Direct Photon Production in pp at LHC Fuming

Direct Photon Production in pp at LHC Fuming LIU IOPP, Central China Normal University Hard Probes 2010 Oct 10 -15, Eilat, Israel

Outline • Motivation: direct photon data from high energy pp collisions • search for a solution: – QGP formation and direct photon production in AA at RHIC – Some LHC data imply a QGP formation in pp at LHC – How can a QGP be formed in pp? • Direct photon results • Conclusion and outlook

Direct photon data in pp (ppbar) PDG review Present data are generally in good agreement with NLO QCD prediction. But a tendency for the data to be above (below) theory for lower (large) pt. 3

Discrepancy at small pt Diana, Rojo, Richard and Ball, PLB 693: 430 -437, 2010 TEVATRON: small pt ~ 20 Ge. V LHC: small pt ~500 Me. V Discrepancy may be huge! High order enhanced terms ------Resummation But, only a few percent for pt<10 Ge. V at LHC

Search for a solution

Au+Au -> direct photons at 200 AGe. V FML, T. Hirano, K. Werner, Y. Zhu, Phys. Rev. C 79: 014905, 2009 Direct photon production from Au. Au collisions at top RHIC energy is well understood in a large pt range at all centralities. Thermal photons make a huge enhancement at small pt !

Direct photon main sources 1. Leading Order contribution 2. Fragmentation contribution: Jets lose energy in plasma NLO 3. Thermal contribution 4. Jet-photon conversion 7

QGP and hydrodynamics described with 3+1 D ideal hydrodynamics Initial condition: thermalized QCD matter at rest at • parameterized based on Glauber model • string overlapping and melting Evolution: 3 D ideal hydrodynamic equation Eo. S: phase transition at QGP phase: 3 flavor free Q & G gas HG phase: hadronic gas PCE Freeze-out: Cooper-Fry Formulism, May followed with cascade treatment 8

QGP from pp at LHC L >> Lambda is hard to realize? BE correlation by CMS: PRL 105, 032001, 2010 The size of correlated particle emission region Increase with multiplicity! CMS, JHEP 1009: 091, 2010 Azimuthal correlation: a ridge-like structure This resembles similar features observed in AA at RHIC---QGP formation

How to form QGP in pp?

Hadron production in pp Usually, a pair of strings are formed: PYTHIA, HIJING, DPM, FRITIOF, VENUS, NEXUS, … F. M. Liu et al. PRD 67, 034011 (2003) hadron production: string fragmentation, i. e. 11

pp collisions at high E Problem: Two-string picture can not explain observables (~100 Ge. V) 1) 2) 3) 4) High multiplicity (multiplicity dis. predicted is too narrow) Increase of mean pt High pt jets Rise of central rapidity density Solution: Multiple scattering becomes important at high energies More Pomerons/strings are added Pomeron = a pair of strings

If the collision energy is very big Rapidity Multiple elementary interactions (Pomerons) in NEXUS/EPOS: Prob(ν): H. J. Drescher et al, Phys. Rept. 350, 93(2001) F. M. Liu et al. PRD 67, 034011 (2003) Pomeron number ν has a certain prob. to be very big (high multiplicity events) String overlapping QGP formaton! 13 13

QGP and hydrodynamics in pp EPOS, K. Werner 14

Direct photon results

A huge enhancement at small pt!

Focus on high multiplicity events Rapidity distr. of direct photons (thermal photons) from pp at 7 Te. V. 2500. 5 Pt>1 Ge. V/c

Conclusions and outlook 1. Hugh discrepancy between data and NLO cal. is expected for direct photon production in pp at LHC at small pt region 2. A QGP is very possibly formed in pp collisions at LHC (high multiplicity events). Then thermal photons can make up some above discrepancy 3. Further study is needed for those high multiplicity events: prompt photon and jet production energy loss and jet photon conversion gamma/jets – hadrons correlations 18

Thank you! 19