High Multiplicity Events in p+p Collisions at LHC Energy Masahiro Konno for the ALICE Collaboration (University of Tsukuba) JPS @ Yamagata, 9/21/2008

p+p Collisions at LHC LHC: Proton+Proton collider at maximum energy √s = 14 TeV (Luminosity: 1030 – 1034 cm-2s-1 => Min. Bias rate: 50 kHz – 500 MHz) First circulating beam performed on Sep. 10, 2008 => First p+p collisions will be soon... Physics topics: Higgs, SUSY, low-x physics, reference data for heavy ion First Pb+Pb beam expected by end of 2009 What else can one do with p+p data while waiting for Pb+Pb? => Heavy Ion Physics with p+p collisions at LHC energy Particle density dNch/dη ~ 50-100 can be reached even in p+p (central S+S @ SPS, mid-central Cu+Cu @ RHIC) Predictions p+p 14 TeV <Nch> Ntot dN/dη(η=0) PYTHIA6.214 91.0 158.4 6.8 PHOJET1.12 69.6 115.1 5.1 Multiplicity distributions (PYTHIA, 14 TeV p+p)

High Multiplicity Events in p+p Collisions In high multiplicity events in p+p collisions, one expects secondary collisions of particles to take place – rescattering. Rescattering may occur among partons early in the collisions, and also among hadrons later in the collisions. Both kinds of rescattering can lead to collective behavior among the particles. - Hadronic FS interactions - Multiple parton interactions => Collective flow, Thermal behavior => Small QGP? (not new, explored even at Tevatron...) Increasing the center-of-mass energy increases the parton fluxes in p+p collisions. At very high energies, multiple-parton interactions become important. Energy density vs. Npart Upper: Central Lower: MB

Particle Rapidity Density p+p (p+pbar) Cu+Cu @ RHIC arXiv:0709.4008 High multiplicity events in p+p Particle density dNch/dη increases with √s. This is due to the increase of the rate of multiple parton scatterings. dNch/dη/(Npart/2) shows different trends for p+p (p+pbar) and heavy ion data – effective energy for transverse particle production. LHC energy

ALICE Experiment A dedicated experiment for heavy ion physics at LHC Point 2 in LHC ring ALICE detector A dedicated experiment for heavy ion physics at LHC Sensitivity to low pT (cutoff ~0.2 GeV/c) thanks to low magnetic field (0.5 T) and small amount of material (X/X0=7%, ITS) before TPC. - Particle identification capability: dE/dx (ITS/TPC), TRD, TOF, HMPID, PHOS

ALICE Detector TPC Time Projection Chamber Silicon Pixel Detector SPD 510 cm 88µs 560 cm Main tracking device (|η|<0.9) dE/dx measurement for PID Drift time: ~88 μs Silicon Pixel Detector Inner two layers of ITS (|η|<2.0, 1.4) 1200 chips, 9.8 M pixels Tracking, Multiplicity, Vertex, Trigger SPD Sector: 4 (outer) + 2 (inner) staves Half-Stave: 10 chips SPD: 10 sectors (1200 chips) Beam-gas event

Triggers – Min. bias, High multiplicity Minimum bias trigger is realized using signals from V0 and SPD detectors. The trigger is based on the number of fired chips in two layers of SPD (Fast-OR). Trigger efficiency: >99% for non. diff events with minimized background (beam-gas). In ALICE, max luminosity: <5x1030 cm-2s-1 (~200 kHz) to avoid unacceptable pile-up. ALICE can collect a large sample of p+p events (~109) in the first year. High multiplicity triggers should be employed to record events with large multiplicities to study multiple parton interactions, parton saturation effects, etc. The trigger thresholds on the number of fired SPD chips select high-multiplicity events. ~few % order

Multiplicity Measurement Multiplicity can be measured by counting tracks with SPD. The tracklets are extracted by association of clusters in the two layers and the primary vertex. The detector response matrix can be obtained from simulation studies. Correction by taking into account the geometrical acceptance depending on zvertex. ALICE PPR II (PYTHIA 14 TeV p+p) σ/M vs. M Mrec vs. Mgen σ/M ~ 5-10% (|η|<0.5) acceptance correction dNch/dη vs. η

