LEADING NEUTRON PRODUCTION IN CHARGED CURRENT DIS AT HERA

Transcription

1 LEADING NEUTRON PRODUCTION IN CHARGED CURRENT DIS AT HERA Alex Koutsman Tutor: Sjors Grijpink 16 August 2002 Abstract The production of leading neutrons in e p and e + p collisions at HERA has been studied with ZEUS detector and the FNC. This study was used to get a better understanding and to test the OPE model. A research was conducted into the possible difference between electron and positron scattering. The theoretical aspects of this project, such as CC DIS and leading neutron production in CC DIS and the way to kinematically describe these interactions are discussed in this paper. The detector setup, data selection requirements and reconstruction technics are also viewed in detail. The results show that with the error marges included there seems to be a difference between electron and positron scattering. The ratios of leading neutrons to CC events are rcc LN = ± for electrons and rcc LN = ± for positrons. The ratios of leading neutrons cross section to the CC cross section are RCC LN = ± for electrons and RCC LN = ± for positrons. A comparison of the neutron energy distributions in the FNC for electron and positron scattering seems to prove the OPE model. However more data and research are needed to conclusively prove the above mentioned results. 1

2 CONTENTS 2 Contents 1 Introduction 3 2 Theory Kinematics Leading Neutron Production Experimental Setup The HERA Particle Accelerator The ZEUS Detector The Forward Neutron Calorimeter Data Selection Reconstruction of Kinematic Variables Charged Current Event Selection FNC Neutron Candidates Data Characteristics Charged Current Data Implementing the Final Cuts Conclusions 20

3 1 INTRODUCTION 3 1 Introduction This is a report of my third year project at NIKHEF. I studied leading neutron production in charged current DIS at HERA. First I will discuss the theoretical aspects of my project. Kinematic variables and the needed Feynman diagrams for the process will be discussed. Then I will go through the detector setup. Starting with the HERA accelerator, going to the ZEUS detector and finishing with a detailed discription of the FNC. The data selection requirements and reconstruction technics are discussed in chapters 4 and 5. And finally I will show the results and draw conclusions.

4 2 THEORY 4 2 Theory 2.1 Kinematics Deep inelastic scattering (DIS) is a process in which constituents of a proton (the quarks) are probed, by means of lepton-proton scattering. It is called inelastic when a quark is knocked out, and the proton is broken up in the final state. It is called deep when the proton is probed with a small wavelength virtual boson, so the resolution is very high. Such bosons are γ, Z 0 and W ±. Processes which include γ and Z 0 exchange are called Neutral Current (NC) scattering. The exchange of a charged boson W ± is called Charged Current (CC) scattering. As my title gives it away, I will be focusing on the latter. For CC scattering we get a neutrino in the final state, which interacts extremely weakly with its environment. A W + -particle is the interaction boson for a positron in CC DIS, since it is the only boson for DIS that has positive charge. The electron has only the W -particle as the interaction boson for CC DIS. In NC scattering we get an electron or a positron in the final state as well, and neutrally charged γ and Z 0 are the interaction bosons. Figure 1 shows the Feynman diagram for this process. It is conventional to use the following Lorentz-invariant variables to define the kinematics of DIS. The four-momentum transfer or also called virtuality is given by: Q 2 (k k ) 2 = q 2 (1) This gives us the length scale at which we probe the proton, and be written as: λ = 1/ Q 2 (2) The four-momentum fraction carried by the struck quark is defined as: The inelasticity, x Q2 2q P y q P (4) k P is the fraction of the energy transferred from the lepton to the struck quark, in the proton rest frame. These kinematic variables of are not independent. We need only two out of Q 2,x and y to fully describe the kinematics a DIS event. Neglecting the masses of the proton and the electron provides us with the following relation of the kinematic variables: (3) Q 2 = sxy, (5) with s the centre-of-mass energy squared of the lepton-proton system, s (p + k) 2. And s also being the maximum Q 2, since 1 is the maximum of both x and y. The invariant mass of the hadronic final state is given by W 2 = Q 2 1 x + m 2 p x, (6) where m p is the mass of the proton.

