- Email: [email protected]
Nuclear counter effect and π-e misidentification
Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
Nuclear counter e!ect and p-e misidenti"cation Dario ZuK rcher!,",* !Institute for Particle Physics (IPP). ETH Zu( rich, CH-8093 Zu( rich, Switzerland "Paul Scherrer Institut (PSI), CH-5232 Villigen PSI, Switzerland
Abstract The eB/pB discrimination within the CMS(1) ECAL is investigated using GEANT simulations and the 1998 test beam results. If one takes into account the energy left in the ECAL crystals alone (i.e. without read-out e!ects), the probability that a pB leaves more than 95% of its initial energy decreases from about 0.01% for 10 GeV to about 0.001% for 50 GeV. The Nuclear Counter E!ect within the Avalanche Photo-Diodes (APD) enhances the probability of an electron misidenti"cation. With the expected value of this e!ect (+100 MeV), this probability appears then to be between 0.2% and 0.01% for initial momenta varying, respectively, between 5 and 50 GeV. Important consequences of the pionelectron misidenti"cation could appear in the form of new possible backgrounds for physics channels. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj; 7.20.Fw; 25.80.Gn; 85.60.Dw Keywords: Pion-electron misidenti"cation; Nuclear counter e!ect; Avalanche photodiodes (APD)
1. Introduction The Compact Muon Solenoid (CMS) [1] is designed to measure the energy and momentum of photons, electrons, muons and other charged particles with high precision. Among other detector requirements, an essentially background free identi"cation of electrons is of crucial importance for the ful"lment of practically all the requested physics tasks. In the CMS detector, the electron energy will be measured by the electro-magnetic calorimeter
* Correspondence address. Institute for Particle Physics (IPP), ETH ZuK rich, CH-8093 ZuK rich, Switzerland. Tel.: #41227671588; fax: #41-227678940. E-mail address: [email protected] (D. ZuK rcher)
(ECAL). We investigate the case, where charged pions produce a large signal within the ECAL and could therefore be misidenti"ed as electrons.
2. The energy deposition within the ECAL The interactions between pB and the lead tungstate (PbWO ) crystals can be schematically sub4 divided in two main possibilities. Either the particles produce only simple ionization in the material or they interact hadronically, resulting in a larger amount of deposited energy. In the "rst case, the charged pions behave like `Minimal Ionizing Particlesa (MIP) and deposit about 280 MeV of energy within the crystals. When some hard interactions like pBAPAHpo2pB2 occurs (where A means an atom and AH a modi"ed one),
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 2 6 9 - 3
SECTION X.
434
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
One expects a value dn/dx of about 80}100 e/holes pairs per lm [2,3]. To obtain the amount of electron}hole pairs given by charged particles crossing the APD, its ewective thickness d has to be known: %&& d Q d " PIN ) APD +5 lm (3) %&& Q M PIN where Q and Q are, respectively, the charges PIN APD collected by the PIN diode and the APD, whose gain is M. A comparison of the number of electron}holes pairs produced by the Nuclear Counter E!ect with the number of electron}holes pairs produced by scintillation light, allows to calculate the corresponding fake energy signal. One MIP crossing the APD will then give a signal equivalent to the one given by a photon in the crystal with energy:
Fig. 1. GEANT simulation of the fractional amount of energy left by charged pions and electrons within the ECAL.
the presence of p0's gives instead rise to electromagnetic showers within the ECAL crystal. Fig. 1 shows the energy deposited by pB and eB within a 9]9 matrix of crystals. The eB's leave all their energy producing a narrow peak at +100% fractional energy. On the contrary, non interacting pB's give rise to the peak on the left side of the plots. Furthermore, a small fraction of the hard interaction events, shows a deposited energy of more than 95% of the initial one.
