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Electromagnetic Dissociation Measurement at LHC with ALICE ZDC

Electromagnetic Dissociation Measurement at LHC with ALICE ZDC. N. De Marco INFN Torino (Italy) for the ALICE Collaboration. Outline ZDC description EM dissociation cross sections of Pb nuclei at s = 2.76 ATeV Single and Mutual EM dissociation. ZDC for the ALICE experiment.

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Electromagnetic Dissociation Measurement at LHC with ALICE ZDC

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  1. Electromagnetic Dissociation Measurement at LHC with ALICE ZDC N. De Marco INFN Torino (Italy) for the ALICE Collaboration Outline • ZDC description • EM dissociation cross sections of Pb nuclei at s = 2.76 ATeV • Single and Mutual EM dissociation PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  2. ZDC for the ALICE experiment D1 Dipole Separator Dipole Corrector Vacuum Chamber Quadrupoles ZEM ZNA ZPA Interaction point 7.35 m not in scale 114 m • The ZDC detector is made by 2 identical sets of calorimeters, located at opposite sides with respect to I.P, 114 meters away from it. Each ZDC set consists of 2 hadronic quartz fiber calorimeters: 1 for spectator neutrons (ZNA on A side, ZNC on C side) placed at 0° with respect to LHC axis (|η| > 8.8) 1 for spectator protons (ZPA, ZPC) positioned externally to the outgoing beam pipe. 2 forward EM calorimeters (ZEM1, ZEM2) placed at 7.35 m from I.P, on A side (4.8 < η < 5.7). 7x7x100 cm3 Sketch view of A side ZN ZP Beam pipes PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  3. EM Dissociation • When two nuclei A and B collide at b>RA+RB the interaction is purely electromagnetic. • At ultrarelativistic energies, each nucleus experiencies the strongly Lorentz-contracted Coulomb field of the other nucleus -> equivalent photon flux  Z2 (Weizsacker-Williams method) • Two Coulomb induced processes provide stringent limits on the heavy-ion beam lifetime in colliders: • bound-free pair production • electromagnetic dissociation • These cross sections at LHC energies far exceed the hadronic one had~8 b, e+e-~281 b, EMD~226 b for Pb-Pb @ s = 5.54 A TeV, R. Bruce et al., Phys.Rev. 12 071002 (2009) • Main contribution to EM dissociation ->GDR excitation and subsequent decay in the neutron channel • The neutrons from electromagnetic dissociation are expected to be emitted very close to beam rapidity and with an average energy close to the one of the beam. -> ZN acceptance for neutrons emitted in EMD of Pb nuclei at s = 2.76 ATeV -> ~99% PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  4. RELDIS Model Relativistic ELectromagnetic DISsociation Model I.A. Pshenichov et al. Phys. Rev. C 60 044901 (1999), I.A. Pshenichov, Phys. Part. Nuclei 42 215 (2011) RELDIS code describes the EMD of ultra-relativistic nuclei including: • single and double virtual photon absorption by nuclei • intranuclear cascades of produced hadrons • statistical decay of excited residual nuclei (evaporation, fission and multifragmentation) -> good description of existing data on lead at CERN SPS and gold at RHIC RELDIS x-section predictions for Pb-Pb @ s = 2.76 ATeV • Baltz Single EMD prediction 5% more w.r.t. RELDIS prediction A.J. Baltz et al., Physics Reports 458 (2008) 1 • difference included as a systematic error on RELDIS Single EMD cross section • STARLIGHT generator A. J. Baltz et al. Phys. Rev. C 80 (2009) 044902 PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  5. van der Meer scan During 2010 Pb van der Meer scan ZDC provided 3 different signal logics (low thresholds). Preliminary results from the PbPb @ s = 2.76 ATeV van der Meer scan • Very preliminary systematic errors on van der Meer results (1 σ): • 4% from vdM data analysis (ALICE) • 3% from beam current measurement (LHC) • 0% + 11% from ghost charge fraction (LHC) • 4% (ALICE), -3% +11% (LHC) PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  6. EMD DATA – ZNC vs ZNA Dedicated PbPb @ s = 2.76 ATeV EMD run: 2.8 106 events collected Trigger: minimum energy deposition in at least one of the two ZNs (ZNC OR ZNA) energy thresholds: ZNC~500 GeV, ZNA~450 GeV (3 below 1n mean value=1.38 TeV) Mutual EMD at least 1 n emitted by BOTH nuclei Single EMD at least 1 n emitted by one of the 2 Pb nuclei 1n,2n 1n,2n 0n,1n PLHC 2011 - Perugia, Italy, June 6-11th, 2011 1n,1n 1n,1n 1n,0n 2n,1n 2n,1n