5 2 THEORY 5 "! (a) (b) Figure 1: (a)the Feynman diagram for a DIS process; (b) The kinematic variables used to describe the interaction. 2.2 Leading Neutron Production In the case of DIS with a neutron in the final state originating directly from the proton one speaks of leading neutron (LN) production. An approach to leading neutron production is to see it as a charge-exchange process (p n) in terms of the exchange of virtual particles. In the language of particle exchange models, isovector exchange (π, ρ and a 2 ) is required, and isoscalar exchange (f and ω) is absent for direct neutron production. Due to the small mass of the π-particle, the cross section for π exchange is much larger that that for ρ or a 2 exchange. Thus in the case of a LN in the final state, it is predominantly the structure of the pion that is being probed. In general terms a proton splits up in a neutron and a π + -particle. Where the π + - particle interacts with a virtual boson from the lepton, and we see a LN in the final state. This is also refered to as One Pion Exchange model (OPE). The Feynman diagram for the OPE in CC is shown in Figure 2. If we consider the model of partons consisting of only valence quarks, the proton would consist of two up quarks and one down quark, the neutron would consist of two down quarks and one up quark and the π + -particle of one up quark and one anti-down quark. Since the W ± -particles couple to electrical charge, and the up and the anti-down quarks have a positive charge, the cross section of CC OPE should be zero in the case of positron scattering, as the W + -particle has a positive charge and does not couple with the quarks in the π + -particle. For the OPE process in electron scattering the cross section could be larger than zero, as the negative W -particle couples to the quarks in the π + -particle, in contrast to the W + -particle. With the consideration of sea quarks interacting as well, which is a more realistic approach, the cross section for positron scattering could also be larger the zero. Since the sea quarks consist of both positively and negatively charged quarks. To kinematically describe the process of LN production in DIS we need two more variables. One is the laboratory production angle of the neutron θ n, and the other is the

6 2 THEORY 6 υ W + - Figure 2: The Feynman diagram for CC DIS leading neutron production. energy fraction of the proton carried by the produced neutron, defined as: x L = N k P k E n E p, (7) where E p is the proton beam energy and E n is the neutron energy. The transverse momentum of the neutron is then given by p T x L E p θ n, since for a small angle at which the neutron is produced sinθ n θ n.

7 2 THEORY Experimental Setup The HERA Particle Accelerator The Hadron-Elektron-Ring-Anlage (HERA) was built from 1984 to 1992 at the Deutche Elektronen Synchrotron (DESY) laboratory in Hamburg, Germany, and is the world s largest Electron-Proton collider. It consists of a storage ring for protons and another one for electrons (or positrons). Its near-circular tunnel has a circumference of km and is built under the surface. There are four experimental underground halls, of which two are used by general electronproton collision experiments, H1 and ZEUS. The other two halls are used for fixed target experiments, HERMES and HERA-B. In the proton ring super conducting magnets are used to store the protons which have an energy of 920 GeV. To accelerate and compensate for the loss of energy due to synchrotron radiation super conducting R.F. cavities are used in the electron ring. The electron (positron) beams have an energy of GeV. Both the positron and proton beams are stored in a total of 220 R.F. buckets. This gives a bunch structure with a time of 96 ns between two successive e-p bunch passings. Not all of the 220 bunches are filled, which gives possibility for the study of background due to cosmic rays and beam gas interactions. Hall North H1 360m R=797m 360m Hall East HERMES e p Hall West HERA B 40 GeV Protons HERA PETRA p e 820 GeV Protons 27.5 GeV Positrons 14 GeV Positrons Hall South ZEUS Figure 3: A schematic view of HERA accelerator and experiments.

8 2 THEORY The ZEUS Detector The ZEUS detector is a multi-purpose e-p collision detector. The first thing that draws attention, when looking at the design (Figure 4) is the assymetry. This is due to the fact that the centre-of-mass system does not coincide with the laboratory system. The protons coming from the right will generally boost the particles in the final state to the left, so a detector elongated in that direction is required. Figure 4: A schematic view of the ZEUS detector. Closest to the interaction point was situated a vertex detector (VXD), which is surrounded by a driftchamber (CTD) and two tracking devices for forward and very backward