dn d %&& . E " (4) NCE dx N %@) For example, a k (MIP) deposits about 280 MeV in the crystal. With N +4 e/h per MeV one gets %@) about 1120 e/h from scintillation light. In addition, about 500 e/h are produced if the k crosses the APD. The size of the fake signal given by the Nuclear Counter E!ect is then E "500/1120] NCE 280+125 MeV. Fig. 2 shows a scheme, which point out the APD working principle and the electron avalanches arising from light collection and from the Nuclear Counter E!ect. The size of this e!ect was measured with the help of the test beam
3. Energy read-out and nuclear counter e4ect The number of electron}hole pairs generated by scintillation light is given by N "¸>e f. (1) %@) Q With the 2 APDs read-out ( f"10%) of the barrel crystals one expects to see about 4 photoelectrons for every MeV. In addition, electron/holes pairs can also be produced by the passage of ionizing radiation within the APD (`Nuclear Counter E!ecta): dn/dx"(dE/dx)o(1/E ). %@)
(2)
Fig. 2. APD working principle and Nuclear Counter E!ect.
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
435
clear Counter E!ect, then a 10 GeV fake signal is observed (starting from a 20 GeV pion!). Then, adding the energy deposited in the crystals, these pions can give rise to signals of more than 20 GeV, that is to E /p ratios larger than one. ECAL * The test beam data and the GEANT simulations show that the fraction of e-like pions is strongly increased by the Nuclear Counter E!ect within the APD, as shown in Figs. 4 and 5. Table 1 summarizes the results of these simulations. 5. Discrimination methods for isolated pB and eB
Fig. 3. Muon energy collected by APD's during the test beam runs as a function of the track position.
data by comparing the observed energy of tracks directed towards the APDs with the one of tracks directed outside this area, as shown in Fig. 3. These results have shown a larger Nuclear Counter E!ect, which has been qualitatively explained as an e!ect of the bad contacts between crystals and APDs. However, these results have allowed a comparison between simulations and data, con"rming the accuracy of the GEANT program.
In contrast to eB the E /p ratio for pB is ECAL * generally very small. Consequently, the "rst possibility of pion-electron discrimination consists in limiting the E /p ratio and a lower cut eliminECAL * ates almost all events which originate from pion tracks. For e-like pB-events a large number of hadrons survives the ECAL and enters in the HCAL (for e with an energy below 1 TeV essentially no leakage is observable). Another possibility is then to use the signal coming out of the Hadronic Calorimeter (HCAL) to put a veto in the selection, when a given value is exceeded [4].
4. pB as `fake-ea candidates For crystal sizes of about 25 radiation lengths, very small shower leakage is expected for electrons and photons with energies below 1 TeV. On the contrary, for charged pions, a hard interaction is less likely and the shower can start all along the crystal length. The leakage of particles could then result in problems for the corresponding energy measurement. In case that a hard interaction occurs in the last part of the PbWO crystal, a large number of 4 particles might eventually pass through the APDs and produce a remarkable fake energy signal. If for example 100 particles cross these photodevices and each one gives a 100 MeV large Nu-
Fig. 4. E /p signal generated by 20 GeV pB crossing the ECAL * ECAL crystals (with or without Nuclear Counter E!ect).
SECTION X.
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D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
Table 1 GEANT (CMS design): charged pions as `fake electronsa candidates (the error is only statistical) No NCE
100 MeV NCE (2 APDs per crystal)
E -(GeV) p
0.95(E
0.95(E
5 10 20 50 100
0.006$0.004 0.01$0.005 0.018$0.005 0.001$0.001 (0.004
/p (%) ECAL *
/p (1.2 (%) ECAL *
0.11$0.02 0.06$0.01 0.06$0.01 0.011$0.005 0.008$0.004
E /p '1.2 (%) ECAL * 0.05$0.01 0.02$0.01 0.007$0.003 0.001$0.001 (0.004
6. Misidenti5ed pB as source of new backgrounds Fig. 6 shows a Pythia simulation of the main QCD processes which produce pB and of two possible physics channels, which could be a!ected by this background. The total amount of pB produced at LHC in a day at luminosity L"1033 cm~2 s~1 by scatterings between coloured partons is shown. The total rate of `fake-ea above a p of 20 GeV is about 10 Hz. This could be 5 a real danger for the o!-line analysis where the
Fig. 5. Same as Fig. 4 but with 50 GeV pB.
