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Global event characterization

Global event characterization. E. Scomparin – INFN Torino (Italy). 1 st Physics ALICE Week Erice (Italy), December 4-10, 2005. Introduction: the observables Pb-Pb collisions: centrality determination Method Accuracy Systematic errors p-A collisions: updates on gray/black nucleons

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Global event characterization

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  1. Global event characterization E. Scomparin – INFN Torino (Italy) 1st Physics ALICE Week Erice (Italy), December 4-10, 2005 • Introduction: the observables • Pb-Pb collisions: centrality determination • Method • Accuracy • Systematic errors • p-A collisions: updates on gray/black nucleons • EM dissociation: status

  2. Physics issues A global view on global observables • Measurement of inclusive observables (no PID) • Multiplicity  pp, AA • Hadroproduction models (hard vs soft) • Rapidity spectra  pp, AA • Transparency • Transverse momentum spectra  pp, AA • Thermal freeze-out • Approach to quenching scenarios • Nuclear flow  AA • Details on medium properties • Collective motion of the expanding system, pressure, etc. • Event geometry • Centrality  pA, AA • Event selection • Threshold effects (Many of these) topics covered in section 6.1 of the ALICE PPR

  3. Centrality in A-A collisions • Fixed target experiment • Transverse energy distributions (NA38) • Multiplicity distributions (NA57) • Forward energy distributions (NA49, NA50) • All more or less equivalent, because of WNM • But • Additional (physics) fluctuation in ET and Nch measurements • to be foled with the detector resolution • Not present for EZDC (only detector resolution) • At collider • ET and Nch do not scale any more linearly with Npart • (but are still monotonically correlated) • EZDC still linearly connected with Npart but there are loss due to • fragments (no more monotonic) It is difficult to say “a priori” which is the best strategy for centrality determination at ALICE  detailed simulation needed to understand the centrality resolution for the various estimators

  4. VACUUM CHAMBER D1 DIPOLE SEPARATOR QUADRUPOLES EM ZDCs NEUTRON ZDC PROTON ZDC INTERACTION POINT DIPOLE CORRECTOR X:Y=200:1 7 m 116 m ZP ZN Centrality measurement with the ZDCs • Detailed (full) simulation exists • Propagation of 2.7 TeV nucleons • Beam line as a magnetic spectrometer • Understand acceptance (~75%) • Knowledge of fragmentation required • Use past experimental results

  5. Is fragmentation understood ? • From a phenomenological point of view, yes ALADIN results (0.4 – 1 GeV/nucleon) In agreement with higher energy experiments NA49: fragment measurements done RHIC: maximum number of free neutrons in agreement with low-energy observations • Would the picture be still correct at LHC energies ? • Likely to be so: nuclear fragment emission seen as a late • de-excitement of the spectator nucleons system Fragmentation model (coded in AliRoot)  used also for CBM studies

  6. Main results • Full simulation based on a significant, but not too large sample • (103 HIJING events, plus a sample of 104 2.7 TeV spectator nucleons) 0-3.6 % • Correct attribution of Npart range needs trig = 100% • (or trigger inefficiency correctly evaluated)

  7. Fast simulation • Assume for the moment trig =100% • Use a fast simulation (based on a parameterization of detector response) • Binning in fraction of • inelastic Pb-Pb cross section • (most usual choice) • 10 centrality classes • have been defined Study the corresponding Npart distributions

  8. Npart distributions • Use the sum of hadronic energies on the two sides Generated Npart Reconstructed Npart

  9. Resolution on Npart • Which is the Npart smearing • necessary to go from the • generated to the reconstructed • distribution ? • Fit the reconstructed spectra • with the smeared generated • spectra Example: 5-10 % centrality bin Npart ~ 15 (little dependence on centrality)

  10. Pb-Pb triggering efficiency • No quantitative estimate found • Words are in general very reassuring (Forward Detectors TDR) • trigshould be known • quantitatively • otherwise the • Npart assignment • could be biased Does a Pb-Pb simulation exist ?

