantly, make it possible to perform alignment using reconstructed tracks. Similar to the IT, the Outer Tracker (OT) performs track measurements downstream of the magnet, allowing to determine the momenta of charged particles. The OT covers the outer region of the three tracking stations T1-T3. Since the track density further away from the beam pipe is lower, straw tubes are used. The total active area of one station is 5971 485 mm, and the OT and the IT together cover the full acceptance of the experiment. As is the case for the IT, these layers are also arranged in an x-u-v-x geometry, as shown in Figure 3.8. The OT is designed as an array of individual, gas-tight straw-tube modules. Each module contains two layers of drift-tubes with an inner diameter of 4.9 mm. The frontend (FE) electronics measures the drift time of the ionization clusters produced by charged particles traversing the straw tubes, digitizing it with respect to every bunch crossing. Given the bunch crossing rate of 5 ns and the diameter of the tube, and in order to guarantee a fast drift time (below 5 ns) and a sufficient drift-coordinate resolution ( µm), a mixture of Argon (7%) and CO (3%) is used as counting gas. To improve the momentum estimate of charged particles, track measurements are performed before these enter the magnet. Therefore, the Tracker Turicensis (TT), a planar tracking station, is located between the VELO and the LHCb dipole magnet. It is also used to perform the track measurements of long lived neutral particles which decay after the VELO. In addition, by using the weak magnetic field inside the tracker, track inform- 3
3. The LHCb detector Figure 3.8: Layout of the OT. ation from the TT is used by the High Level Trigger to confirm candidates between the VELO and the tracking stations. In order to cover the full acceptance of the experiment, the TT is constructed 15 cm wide and 13 cm high. It consists of four detector layers, with a total active area of 8.4 m, with 14336 readout channels, up to 38 cm in length. To improve track reconstruction, the four detector layers are arranged in two pairs that are separated by approximately 7 cm along the LHCb beam axis. And again, like the IT and the OT, the TT detection layers are in an x-u-v-x arrangement. The layout of one of the detector layers is illustrated in Figure 3.9. Its basic building block is a half module that covers half the height of the LHCb acceptance. It consists of a row of seven silicon sensors, named a ladder. The silicon sensors for the TT are 5 µm thick, single sided p + -on-n sensors, as for the IT. They are 9.64 cm 9.44 cm long and carry 51 readout strips with a strip pitch of 183 µm. 33
3 The LHCb experiment Figure 3.9: Layout of one of the stereo plane detector layers of the TT 3.. Particle identification (PID) 3...1 RICH system LHCb includes two Ring Imaging Cherenkov detectors, aiming at different momentum ranges. The upstream detector, RICH1, covers the low momentum charged particle range 1 6 GeV/c using aerogel and C 4 F 1 radiators, while the downstream detector, RICH, covers the high momentum range from 15 GeV/c up to and beyond 1 GeV/c using a CF 4 radiator. RICH1 has a wide acceptance covering the full LHCb acceptance from ±5 mrad to ±3 mrad (horizontal) and ±5 mrad (vertical). RICH is located downstream of the magnet and has a more limited angular acceptance of ±15 mrad to ±1 mrad (horizontal) and ±1 mrad (vertical). Figure 3.1 shows a picture of both RICH1 and RICH. In both RICH detectors the focusing of the Cherenkov light is accomplished using a combination of spherical and flat mirrors to reflect the image out of the spectrometer acceptance. Hybrid Photon Detectors (HPDs) are used to detect the Cherenkov photons in the wavelength range 6 nm. The HPDs are surrounded by external iron shields. Figure 3.11 represents a typical event at RICH, both with the event display and the pattern of rings produced. 34
3. The LHCb detector Figure 3.1: RICH1 (left) and RICH (right) layouts. 3... Calorimeter system The LHCb calorimeter system has the function of selecting high transverse energy hadron, electron and photon candidates for the first trigger level (L) and providing the identification of electrons, photons and hadrons as well as the measurement of their energies and positions. It is composed by a Preshower detector (PS) and a Scintillator Pad Detector (SPD) plane before the PS, and an Electromagnetic Calorimeter (ECAL) to which it follows a Hadron Calorimeter (HCAL). The reason for these four substructures is to discriminate between hadrons, electrons and photons, since the energy deposition in each of the calorimeter components will depend on the nature of the particles. Figure 3.1 shows a sketch of the energy loss by different particles in SPD, PS, ECAL and HCAL. This information is used by the calorimeter PID to correctly identify the particles. In the whole calorimeter system, the scintillation light is transmitted to Photo-Multipliers (PMTs), that turn this light into an electric signal. The SPD/PS detector consists of a 15 mm lead converter, surrounded by two almost identical planes of rectangular scintil- 35
3 The LHCb experiment Figure 3.11: Example of typical event at RICH1 (left) and rings produced by the different particles (right). lator pads. The sensitive area of the detector is 7.6 m wide and 6. m high. The ECAL employs alternating scintillating tiles and lead plates, with overall dimensions 7.8 6.3 m. Finally, the HCAL, consists of thin iron plates interspaced with scintillating tiles arranged parallel to the beam pipe. Its dimensions are 8.4 6.8 m. 3...3 Muon system The muon system for the LHCb experiment consists of five tracking stations placed along the beam axis, as shown in Figure 3.13. The first station (M1) is placed in front of the calorimeter preshower, while the remaining four stations (M, M3, M4 and M5) are located downstream the calorimeter, interleaved with three iron filters. The inner and outer angular acceptances of the muon system are (16) mrad and 36 (58) mrad in the bending (non-bending) plane, similar to that of the tracking system. This provides a geometrical acceptance of about % for muons from b decays relative to the full solid angle. Each station is subdivided in four regions with dimensions and logical pad size which scales a factor of two from one region to the next one. Since the multiple scattering in the absorber increases with the distance from the beam axis, limiting the spatial resolution of the detector, the granularity of the detector varies accordingly. The logical pad dimension has been chosen such that its contribution to the transverse 36