Separation of Soft and Hard Events Event Selection in p+p collisions - Min. bias events: usually NSD in experiments. soft is dominant. - Jet events: particle showers from hard scattering - High multiplicity events: jets (hard), not-jets (soft) We usually focus on the hard scattering in p+p, but should not forget the underlying event or minimum bias events. The identification of soft and hard interactions is largely a matter of definition. Can one separate the ‘soft’ from the ‘hard’? => Veto on jets (In CDF, ‘soft’ events contain no cluster with ET > 1.1 GeV based on calorimeter. All other events are ‘hard’) In ALICE, only charged jets can be reconstructed with tracking system. (EMCAL will be added later to enhance the trigger capability.) Ref: PRD65 072005 (2002) PRD65 092002 (2002) Nch vs. pT(jet)

Multiplicity Distributions PRD69 034007 (2004) Multiplicity distribution is a most basic observable to study the particle production. At lower energies, the data follow KNO scaling: At higher energies, the scaling is found to be broken. This is explained as follows: (1) contributions from jets (2) multiple parton interactions CDF report: in soft events KNO scaling is valid. Even in a single parton scattering, there are high multiplicity events. Thus, multiplicity measurements at LHC will bring a first indication of the importance of MPI and soft-hard interplay. Will be a first paper in ALICE. E735, UA5 1800 GeV 900 GeV 546 GeV 200 GeV charged multiplicity distributions

pT Spectra Transverse momentum of charged particles can be measured with TPC and ITS. In 14 TeV p+p collisions, according to PYTHIA, 12% of Min. bias events have pT(hard)>10 GeV/c. (c.f. 1% at 1.8 TeV) For 14 TeV p+p, a considerable part of collisions is expected to have more than one hard (or soft ) scattering. - CDF report: in soft events, pT distribution is only determined by multiplicity, and is energy invariant. In hard events, pT distributions rise with √s. Data points: CDF Lines: PYTHIA 14 TeV 1800 GeV 630 GeV pT resolution vs. pT (ALICE PPR II)

Mean pT vs. Multiplicity The correlation between <pT> and multiplicity is attributed to (a) the increase of jet contribution, (b) radial flow effect (in A+A). - Since high multiplicity jets have a high mean pT, it could be expected that high multiplicity events have higher mean pT. - CDF report: <pT> also increases with multiplicity even in soft events. K π PRD65 072005 (2002) soft hard CDF data <pT> follows the same trend on multiplicity dNch/dh in both p+p and A+A for pi/K/p.

Elliptic Flow Heavy Ion Physics (SPS, RHIC): v2, ε/2 vs. Centrality Heavy Ion Physics (SPS, RHIC): Elliptic flow is sensitive to early parton dynamics, and HBT is instead affected by late hadronic interactions and freeze-out. Does size and shape (parton spatial distribution) matter for a proton? => Measure elliptic flow, HBT in p+p collisions. - Centrality dependence of v2 PHENIX Au+Au at √sNN = 200 GeV Ref: arXiv:0806.0523 Higher v2 can be obtained in peripheral p+p, but the multiplicity is lower, the measurement of v2 is not so easy. Also selecting soft events is required to distinguish jet contribution. p+p at LHC (prediction)

HBT Measurement Momentum correlations can be measured at ALICE. The Radii (STAR) scale with charged multiplicity dNch/dη including various collisions systems (p+p, Au+Au, etc). The radii measured in p+pbar (E735) increase with charged multiplicity. The observed behavior may be related to the space-time evolution of jet hadronization. C2 vs. qs (ALICE MC) R vs. dNch/dη (E735) STAR Ref: PRD48 1931 (1993)

Summary LHC is Proton+Proton collider, but also heavy ion collider. What can one do with p+p data while waiting for Pb+Pb? => Heavy ion physics with p+p collisions at LHC energy (... not just reference to heavy ion data) => High multiplicity events in p+p collisions Since the particle density dNch/dη ~ 50-100 can be reached even in p+p, it is comparable to mid-central Cu+Cu collisions at RHIC (√sNN = 200 GeV). In ALICE experiment, min. bias and high multiplicity triggers are available. Multiplicity measurement can be done with silicon detector. A possible way : Selection between soft and hard events a la CDF/STAR -- with/without charged jets reconstructed with TPC - Observables: multiplicity, pT spectra, <pT>, elliptic flow, HBT, etc.