Charged-particle multiplicity with ALICE at LHC International Workshop on Interplay between Soft and Hard interactions in particle production at ultra-relativistic energies Catania (Italy) – September 8-10, 2010 Charged-particle multiplicity with ALICE at LHC Domenico Elia, INFN Bari for the ALICE Collaboration D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Outline ALICE @ LHC: detector overview motivation for pp measurements Charged multiplicity measurements in pp: minimum bias proton-proton data tracklet reconstruction method pseudorapidity density and multiplicity at 0.9, 2.36 and 7 TeV Beyond pp minimum bias running: high multiplicity pp events heavy-ion multiplicity measurement Summary D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 ALICE layout low-momentum cutoff (<100 MeV/c) excellent tracking and PID dedicated di-electrons and di-muons high resolution calorimeter for direct photons efficient min-bias trigger, additional triggers ITS D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

ALICE layout SPD ITS Silicon Pixel Detector low-momentum cutoff (<100 MeV/c) excellent tracking and PID dedicated di-electrons and di-muons high resolution calorimeter for direct photons efficient min-bias trigger, additional triggers ITS Silicon Pixel Detector SPD two innermost layers of the ITS radii 3.9 / 7.6 cm |h| < 2.0 / 1.4 10 M pixels 425 x 50 mm2 pixel size 80% active during data taking MB and HM triggers (FastOr) tracklets (down to pT  30 MeV/c) D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Motivation for pp physics ALICE dedicated to heavy-ions: study strongly interacting matter (QGP) LHC: x 30 RHIC energy first Pb-Pb collisions expected Nov. 2010 pp collisions needed as “comparison data” Physics programme with pp: comprehensive study of MB@LHC comparison with models, tuning MC search for collective effects at partonic level multiplicity dependence of measurements soft & semi-hard QCD complementary to other LHC experiments address specific issues of QCD D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Trigger and data samples Minimum bias trigger: pp @ 0.9 and 7 TeV SPD (|h|<2) or V0-A or V0-C (at least one particle in 8 units of h) in coincidence with passing bunches (BPTX beam pickups) also control triggers to measure beam-induced and accidental background pp @ 2.36 TeV SPD only + BPTX Collected MB pp samples: 2009: 0.9 and 2.36 TeV, 0.5 M events (10.3 mb-1) 2010: 0.9 and 7 TeV, 700 M events (9.4 nb-1) of which 10 M events @ 0.9 TeV D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Event classes 0.9 and 2.36 TeV data: Inelastic (INEL) and Non-Single-Diffractive (NSD) fraction of diffractive processes in MC from measured cross sections (UA5 @ 0.9 TeV and CDF/E710 @ 1.8 TeV) Pythia and Phojet used to asses effects of different kinematics of diffractive processes 7 TeV data: diffraction cross sections unknown hadron level definition (INEL>0): at least one charged particle in |h| < 1 (similar to ATLAS) minimizes model dependence INEL: MBOR (SPD or V0-A or V0-C) and offline bkg suppression NSD: MBAND (V0-A and V0-C) and offline bkg suppression INEL>0: INEL and at least one charged particle in |h | < 1 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Multiplicity with the SPD Primary vertex reconstruction The “tracklet” algorithm: looks for pair of clusters aligned with vertex Dj (bending plane) and Dq (polar angle) cuts optimized both for pp and PbPb iterates to find best combinations in pp: efficiency 99%, bkg of few % Corrections: trigger bias, vertexing inefficiency detector and trackleter inefficiencies combinatorial background contamination from decays, conversions, secondaries D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 First dN/dh measurements First measurement @ 0.9 TeV: 284 events, 23rd November 2009 magnetic field off consistent with pp systematic error 7% (fraction and kinematics of diffractive processes) first paper at LHC Following measurements: dN/dh and multiplicity distributions @ 0.9, 2.36 and 7 TeV: first paper @ 7 TeV all three first multiplicity papers published on EPJC EPJC: Vol. 65 (2010) 111 EPJC: Vol. 68 (2010) 345 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 