3.3 LHCb analysis workflow Figure 3.1: Energy deposition on the subelements of the calorimeter system depending on the nature of the particle. momentum resolution is approximately equal to the multiple-scattering contribution. The muon stations are equipped with Multi Wire Proportional Chambers (MWPCs), operating with a gas mixture of Ar (45%), CO (15%) and CF 4 (4%), with the exception of the inner part of the most upstream station where GEM (Gas Electron Multiplier) chambers are installed. 3.3 LHCb analysis workflow Data taken by LHCb from LHC p-p collisions, at a rate of several million events per second, must be selected in the most possible efficient way for its subsequent analysis. This process is performed using several C++ tools and algorithms, grouped in different projects. The package which serves as a framework for all these projects in LHCb is called GAUDI [3]. The sequential steps leading from raw detected data to the physics results are: Trigger. The amount of data generated by LHC collisions is too high to be directly stored. The trigger selection is an online process, i.e. it takes place almost at the same time data is being recorded by the detector. This is an initial and fast selection which allows to discard most of the events that are not interesting for the physics analysis. The LHCb trigger reduces the rate from several millions of events per second to just a few thousand. Reconstruction. Data selected by the trigger, pure electronic signals recorded by the different subdetectors, are transformed by different mathematical algorithms 37
3 The LHCb experiment Figure 3.13: Side view of the muon sistem. in an ensemble of tracks and vertices. Tracks correspond to the charged particles trajectories produced in the collisions (or by decays of other particles) which go through the detector, while vertices are the points where the p-p collisions (PVs), or the decay of a particle in two or more daughter tracks, took place. Vertices are built from the crossing point of two or more tracks. The group of C++ LHCb libraries which contain the relevant tools is called Brunel. Stripping. Once all the triggered events have been reconstructed, it becomes necessary to separate them according to their physics content. Stripping is the centralized selection of interesting events run after the reconstruction. This is an offline process, i.e. it takes place with the data already on tape, and it is performed by selecting the different decays using their particular features. A set of physicsdriven streams is created, i.e. a group of selections of a similar type, selecting similar events (e.g. the DiMuon stream, is to select events containing at least a couple of high-transverse momenta muons with opposite charges). Each of these streams 38
3.3 LHCb analysis workflow contains several stripping lines, each of which embeds the particular selection cuts required for a typical search (in the DiMuon stream, two different lines could be: one attempting to select D + π + µ + µ events and another one simply attempting to select Bs µ + µ events). Finally, the triggered, reconstructed and stripped dataset has to be then distributed to a series of computing centers spread all over the world. A copy of the raw data from detectors is also saved, with the idea of allowing a later re-reconstruction and stripping once the relevant algorithms have been improved. This both ensures that the data cannot be lost and allows physicists an easy access to a distributed computing system of huge power. This distributed system is called the Grid. The stripped data are then ready to be analysed (offline), and the the group of C++ LHCb libraries which contain the relevant tools for the analysis is called DaVinci. 3.3.1 Trigger One of the most important challenges for LHCb is the need of a trigger system capable of reducing the rate of events from proton proton collisions from the nominal 3 MHz to a maximum rate of about 3 khz, which is the maximum permitted by the available resources for long term storage. At the same time, this must be achieved with the minimum possible loss of interesting events for the physics analysis (mainly involving B and D mesons). Two main signatures allow the identification of these kind of events: tracks with high transverse momenta with respect to the beam axis, and tracks with non-zero impact parameters. The LHCb trigger is divided in two different levels. The first level (L), is a hardware trigger, implemented using custom made electronics to reduce the input rate to a maximum of 1 MHz. At this rate, the whole detector can be read out. The second trigger level (High Level Trigger, HLT) is a C++ application running on an Event Filter Farm (EFF) composed of several thousands CPU nodes. It reduces the L output rate to a maximum rate of about 3 khz. The HLT selected events are then saved on permanent storage. The HLT itself is divided in two parts: HLT1 and HLT. HLT1 reduces its input rate to about 4 khz using a partial reconstruction of the data to save computing time. At the HLT level, events are reconstructed and selected by a set of inclusive and exclusive algorithms. The reconstruction performed in HLT is as similar as possible to the one 39