  7. Single EMD plus Hadronic Trigger: ZNC OR ZNA Spectra (pedestal + 1n,2n,3n,4n,5n peak) fitted with the sum of 6 gaussians: pedestal (3 free parameters)+ 1n peak (3 free parameters)+ 4 gaussians (free normalization, μn = n*μ1n to take into account pedestal width affecting 1n peak width) -> Resolution on 1n peak (1.38 TeV)  ~ 20.0% (NevTOT(OR) – NevPED) -> Single EMD + Hadronic interactions 1n 1n 1n 2n 2n ZNC ZNA 3n 3n 4n 4n 5n 5n PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  8. Single EMD plus Hadronic The cross section for the considered process can be calculated using the vdM cross section: σsEMD+had = σ(OR)vdM  (NevTOT(OR) - NevPEDESTAL)/NevTOT(OR) where σ(OR)vdM and NevTOT(OR) refer to the trigger ZNAorZNC Systematic error sources: • different fits of ZN spectra: ~ 0.2% • side A vs. side C difference: ~ 2.9% • difference in background in vdM fill w.r.t. background in analysed RUN: ~ 0.8% • systematic error on vdM x-section: 4%(ALICE), -3% +11%(LHC) σ sEMD+had = (193.1  0.1 stat. syst.) b +23.9 -11.6  To compare  values with RELDIS model predictions, experimental values must be corrected for the detection probability obtained from MC: +24.2 -11.7 PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  9. Single minus Mutual EMD Event selection: signal in one ZN but NOT on the other • Single EMD with emission only from 1 Pb nucleus  Single EMD but NOT Mutual EMD Hadronic contamination negligible (~2% of hadronic events survive to this event selection) Spectra (1n,2n,3n,4n,5n peak) fitted with the sum of 5 gaussians: 1n peak (3 free parameters)+ 4 gaussians (free normalization, μn = n*μ1n , with σped fixed from previous fits) 1n 1n 2n 2n 3n ZNC ZNA 3n 4n 4n 5n 5n PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  10. Single minus Mutual EMD Using the vdM cross section and the number of events collected with the same trigger: σsEMD-mEMD = σ(OR)vdM * NevsEMD-mEMD/NevTOT(OR) Systematic error sources: • different fits of ZN spectra ~0.8% • side A vs. side C difference ~1.7% • difference in background in vdM fill w.r.t. background in analysed RUN ~ 0.8% • systematic error on vdM x-section: 4% (ALICE), -3% +11% (LHC) +21.3 -10.5 σsEMD-mEMD = (174.7  0.1 stat. syst.) b  Detection probability correction (from MC): +21.6 -10.6 PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  11. Single minus Mutual EMD Signal in ONLY one of the two ZNs -> hadronic contamination negligible in the sample • neutron multiplicities can be compared with the predictions from the RELDIS model • 1n and 2n emission channels main contribution to EMD • The model overpredicts especially the fraction of 3n events • Partial neutron emission need a better description PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  12. Mutual EMD plus Hadronic Event selection Mutual EMD Hadronic ZNA ZNC Events ZNC signal (ADC ch.) ZNA signal (ADC ch.) • Less contamination of EMD events in the Hadronic sample on A side -> ZEM are only on A side • The hadronic contribution increases with increasing neutron multiplicity PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  13. Mutual EMD plus Hadronic Mutual EMD sample  residual