dN/dh vs other experiments High statistics measurements @ 0.9 and 2.36 TeV: normalized to INEL and NSD events consistent with UA5 (0.9 TeV) and CMS (only NSD) systematic error 2-3% increase from 0.9 to 2.36 TeV 24% (NSD) well above model predictions EPJC: Vol. 68 (2010) 89 0.9 TeV 2.36 TeV EPJC: Vol. 68 (2010) 89 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 dN/dh vs energy First measurement @ 7 TeV: normalized to INEL>0 (INEL with at least 1 charged particle in |h|<1) minimized model dependent corrections and systematics (3%) increase from 0.9 to 7 TeV 57% even more above model predictions EPJC: Vol. 68 (2010) 345 Energy dependence: power law fits well EPJC: Vol. 68 (2010) 345 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Multiplicity distributions Measurements @ 0.9 and 2.36 TeV: distributions in three h-intervals very good agreement with UA5 (|h|<0.5) and CMS fits with one negative binomial work well in limited |h| regions difference between INEL and NSD in the low-multiplicity region 0.9 TeV: ALICE vs UA5 EPJC: Vol. 68 (2010) 89 0.9 TeV: INEL vs NSD D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Multiplicity vs MC Comparison with MC @ 0.9, 2.36 and 7 TeV: tails increase faster wrt MC, not satisfactory @ 7 TeV PHOJET: good @ 0.9 TeV, fails @ 2.36 and 7 TeV PYTHIA ATLAS tuning: fails @ 0.9 TeV, reasonably close @ 2.36 and 7 TeV but deviations around 10-20 PYTHIA D6T and Perugia-0: far from data @ all energies EPJC: Vol. 68 (2010) 89 EPJC: Vol. 68 (2010) 345 0.9 TeV 2.36 TeV 7 TeV D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 High multiplicity studies Motivation for high multiplicity pp: measure the multiplicity distribution up to highest reachable value 10 times average pp multiplicity  potentially new physics study collision properties vs multiplicity: topology (pt distribution, jets, sphericity, flow etc) strangeness, baryon, charm content, HBT etc vs multiplicity High multiplicity event pp @ 7 TeV Nch = 120 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 High multiplicity studies Data collection: exploit the SPD FastOr: 400+800 individual chips trigger algorithms: FastOr multiplicity based 15 M pp @ 7 TeV data being taken, studied Data sample preparation: events usable for physics studies multiple collisions (pile-up): removal (tag double vertex events) estimate residual background, bias beam-bkg removal at high multiplicity Sector: 4 (outer) + 2 (inner) staves Half-Stave: 10 chips SPD: 10 sectors (1200 chips) Pile-up event pp @ 0.9 TeV D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Perspectives for heavy ions First physics measurement in PbPb: central collisions dN/dh, first estimate of the energy density dNch/dy = 2600 dNch/dy = 1200 ln(√s) extrapolation saturation model Eskola hep-ph/050649 Integrated multiplicity distributions from AuAu/PbPb collisions and scaled pp collisions Most of the predictions before RHIC considerably higher ALICE designed for dNch/dy=8000 D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Perspectives for heavy ions First physics measurement in PbPb: central collisions dN/dh, first estimate of the energy density first collisions expected by November: PbPb @ √sNN = 2.76 TeV, 20 days @ 50 Hz  1.5 107 min bias 102 central events enough for first dN/dh preparation within HI First Physics task force ongoing 500 Hijing PbPb @ 2.76 A TeV 5% centrality Ratio generated/corrected dN/dh D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Summary Multiplicity measurements in pp: about 700 M events collected results published at the three available energies: INEL and NSD events @ 0.9 and 2.36 TeV INEL>0 @ all three energies comparison with other experiments: good agreement with UA5 (0.9 TeV) and CMS (NSD only) comparison with MC: PHOJET good @ 0.9 TeV, all unsatisfactory @ 2.36 and 7 TeV energy dependence not described (increase faster than predicted) Beyond pp minimum bias: high multiplicity pp sample under study eager for first PbPb … D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010

WISH 2010 / Catania (Italy) - September 8-10, 2010 Thank you! D. Elia (INFN Bari) WISH 2010 / Catania (Italy) - September 8-10, 2010