The ALICE Experiment Event by Event fluctuations ALICE TOF Calibration

1. QGP Advanced computingChiara ZampolliMuseo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi & INFN & University of Bologna The ALICE Experiment Event by Event fluctuations ALICE TOF Calibration Chiara Zampolli

2. The Quark-Gluon-Coloured-World (QGCW) • Collisions between heavy nuclei (such as 208Pb82+) at a very high energy (such as ~1150 TeV, the maximum available so far) are expected to produce a “new world”, the Quark-Gluon-Coloured-World (QGCW) LHC supercollider @ CERN. • In such collisions, many QCD open-colour states may be produced, much more than the number of baryons and mesons. • Totally unexpected events are likely to be detected!! Event-by-Event physics becomes crucial. Chiara Zampolli

3. Experiments at the LHC CMS LHC Designed for high pT physics in p-p collisions Dedicated LHC HI experiment ~ 9 km ATLAS ALICE CERN Chiara Zampolli

4. The ALICE Physics Program • Heavy ion observables in ALICE • Probes of deconfinement & chiral symmetry restoration • Global characteristics of the fireball (Evt by Evt) -Multiplicities & Et distributions, -HBT Correlations, elliptic and transverse flow, -hadron ratios and spectra, -Evt-by-Evt fluctuations -… -Charmonium and Bottomonium states, -strangeness enhancement, resonance modification, -jet quenching and high pt spectra, -open Charm and Beauty -thermal gradiation,… • p-p and p-A physics in ALICE • Physics of ultra-peripheral heavy ion collisions • Contribution of ALICE to cosmic-ray physics Chiara Zampolli

5. ALICE E-by-E Program Thanks to the very high charged particle multiplicity expected per event, E-by-E studies will be feasible with the ALICE detector for many observables: • Temperature • Mean pT • Particle Ratios • Multiplicity • Conserved Quantities (Charge) • HBT radii • Balance Function • Flow • DCC • ... Particle IDentification plays a crucial role! http://aliceinfo.cern.ch/, ALICE PPR II Chiara Zampolli

6. ALICE PID separation @ 3s separation @ 2s (dE/dx) Chiara Zampolli

7. K p p Combined PID – ITS || TPC || TOF 0.15 < pT < 4 GeV/c efficiency contamination Chiara Zampolli

8. p K p Generated vs Identified Spectra Generated Identified (t + w) Identified (w) Chiara Zampolli

9. Fitting of the Spectra • Correction of the identified spectra taking into account: • Limited acceptanceand reconstruction efficiency of the detectors: εacc • Transverse momentum reconstruction efficiency: εp • PID efficiency: εPID • PID contamination: CPID • Event by event fitting procedure for pT spectra: exponential function ,T = slope parameter, connected to the kinetical freeze-out temperature Chiara Zampolli

10. p K p Results – Single Event, pT spectra Generated Reconstructed i.e. corrected! Fit range: 0.25 < pT < 2 GeV/c Chiara Zampolli

11. p p K = 226 MeV sT = 13 MeV Results – T Distributions = 182 MeV sT=3 MeV = 303 MeV sT = 21 MeV sT/T ~ 0.5% sT/T ~ 7% sT/T ~ 6% Chiara Zampolli

12. Particle Ratios K/π: R = 0.106 σR = 0.009 p/π: R = 0.055 σR = 0.006 σR/R ~ few % Chiara Zampolli

13. p K p = 451 MeV spT=6 MeV = 578 MeV spT=24 MeV = 744 MeV spT=50 MeV Mean pT spT/pT~ 1% spT/pT~ 7% spT/pT~ 4% Chiara Zampolli

14. ALICE TOF Calibration • A more “technical” work has been carried on concerning the ALICE TOF detector calibration. • Corrections to be introduced, wrt two main sources of uncertainty: • TIME DELAYS: Online Calibration an equalization of the channels of the TDCs is necessary because of the delays introduced by the electronics (mainly cable lengths and pulse line lengths). • TIME SLEWING : Offline Calibration time slewing is caused by the finite amount of charge necessary to trigger the discriminator → charge fluctuations generate a time walk. • TOF channel status to be determined (from HW map, pulser runs, noise runs). Chiara Zampolli

15. TOF Online Calibration • Determine the relative delays for each TOF channel using the tTOF-texp spectra • tTOF = time measured by TOF • texp = expected time of flight, from TOF geometry, assuming β= 1 and a straight line trajectory No delay, edge @ ~ 0 • Sharp edgeexpected at the delay value from fastest particles (β~ 1) Chiara Zampolli

16. TOF Offline Calibration • Dependence of the measured times vs time over threshold used. • To unfold the time spread due to the momentum spectra, to the particle types, and to the different track length,the algorithm is based on the comparison between the times of the reconstructed tracks (from track length and momentum measurements) and the measured times (tEXP-tTOF). The identity of the particle (needed to choose the proper tEXP) is determined via a combinatorial algorithm. Chiara Zampolli

17. TOF Offline Calibration – a Feeling • ToT  amplitude of the signal • Test beam results used in simulation Profiling and fitting procedure – 5th order polynomial function Chiara Zampolli

18. TOF Offline Calibration - Performance decalibration σa = 92.7 ps σb = 111.4 ps σc = 93.2 ps calibration σcal = ~ 10 ps Results obtained with 500 tracks from 2 HIJING central Pb-Pb ev; simulated time delay = 2 ns. Chiara Zampolli

19. Conclusions • Event by event fluctuations studies are an important tool to explore the QCD phase diagram, searching for the QGP, and the QCD critical point. • Thanks to its very high particle yield per event, and to the excellent PID capabilities, ALICE will be able to study fluctuations measuring the identified particle spectra (π, K, p) and the particle ratios (K/ π, p/ π) on an Event-by-Event basis. • As data taking is approaching (a first cosmics run period is foreseen for December), the calibration tools have to be ready within ALICE framework to comply with the experiment requirements. • A lot of work is ongoing, both from the analysis point of view and as more technical software development, waiting for LHC data... Chiara Zampolli

20. BACK-ups Chiara Zampolli