Another selection method (signal timing) is founded on the assumption that the pulse shape of the APD-signals di!ers between the one produced by scintillation light and the one given by the Nuclear Counter E!ect. Finally, the `shower shape selectiona bases itself on the possibility that the distribution of the deposited energy within the crystals is di!erent for pB and eB tracks [5]. The shower developing out of pB tracks can start all along the crystal and its shape could therefore be di!erent compared to the one produced by electrons. Moreover, e-like events often arise from a particle shower with a tial entering one APD, giving an abnormal energy deposition in one crystal.
Fig. 6. Pythia simulation of di!erent processes at LHC, where charged pions and electrons are generated (shown as a function of their transverse momentum). pB and eB have pseudorapidities DgD smaller than 2.5 and one isolated pion over thousand is considered to be a `fakea electron.
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
maximum accepted rate is 100 Hz. The process ppPWBPeBv, for example, has a rate of few Hz. If one then wants to put the p threshold at 10 GeV, 5 the amount of e-like pB's increases to hundreds of Hz! In general, the pion-electron misidenti"cation should be considered as an additional source of backgrounds for several types of events which require isolated electrons.
References
437
[2] CERN/LHCC 97-33, CMS TDR 4, The Electro-magnetic Calorimeter Project, Technical Design Report, 15. December 1997. [3] F. Cavallari, CMS Conference Report 1997/010, Progress on avalanche photo-diodes as photon detectors for PbWO 4 crystals in the CMS experiment. [4] M. Fabrice Gautheron, These de doctorat No. 13097, Etudes de cristaux scintillants de tungstate de plomb et de prototypes de calorime`tre eH lectromagneH tique pour le project CMS au LHC, UniversiteH Claude Bernard, Lyon, 1997. [5] C.J. Purves, CMS Technical Note 1996/006, New values for electron/charged-pion Discrimination from the '95 CMS Electro-magnetic Calorimeter Prototype.
[1] CERN/LHCC 94-38 LHCC/P1, The Compact Muon Solenoid, Technical Proposal, December 1994.
SECTION X.
Nuclear counter e!ect and p-e misidenti"cation Dario ZuK rcher!,",* !Institute for Particle Physics (IPP). ETH Zu( rich, CH-8093 Zu( rich, Switzerland "Paul Scherrer Institut (PSI), CH-5232 Villigen PSI, Switzerland
Abstract The eB/pB discrimination within the CMS(1) ECAL is investigated using GEANT simulations and the 1998 test beam results. If one takes into account the energy left in the ECAL crystals alone (i.e. without read-out e!ects), the probability that a pB leaves more than 95% of its initial energy decreases from about 0.01% for 10 GeV to about 0.001% for 50 GeV. The Nuclear Counter E!ect within the Avalanche Photo-Diodes (APD) enhances the probability of an electron misidenti"cation. With the expected value of this e!ect (+100 MeV), this probability appears then to be between 0.2% and 0.01% for initial momenta varying, respectively, between 5 and 50 GeV. Important consequences of the pionelectron misidenti"cation could appear in the form of new possible backgrounds for physics channels. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj; 7.20.Fw; 25.80.Gn; 85.60.Dw Keywords: Pion-electron misidenti"cation; Nuclear counter e!ect; Avalanche photodiodes (APD)
1. Introduction The Compact Muon Solenoid (CMS) [1] is designed to measure the energy and momentum of photons, electrons, muons and other charged particles with high precision. Among other detector requirements, an essentially background free identi"cation of electrons is of crucial importance for the ful"lment of practically all the requested physics tasks. In the CMS detector, the electron energy will be measured by the electro-magnetic calorimeter
* Correspondence address. Institute for Particle Physics (IPP), ETH ZuK rich, CH-8093 ZuK rich, Switzerland. Tel.: #41227671588; fax: #41-227678940. E-mail address: [email protected] (D. ZuK rcher)
(ECAL). We investigate the case, where charged pions produce a large signal within the ECAL and could therefore be misidenti"ed as electrons.