9 2 THEORY 9 going particles (FDET and RTD). The vertex detector has actually been removed in The tracking detectors are surrounded by a super conducting solenoid (COIL), which produces a magnetic field of T. All these are enclosed by the segmented scintillator-uranium calorimeter, FCAL for forward, BCAL for barrel and RCAL for the rear sectors. An iron yoke that acts as a return path for the magnetic flux is equipped with a backing calorimeter (BAC). The inside and outside of the return yoke are instrumented with muon chambers (FMUON, BMUI, BMUON, RMUI and RMUON). For the beam the magnet system is compensated by an opposite field from a special magnet (Compensator). The Vetowall protects the detector from the particles in the proton beam halo and provides a veto for beam-gas interactions. A more complete description of the ZEUS detector can be found in [1] The Forward Neutron Calorimeter A forward neutron calorimeter (FNC) was installed in the HERA tunnel in It was placed at zero degrees from the incoming proton direction and at a distance of 106 m from the ZEUS detector, as you can see in Figure 5. Y B77 B72 B67 Q51,55,58 B47 Q42 Q30,34,38 B26 B18,22 Q6-15 ZEUS FNC S6 S5 S4 S3 S2 S1 Figure 5: A schematic view of the forward proton beam line from the central detector to the FNC. The calorimeter is a finely segmented, high resolution, compensating, sampling calorimeter. It has 134 layers of 1.25 cm lead plates as absorber and 0.26 cm scintillator plates as the active material. The scintillator can be read out on each side with wavelengthshifting light guides, which are coupled to photomultiplier tubes (PMTs). It is segmented into two sections, a front section, seven interaction-lengths deep, and a rear section, three interaction-length deep. The front section is divided into 14 towers, each 5 cm high, as can be seen in Figure 6. The relative energy resolution for hadrons, as measured in a test beam, was σ/e = 65%/ E(GeV ). As we also see in Figure 6, the proton beam is completely surrounded by the FNC. In towers 11 and 12 there is a 10x10 cm 2 hole for the beam to pass. Three planes of scintillation counters, each 70x50x2 cm 3, are located 70, 78 and 199 cm in front of the calorimeter. These are counters which are used to identify charged particles. As there are neutrons that interact in front of the FNC in inactive material such as magnet support structures, the beam-pipe wall etc., charged particles that are produced during the interactions get

10 2 THEORY Figure 6: A schematic view of the FNC, looking towards the interaction point. The zerodegree point is marked by an X. Around it is the window of geometric acceptance defined by the apertures of the HERA beam-line elements. detected in the scintillation counters. The counters completely cover the bottom front face of the calorimeter.

11 3 DATA SELECTION 11 3 Data Selection 3.1 Reconstruction of Kinematic Variables In the following section some methods to select charged current events will be discussed. The extraction of kinematic variables using different combinations of measured variables will be shown. Electron Method This method only uses the information from the lepton. y e = 1 E e 2E e (1 cosθ e ) (8) Q 2 e = 2E ee e (1 + cosθ e) (9) Where θ e is the polar angle of the scattered lepton. And E e is the energy of the incoming lepton and E e of the scattered one. This method is quite useful since it does not use any information of the hadronic system. However in our case it proves of little help as our scattered lepton stays undetected. This method is used for NC analysis at ZEUS. Jacquet-Blondel Method The Jacquet-Blondel method is very useful for the CC leading neutron events, because it only uses the hadronic information. So we do not need information from the neutrino which is undetected. y JB = δ h 2E e (10) Q 2 JB = p2 T 1 y JB (11) Where δ h = i E i p Z,i, summed over all energy deposits in the calorimeter of the hadronic system. The transverse momentum of the hadronic system is calculated by p 2 T = p 2 X + p2 Y = ( i p X,i) 2 + ( i p Y,i) 2. The accuracy with which the hadronic energy, and thus p T, can be measured is relatively poor. But this method does provide an accurate estimate of y. This because of δ h being in first order stable against hadronic energy fluctuations.

