High Multiplicity Events in p+p Collisions at LHC Energy

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

2. 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 Multiplicity distributions (PYTHIA, 14 TeV p+p)

3. 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

4. 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

5. ALICE Experiment 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

6. Sector: 4 (outer) + 2 (inner) staves Half-Stave: 10 chips SPD: 10 sectors (1200 chips) ALICE Detector TPC Time Projection Chamber 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 E E 88µs SPD 560 cm 510 cm Beam-gas event

7. 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

8. 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

9. 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)

10. 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

11. 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)

12. 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) hard soft CDF data <pT> follows the same trend on multiplicity dNch/dh in both p+p and A+A for pi/K/p.

13. Elliptic Flow 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)

14. 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)

15. 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.