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Direct Photon Production in pp collisions at the LHC

Direct Photon Production in pp collisions at the LHC. F.M. Liu IOPP/CCNU, Wuhan, China K. Werner Subatech, Nantes, France. Théorie LHC France 06 April 2010 IPN Lyon. outline. pp collisions at extremely high energy ( i.e. 14TeV) Multiple scattering, many Pomerons / strings involved

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Direct Photon Production in pp collisions at the LHC

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  1. Direct Photon Production in ppcollisions at the LHC F.M. Liu IOPP/CCNU, Wuhan, China K. WernerSubatech, Nantes, France Théorie LHC France 06 April 2010 IPN Lyon

  2. outline • pp collisions at extremely high energy (i.e. 14TeV) • Multiple scattering, many Pomerons / strings involved • How to treat the afterward secondary scattering? • Direct photon production • RHIC Au+Au data are explained with 4 sources • Hydrodynamic description of the hot dense matter in AA • Hydrodynamic treatment of the secondary scattering in pp • Results and discussion • pQCD + plasma contribution • Is it possible to form a QGP in pp collisions? PHOS@ALICE will answer. pt 100Mev~100GeV .

  3. Particle production in pp Usually, a pair of strings are formed via a) longitudinal excitation b) color exchange hadron production: string fragmentation, i.e.

  4. pp collisions at high E • Problem: Two-string picture can not explain observables (~100GeV) • 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 H.J.Drescher et al, Phys.Rept.350,93(2001). F.M.Liu et al. PRD 67, 034011 (2003)

  5. pp at extremely high E, ie,E=14TeV Multiple elementary interactions (Pomerons) in NEXUS/EPOS: Rapidity Pomeron number might be very big. We need a post-collision evolution to treat the many-body system. Question: How to treat the afterward secondary scattering ? on parton level ? or on hadron level? or string interaction? or Pomeron interaction?

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

  7. preliminary pp-> gamma with NLO pQCD W.Vogelsang & M.R.Walley 1997 JPG: 23,A1-A69 1. Leading Order contribution 2. Fragmentation contribution: High order contribution D0 and CDF can only measure pt > 10GeV. Should the secondary interaction be responsible for this deviation? Saturation makes a decrease of PDF at lowx. can not be.

  8. Secondary scattering in AA Evolution of core region, or huge number of secondary collisions, can be treated with hydrodynamics.

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

  10. 4 main sources in AA • Leading Order contribution 2. Fragmentation contribution: Jets lose energy in plasma 3. Thermal contribution 4. Jet-photon conversion

  11. Au+Au -> direct photons at 200AGeV FML, T.Hirano, K.Werner, Y. Zhu, Phys.Rev.C79:014905,2009 Direct photon production from AuAu collisions at top RHIC energy is well explained in a large pt range at all centralities.

  12. Plasma effect in AA What we learn from AA: The thermal contribution makes an evident increase of the production at low pt region! And energy loss will reduce fragmentation contribution at high pt region. The enlightenment for pp: 1) So secondary collisions can be responsible for the deviation between direct photon data and NLO pQCD prediction: The data will be above (below) the theory for lower (large) pt for high energy pp collisions! 2) When pp collision energy E is extremely high, ie, at 14TeV There is a great number of secondary collisions. Let’s treat with hydrodynamics. A QGP might be formed.

  13. Plasma evolution in pp (EPOS)

  14. Pt =1GeV/c 2GeV/c 3GeV/c pQCD plasma plasma pQCD Plasma contribution to direct photon preliminary Effect from secondary scattering: PHOS@ALICE Pt range: 100Mev~100GeV, detectable!

  15. Conclusion • A great number of Pomerons(strings) are involved in pp collisions at very high energies. Therefore secondary scattering of produced particles should not be ignored. • Hydrodynamic approach is proposed to treat the secondary scattering. Based on this approach we compare direct photon’s pQCD production and plasma production in pp collisions at 14TeV. • We find direct photon is a very useful probe if a QGP can be formed in pp@LHC. PHOS@ALICE is able to test.

  16. Thank you!

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