12 3 DATA SELECTION 12 Other Methods There are also other methods which use both hadronic and lepton information. The p T -method is a frequently used one. In this method the transverse momentum of the hadronic system is replaced by the transverse momentum of the lepton (so the lepton must be detected, thus NC DIS at HERA). This is done because the measurement of the lepton s p T is more accurate then that of the hadronic system. So for example the value of y JB can be corrected. The double angle method also uses the hadronic and the lepton information, or actually the angular part of it. This method has the advantage of being independent of the energy scale, which is difficult to measure. 3.2 Charged Current Event Selection A large missing p T is one of the most important requirements for selecting a charged current DIS candidate. A reconstructed event vertex consistent with an e p or e + p interaction also serves as a good selection requirement. However there is also a lot of background after you apply the above stated selection criteria. Main sources of background come from NC scattering and high-e T photoproduction. Here a significant missing transverse momentum can be caused by the energy resolution of the CAL or the energy that simply escapes detection. Other sources (non e p or e + p interactions) that can cause a p T -imbalance are beam-gas interactions, beam-halo muons or cosmic rays. To separate CC events from the background it is useful to introduce a new variable, γ 0, the polar angle of the hadronic system, measured with respect to the nominal interaction point. When γ 0 is large, the current jet lies in the central region of the detector, the CTD tracks can be used to reconstruct an event vertex, and tracking can be used to give a strong handle on separating the CC events from the background. For a small γ 0, the situation becomes more difficult as the charged particles from the hadronic final state lie outside of the acceptance of the CTD. In this case the timing information of the ZEUS calorimeters was used for reconstructing an event vertex, and more stringent cuts were applied for removing background events. 3.3 FNC Neutron Candidates For the selection of LN in the CC sample, the FNC was used. neutron candidate the following selection cuts were applied: To select a good FNC 184 < E F NC < 920GeV, where E F NC is the energy deposited in the FNC, and which is equal to 0.2 < x L < 1.0; The condition rests on the idea, that first of all the energy of the neutron cannot be higher then 920 GeV, since that is the proton beam energy. Particles with higher energies are not created at HERA, and are thus not interesting to us in this context. For a neutron to be a leading neutron, the carried energy fraction of the proton, x L, should be at least 20% of the proton energy, thus giving us the 184 GeV constraint.

13 3 DATA SELECTION 13 the maximum energy deposit in towers 6, 7, 8 or 9 in the bottom front section of the FNC (see back for figure); The condition for the maximum energy deposit in towers 6, 7, 8 or 9 in the bottom front section is used to separate the neutrons from the protons, which are, since they are charged particles, bent into the top towers (10 till 14) by the magnets in the proton beam line. These magnets are vertically bending magnets. The towers 6, 7, 8 and are alligned in a straight line with respect to the ZEUS detector, as can been seen in Figure 6. energy and vertical width of the shower consistent with that of a hadron; The condition is used to distinguish between hadronic and electromagnetic showers. The electromagnetic energy deposits can lead to a wrong detection or rejection of leading neutrons. The width of a typical hadronic shower is 3.0 < shower width < 7.0 cm, where for a electromagnetic shower it is smaller then 3.0 cm. scintillation counters show a energy deposit below that of a minimum ionizing particle. The last condition is used to separate charged particles from neutrons, as explained in the section on the FNC. If a particle passing through the scintillation counters has an energy deposit value which is higher then or equal to the minimum ionizing particle energy, it is rejected, since it is then an electrically charged particle.

14 4 DATA CHARACTERISTICS 14 4 Data Characteristics 4.1 Charged Current Data This section will show how the data looked after the charged current selection cuts were implemented. The original data was collected in the years 1998 to 2000, of which 16 pb 1 was collected with electron runs (in 1998 until half of 1999) and 60 pb 1 with positron runs over the remaining time. The figures consequently show two distributions, one for the runs with electrons (left plots) and the other for the runs with positrons (right plots). The energy distribution in the FNC can be seen in Figure 7: events 10 3 Entries Mean RMS events 10 3 Entries Mean RMS (a) E (GeV) 1 (b) E (GeV) Figure 7: (a) Energy distribution in the FNC for the runs with electron scattering; (b) Energy distribution in the FNC for the runs with positron scattering. Both of the distributions peak very close to zero. So the energy cut for the leading neutrons will cut many events out. If we look at the distribution of the showerwidth, Figure 8, we see that in the case of electron scattering as well as positron scattering there seem to be maxima around 15 and 45 cm. This double peaked behaviour and the large shower width have been looked into. No conclusions could be drawn of where they could come from, but as can be seen in Figure 10, after applying the FNC neutron selection cuts this behaviour is not observed anymore. The distribution of the towers with a maximum energy deposit gives us Figure 9. The towers 11 and 12 in both distributions seem to be the minima, which is not very strange as the proton beam pipe passes through them and thus reduces their volume.

