1. Direct Photon Production in pp collisions at the LHC 第 8 届高能物理大会分会 2010.4.18 南昌 F.M. Liu IOPP/CCNU, Wuhan, China

2. 2 Main projects on photons at LHC All 4 detectors of LHC have photon detectors They can measure photons from some hundred MeV to some hundred GeV Goals: 1) SM Higgs search via Higgs -> gamma gamma 2) Measure fragmentation function in pp via gamma triggered events, and study QGP properties in AA Here I am talking about another goal: 1)QGP formation in pp? 2)Determine the dynamics for parallel scattering in pp

3. 3 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.

4. 4 Hadron production in pp Usually, a pair of strings are formed: PYTHIA, HIJING, DPM, FRITIOF, VENUS, NEXUS,… hadron production: string fragmentation, i.e.

5. 5 pp collisions at high E Problem: Two-string picture can not explain observables (~100GeV) 1)High multiplicity (multiplicity dis. predicted is too narrow) 2)Increase of mean pt 3)High pt jets 4)Rise of central rapidity density Solution: Multiple scattering becomes important at high energies More Pomerons/strings are added Pomeron = a pair of strings F.M.Liu et al. PRD 67, 034011 ( 2003 ) H.J.Drescher et al, Phys.Rept.350,93(2001).

6. 6 pp at extremely high E, ie, E=14TeV Multiple elementary interactions (Pomerons) in NEXUS/EPOS : Question: How to treat the afterward secondary scattering ? on parton level ? or on hadron level? or string interaction? or Pomeron interaction? Pomeron number might be very big. We need a post-collision evolution to treat the many-body system. Rapidity

7. 7 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. Direct photon data in pp (ppbar) Latest PDG review

8. 8 pp-> gamma with NLO pQCD 2. Fragmentation contribution: High order contribution 1. Leading Order contribution preliminary D0 and CDF can only measure pt > 10GeV. Should the secondary interaction be responsible for this deviation? Saturation makes a decrease of PDF at low x. can not be. W.Vogelsang & M.R.Walley 1997 JPG: 23,A1-A69

9. 9 Secondary scattering in AA Evolution of core region, or huge number of secondary collisions, can be treated with hydrodynamics.

10. 10 Hydrodynamic treatment Initial condition: thermalized QCD matter at rest at Evolution: 3D ideal hydrodynamic equation described with 3+1D ideal hydrodynamics EoS: 1st order phase transition at QGP phase: 3 flavor free Q & G gas HG phase: hadronic gas PCE Freeze-out: May followed with cascade treatment parameterized based on Glauber model string overlapping and melting

11. 11 4 main sources in AA Jets lose energy in plasma 1.Leading Order contribution 2. Fragmentation contribution: 3. Thermal contribution 4. Jet-photon conversion

12. 12 Au+Au -> direct photons at 200AGeV Direct photon production from AuAu collisions at top RHIC energy is well explained in a large pt range at all centralities. FML, T.Hirano, K.Werner, Y. Zhu, Phys.Rev.C79:014905,2009

13. 13 Plasma effect in 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. 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! What we learn from AA: The enlightenment for pp: 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. 1) 2)

14. 14 Plasma evolution in pp (EPOS)

15. 15 Plasma contribution to direct photon Effect from secondary scattering: PHOS@ALICE Pt range: 100Mev~100GeV, detectable! Pt =1GeV/c 2GeV/c3GeV/c pQCD plasma pQCD preliminary

16. 16 Conclusion 1.A great number of Pomerons(strings) may be involved in pp collisions at very high energies. Therefore secondary scattering of produced particles should be considered. 2.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. 3.We find direct photon is a very useful probe if a QGP can be formed in pp@LHC. PHOS@ALICE is able to test.

17. 17 Thank you!