contamination of Hadronic events (Hadronic events with no signal in ZEM1 or ZEM2) Hadronic sample  residual contamination of Mutual EMD events (EMD events with signal in ZEM1 or ZEM2) E = εE * e + (1-εN) * n E  measured mutual EMD x-section N = (1-εE) * e + εN * n N  measured hadronic x-section e  “true” mutual EMD x-section n  “true” hadronic x-section εE  fraction of EMD events from RELDIS sample with no signal in (ZEM1 or ZEM2) (EZEMi<15 GeV) • N  fraction of Hodronic events from HIJING MB + fragm. sample with signal in (ZEM1 or ZEM2) (EZEMi>15 GeV) 1st x-section estimate: solve the system using cross section values for mutual EMD and hadronic processes from van der Meer scan to evaluate “true” cross sections:  σmEMD = (εN* σmEMDvdM- (1-εN)* σhadvdM)/(εE-(1-εN))  σhad = (εE* σhadvdM- (1-εE)* σmEMDvdM)/(εE-(1-εN)) PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  14. Mutual EMD plus Hadronic: Event Selection Interactions from IP Rejection of background events with collision vertex Z displaced • Events corresponding to interaction between satellite bunches can be identified using the ZN timing information • not all satellite events are synchronized with the ADC gate  signals not correctly integrated in some cases % events from satellite bunch interactions rejected: 3.8% events from mutual EMD sample 2.6% events from the hadronic sample time Zvertex  2nd x-section estimate:use the σ(OR)vdM cross section to evaluate the measured : Mutual EMD  σmEMDMEAS = σ(OR)vdM * NevmEMD w.cut /NevTOT(OR) w.cut Hadronic  σhadMEAS = σ(OR)vdM * Nevhad w.cut /NevTOT(OR)w. cut  correct for the contaminations to extract ‘’true’’ cross sections:  σmEMD = (εN* σmEMDMEAS- (1-εN)* σhadMEAS)/(εE-(1-εN))  σhad = (εE* σhadMEAS- (1-εE)* σmEMDMEAS)/(εE-(1-εN)) PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  15. EMD and Hadronic cross sections +0.7 -0.3 +0.9 -0.4 +0.7 -0.3 +0.9 -0.4 σsEMD can be estimated from previous measurements ssEMD + shad - shad ssEMD – smEMD + smEMD ssEMD +22.6 -11.1 PLHC 2011 - Perugia, Italy, June 6-11th, 2011

  16. Summary and Conclusions Cross sections for EMD processes have been measured in Pb-Pb collisions at s = 2.76 ATeVdetecting the emitted neutrons with the ZDCs (errors are dominated by preliminary systematic errors on vdM x-section). +24.2 -11.7 +21.6 -10.6 +0.7 -0.3 +22.6 -11.1 • Predictions from RELDIS are in very good agreement with experimental results. • Neutron emission measurements in Coulomb breakup of excited Pb nucleustest theoretical predictions used in beam losses estimates. • ALICE ZDCs can provide an independent check of the beam luminosity measuring the rate of neutron emission by EMD processesL = R EMD/EMD • Signal in BOTH the 2 ZNs (smEMD +shad) : beam-residual-gas interaction events suppressed, but nontrivial separation between hadronic and mutual EMD interactions • Signal in ONLY one of the 2 ZNs (ssEMD –smEMD) : hadronic contamination negligible, but beam-residual-gas interaction events present (3-4% w.r.t. to collision rate measured by (ZNA OR ZNC) during Pb2010 data taking) PLHC 2011 - Perugia, Italy, June 6-11th, 2011

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