15 4 DATA CHARACTERISTICS 15 events Entries Mean RMS events 10 2 Entries Mean RMS (a) Showerwidth (cm) (b) Showerwidth (cm) Figure 8: (a)showerwidth in the FNC for electron scattering; (b) Showerwidth in the FNC for positron scattering. events 10 2 Entries Mean RMS events Entries Mean RMS (a) Tower-E max (b) Tower-E max Figure 9: (a) Tower with maximum energy deposit in the FNC for electron scattering; (b) Tower with maximum energy deposit in the FNC for positron scattering. 4.2 Implementing the Final Cuts Before going to the final results, Figure 10 was interesting. It is a figure that shows the width of the showers in the FNC after implementing all the cuts on the data, with the exception of the FNC showerwidth cut. This shows that our candidates for leading neutron events already have a showerwidth

16 4 DATA CHARACTERISTICS 16 events 3 Entries Mean RMS events Entries Mean RMS (a) Showerwidth (cm) 1 (b) Showerwidth (cm) Figure 10: (a)showerwidth in the FNC for electron scattering after implementing the final cuts, except for the showerwidth cut; (b)showerwidth in the FNC for positron scattering after implementing the final cuts, except for the showerwidth cut. which is consistent with that of a hadron. So Figure 10 already shows the leading neutrons as selected by our conditions. Figure 11 shows the energy distributions in the FNC of the final neutron smaple. One events 3 Entries Mean RMS events 3 Entries Mean RMS (a) E (GeV) (b) E (GeV) Figure 11: (a)energy distribution of leading neutrons in the FNC after implementation of the cuts for electrons; (b)energy distribution of leading neutrons in the FNC after implementation of the cuts for positrons.

17 4 DATA CHARACTERISTICS 17 LN r CC e - e + Figure 12: The ratio of leading neutrons to the total amount of CC events. thing to notice from these distributions in that one could argue that there seems to be a tendency for neutrons in electron scattering to have higher energy than those in positron scattering. Figure 12 shows the ratios of leading neutrons to the total amount of charged current events for electrons and positrons. The ratios are calculated using the statistical formula : CC = N CC LN ± σ r, (12) N CC r LN where σ r = N CC LN 1 N CC NCC LN in Figure N CC is the statistical error. The values summarized in the table electrons positrons N CC NCC LN rcc LN σ r Figure 13: The values used for the calculation of the ratio of leading neutrons to the total amount of CC events. It is advantageous to measure the ratio of cross sections. This minimizes the systematic uncertainties, due to the finite acceptance of the central calorimeter and to triggering

18 4 DATA CHARACTERISTICS 18 LN R CC e - e + Figure 14: The ratio of the cross section of LN to the cross section of charged current events. inefficiencies. Also a luminosity measurement is not required. Defining the cross sections as: σ CC = N CC A CC L (13) σ LN CC = N LN CC A CC A F NC L, (14) where L is the luminosity and A the acceptance. The formula for the ratio of cross sections can be seen in equation 15: RCC LN = σln CC σ CC ± σ R = N LN CC A F NC N CC ± σ R, (15) electrons positrons RCC LN σ R Figure 15: The values of the ratio of the LN cross section to the cross section of charged current events and the statistical errors.

19 4 DATA CHARACTERISTICS 19 where σ R is again the statistical error on the ratio. Averaging the FNC acceptance (A F NC ) over the selected x L -range (0.2 < x L < 1.0), you get A F NC = For a more detailed explanation of A F NC and its calculation see [2]. Figure 14 shows the ratio of cross sections of leading neutrons to charged current events. The values are summarized in Figure 15.

20 5 CONCLUSIONS 20 5 Conclusions The results show that there seems to be a difference in CC DIS leading neutron production at HERA between electron scattering and positron scattering. The ratios of events seem to differ with the error marges included, being rcc LN = ± for electrons and rcc LN = ± for positrons. The ratios of the cross sections differ as well with the error marges included. Their values being RCC LN = ± for electrons and RCC LN = ± for positrons. As Figure 11 shows us one could argue that the neutron energy distribution in the FNC for electron scattering seems to tend to have higher average energy than the neutron energy distribution for positron scattering. The OPE model seems to be proven here, as it predicts that in the high x region there would be more leading neutrons for electron scattering than for positron scattering. Since the valence quarks in the pion would couple to the W -particle and not to the W + -particle, thus producing the difference in the high x region, since the OPE model tells us that the valence quarks carry a large fraction of the four-momentum. However more research and data is needed to be conclusive. The statistical error should be minimized by a greater amount of data. And the systematic errors could be minimized by engaging into more research.

21 REFERENCES 21 References [1] Zeus Collaboration, M.Derrick et al., The ZEUS Detector, Status Report 1993,DESY (1993). [2] Zeus Collaboration, S.Chekanov et al., Nucl. Physics B637 (2002)3-56.