2. The energy deposition within the ECAL The interactions between pB and the lead tungstate (PbWO ) crystals can be schematically sub4 divided in two main possibilities. Either the particles produce only simple ionization in the material or they interact hadronically, resulting in a larger amount of deposited energy. In the "rst case, the charged pions behave like `Minimal Ionizing Particlesa (MIP) and deposit about 280 MeV of energy within the crystals. When some hard interactions like pBAPAHpo2pB2 occurs (where A means an atom and AH a modi"ed one),
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 2 6 9 - 3
SECTION X.
434
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
One expects a value dn/dx of about 80}100 e/holes pairs per lm [2,3]. To obtain the amount of electron}hole pairs given by charged particles crossing the APD, its ewective thickness d has to be known: %&& d Q d " PIN ) APD +5 lm (3) %&& Q M PIN where Q and Q are, respectively, the charges PIN APD collected by the PIN diode and the APD, whose gain is M. A comparison of the number of electron}holes pairs produced by the Nuclear Counter E!ect with the number of electron}holes pairs produced by scintillation light, allows to calculate the corresponding fake energy signal. One MIP crossing the APD will then give a signal equivalent to the one given by a photon in the crystal with energy:
Fig. 1. GEANT simulation of the fractional amount of energy left by charged pions and electrons within the ECAL.
the presence of p0's gives instead rise to electromagnetic showers within the ECAL crystal. Fig. 1 shows the energy deposited by pB and eB within a 9]9 matrix of crystals. The eB's leave all their energy producing a narrow peak at +100% fractional energy. On the contrary, non interacting pB's give rise to the peak on the left side of the plots. Furthermore, a small fraction of the hard interaction events, shows a deposited energy of more than 95% of the initial one.
dn d %&& . E " (4) NCE dx N %@) For example, a k (MIP) deposits about 280 MeV in the crystal. With N +4 e/h per MeV one gets %@) about 1120 e/h from scintillation light. In addition, about 500 e/h are produced if the k crosses the APD. The size of the fake signal given by the Nuclear Counter E!ect is then E "500/1120] NCE 280+125 MeV. Fig. 2 shows a scheme, which point out the APD working principle and the electron avalanches arising from light collection and from the Nuclear Counter E!ect. The size of this e!ect was measured with the help of the test beam
3. Energy read-out and nuclear counter e4ect The number of electron}hole pairs generated by scintillation light is given by N "¸>e f. (1) %@) Q With the 2 APDs read-out ( f"10%) of the barrel crystals one expects to see about 4 photoelectrons for every MeV. In addition, electron/holes pairs can also be produced by the passage of ionizing radiation within the APD (`Nuclear Counter E!ecta): dn/dx"(dE/dx)o(1/E ). %@)
(2)
Fig. 2. APD working principle and Nuclear Counter E!ect.
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
435
clear Counter E!ect, then a 10 GeV fake signal is observed (starting from a 20 GeV pion!). Then, adding the energy deposited in the crystals, these pions can give rise to signals of more than 20 GeV, that is to E /p ratios larger than one. ECAL * The test beam data and the GEANT simulations show that the fraction of e-like pions is strongly increased by the Nuclear Counter E!ect within the APD, as shown in Figs. 4 and 5. Table 1 summarizes the results of these simulations. 5. Discrimination methods for isolated pB and eB
Fig. 3. Muon energy collected by APD's during the test beam runs as a function of the track position.
data by comparing the observed energy of tracks directed towards the APDs with the one of tracks directed outside this area, as shown in Fig. 3. These results have shown a larger Nuclear Counter E!ect, which has been qualitatively explained as an e!ect of the bad contacts between crystals and APDs. However, these results have allowed a comparison between simulations and data, con"rming the accuracy of the GEANT program.
In contrast to eB the E /p ratio for pB is ECAL * generally very small. Consequently, the "rst possibility of pion-electron discrimination consists in limiting the E /p ratio and a lower cut eliminECAL * ates almost all events which originate from pion tracks. For e-like pB-events a large number of hadrons survives the ECAL and enters in the HCAL (for e with an energy below 1 TeV essentially no leakage is observable). Another possibility is then to use the signal coming out of the Hadronic Calorimeter (HCAL) to put a veto in the selection, when a given value is exceeded [4].