  11. ZDC trigger efficiency (1) • The hadronic ZDCs can detect even a single proton/neutron • In principle the trigger efficiency is 100% • Problem: there is a huge background • from Coulomb interactions • Useful on one side, since can be used for • luminosity estimates (see later) • Background for the inelastic cross section • evaluation At RHIC, agreement with theory

  12. ZDC trigger efficiency (2) • The ratio geom/tot does not change very much from RHIC to LHC • (tot is the total cross section for breakup of BOTH nuclei) 1.6 – 1.8 increase from RHIC to LHC Other possibility for L0 trigger in Pb-Pb: use ZEM Also for this detector a detailed efficiency simulation still does not exist to be performed

  13. Other centrality-related issues: symmetric ZDCs 1 side 2 sides 1 side Resolution is visibly better when the ZDC information on both sides is used

  14. Asymmetry studies • Small asymmetry present in the • HIJING event washed out by the • (mutually independent) formation • of nuclear fragments HIJING After fragm. • Only for central events the • formation of nuclear fragments is • not important But in this case trivial stochastic fluctuations may hide effects Due to physics correlations

  15. HIJING Glauber Still another point to investigate • Probably due to differences between analytical approach and Monte-Carlo • approach, also observed at RHIC (e.g. Eccentricity calculations)

  16. RHIC situation • ~ 20% systematic uncertainty in the Npart evaluation for peripheral events • Similar to what we observe for the ALICE HIJING vs Glauber comparison

  17. 18% bias NO bias Centrality: what else to do (1)? • Assess in a quantitative way our Pb-Pb trigger efficiency • Effect of a 18% error on trig (at RHIC trig~ 90%) • (equivalent to assuming that we have trig = 0 for b >15 fm) Effect of bias increasingly important towards peripheral events

  18. Centrality: what else to do (2) ? • Investigate in a quantitative way the quality of other centrality estimators • Charged multiplicity via tracklets in the SPD • (done, but w/o vertex smearing) • Forward charged multiplicity (FMD) • Photon multiplicity (PMD) • Use them • Standalone • Correlated to ZDC

  19. pA collisions: centrality • Basic principle already discussed several times • Emission of soft (in the target reference frame!) nucleons • Much more model-dependent wrt centrality determination in A-A • Gray nucleons (0.25 < p < 1 GeV/c, in the target reference frame) • Directly ejected by the collision with the projectile (1st generation) • Knocked-out by 1st generation nucleons • Several models (geometric cascade, intranuclear cascade, polynomial) • Black nucleons (p < 0.25 GeV/c, in the target reference frame) • Free nucleons from the break-up of the excited nuclear remnants • More or less equivalent to A-A spectators (Fermi-like motion)

  20. Gray Black • Ng   • Nb  Ng Gray and black neutron distributions FERMILAB E667 Gray tracks: forward peaked Black tracks: uniform distr. Saturation at high Ng!

  21. Gray/black separation Protons: use rough ZP segmentation separate gray from blacks Gray are mainly emitted forward (in the direction of the proton) Lorentz-boosted with the nucleus Become slower than the black in the ALICE CM frame Detected in the ZDC external zone

  22. Centrality binning Example: 4 (arbitrary) centrality bins Anyway, RHIC experiments use forward multiplicity for centrality tagging in d-Au  to be investigated at ALICE

  23. Luminosity monitoring (E.M. dissociation) • Measure mutual e.m. dissociation of nuclear beams • Use 1n-1n channel to monitor luminosity: 1n-1n=0.7 b (10% accuracy) • Other cross sections (RELDIS) • Single e.m. :215 b • Mutual e.m. (xn-xn) : 7 b • Triggering scheme foreseen • One ZDC enters at level 0  (non-prescaled) trigger rate ~ 2·105 s-1 • ( at L = 1027 cm-2 s-1 ) • Prescaling factor ~ 103prescaled trigger rate ~ 2·102 s-1 • The other ZDC enters at level 1 • Final rate for the 1n-1n process ~ 1 s-1 • Is such a statistics high enough ?

  24. Low neutron-multiplicity events • Narrow pT range: neutron spot very well contained • Energy resolution allows clean separation of 1n-2n-3n contribution

  25. Conclusions • First round of simulation studies on event characterizarion done Chapter 6.1 of PPR • Still missing (or in progress) • Quantitative comparison of the centrality determination using • various estimators • Realistic (and quantitative) evaluation of Pb-Pb triggering efficiency • Alternative solutions for centrality determination in p-A • Forward multiplicity • Many topics concerning event characterization not covered here • See e.g. • Tiziano’s talk on multiplicity dertermination • Nora’s talk on event plane determination with the ZDC • Francesco’s talk on effective energy and multiplicity

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