4. pB as `fake-ea candidates For crystal sizes of about 25 radiation lengths, very small shower leakage is expected for electrons and photons with energies below 1 TeV. On the contrary, for charged pions, a hard interaction is less likely and the shower can start all along the crystal length. The leakage of particles could then result in problems for the corresponding energy measurement. In case that a hard interaction occurs in the last part of the PbWO crystal, a large number of 4 particles might eventually pass through the APDs and produce a remarkable fake energy signal. If for example 100 particles cross these photodevices and each one gives a 100 MeV large Nu-
Fig. 4. E /p signal generated by 20 GeV pB crossing the ECAL * ECAL crystals (with or without Nuclear Counter E!ect).
SECTION X.
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D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
Table 1 GEANT (CMS design): charged pions as `fake electronsa candidates (the error is only statistical) No NCE
100 MeV NCE (2 APDs per crystal)
E -(GeV) p
0.95(E
0.95(E
5 10 20 50 100
0.006$0.004 0.01$0.005 0.018$0.005 0.001$0.001 (0.004
/p (%) ECAL *
/p (1.2 (%) ECAL *
0.11$0.02 0.06$0.01 0.06$0.01 0.011$0.005 0.008$0.004
E /p '1.2 (%) ECAL * 0.05$0.01 0.02$0.01 0.007$0.003 0.001$0.001 (0.004
6. Misidenti5ed pB as source of new backgrounds Fig. 6 shows a Pythia simulation of the main QCD processes which produce pB and of two possible physics channels, which could be a!ected by this background. The total amount of pB produced at LHC in a day at luminosity L"1033 cm~2 s~1 by scatterings between coloured partons is shown. The total rate of `fake-ea above a p of 20 GeV is about 10 Hz. This could be 5 a real danger for the o!-line analysis where the
Fig. 5. Same as Fig. 4 but with 50 GeV pB.
Another selection method (signal timing) is founded on the assumption that the pulse shape of the APD-signals di!ers between the one produced by scintillation light and the one given by the Nuclear Counter E!ect. Finally, the `shower shape selectiona bases itself on the possibility that the distribution of the deposited energy within the crystals is di!erent for pB and eB tracks [5]. The shower developing out of pB tracks can start all along the crystal and its shape could therefore be di!erent compared to the one produced by electrons. Moreover, e-like events often arise from a particle shower with a tial entering one APD, giving an abnormal energy deposition in one crystal.
Fig. 6. Pythia simulation of di!erent processes at LHC, where charged pions and electrons are generated (shown as a function of their transverse momentum). pB and eB have pseudorapidities DgD smaller than 2.5 and one isolated pion over thousand is considered to be a `fakea electron.
D. Zu( rcher / Nuclear Instruments and Methods in Physics Research A 442 (2000) 433}437
maximum accepted rate is 100 Hz. The process ppPWBPeBv, for example, has a rate of few Hz. If one then wants to put the p threshold at 10 GeV, 5 the amount of e-like pB's increases to hundreds of Hz! In general, the pion-electron misidenti"cation should be considered as an additional source of backgrounds for several types of events which require isolated electrons.
References
437
[2] CERN/LHCC 97-33, CMS TDR 4, The Electro-magnetic Calorimeter Project, Technical Design Report, 15. December 1997. [3] F. Cavallari, CMS Conference Report 1997/010, Progress on avalanche photo-diodes as photon detectors for PbWO 4 crystals in the CMS experiment. [4] M. Fabrice Gautheron, These de doctorat No. 13097, Etudes de cristaux scintillants de tungstate de plomb et de prototypes de calorime`tre eH lectromagneH tique pour le project CMS au LHC, UniversiteH Claude Bernard, Lyon, 1997. [5] C.J. Purves, CMS Technical Note 1996/006, New values for electron/charged-pion Discrimination from the '95 CMS Electro-magnetic Calorimeter Prototype.
[1] CERN/LHCC 94-38 LHCC/P1, The Compact Muon Solenoid, Technical Proposal, December 1994.
SECTION X.