1、RHIC和LHC实验中直接光子的产生,2010年7月 乌兰浩特,刘复明 华中师范大学,2,Background,LHC加速器正在进行高能pp碰撞LHC上的四个实验组都安装了高性能的光子探测器物理目标:ATLAS、CMS：通过双光子道寻找标准模型Higgs粒子ALICE: pp通过gamma事件测量碎裂函数 AA研究QGP性质,3,这里我想探索的问题是：1. 流体力学可否应用于高能pp碰撞? 反方：流体力学应用要求系统尺度远大于平均自由程正方：Landau建议 应用于强子强子碰撞 已取得巨大成功 (ISR能区： 快度分布、KNO scaling、）2. 高能PP碰撞可否产生QGP？ 方法：直接光
2、子、Drell Yan 历史：TEVATRON上 E735 group 的失败经验,4,outline,高能pp碰撞所面临的问题：Multiple scattering, many Pomerons / strings involvedHow to treat the afterward secondary scattering? 直接光子的产生 （来自RHIC实验的经验）RHIC Au+Au data are explained with 4 sourcesHydrodynamic description of the hot dense matter in AA Hydrodynamic
3、treatment of the secondary scattering in pp结果和讨论pQCD + plasma contributionIs it possible to form a QGP in pp collisions?,5,Hadron production in pp,Usually, a pair of strings are formed: PYTHIA, HIJING, DPM, FRITIOF, VENUS, NEXUS,hadron production: string fragmentation, i.e.,F.M.Liu et al. PRD 67, 03
4、4011 (2003),6,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 ptHigh pt jetsRise of central rapidity density,Solution: Multiple scattering becomes important at high energies More Pom
5、erons/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).,7,高能极限下的pp碰撞,Multiple elementary interactions (Pomerons) in NEXUS/EPOS:,Question: How to treat the afterward secondary scattering ? on parton level ? or on hadron leve
6、l?,Pomeron number might be very big.,Rapidity,8,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,9,pp- gamma with NLO pQCD,2. Fragmentation contribu
7、tion: 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 at low x！,W.Vogelsang & M.R.Walley 1997 JPG: 23,A1-A69,10,RHIC AuAu碰撞,流体力学的应用取得了巨大胜利：1） Jet qu
8、enching , 高横动量区强子产额的压低2） Elliptic Flow （v2) 随着碰撞中心度的变化规律 低横动量区 n_q scaling 背对背的关联 QGP产生的证据,11,Hydrodynamic treatment,Initial condition: thermalized QCD matter at rest atEvolution: 3D ideal hydrodynamic equation,described with 3+1D ideal hydrodynamics,EoS: 1st order phase transition at QGP phase: 3 f
9、lavor free Q & G gas HG phase: hadronic gas PCE,Freeze-out:,Cooper-Fry Formulism, May followed with cascade treatment,parameterized based on Glauber model string overlapping and melting,12,AuAu碰撞中直接光子的来源,Jets lose energy in plasma,Leading Order contribution,2. Fragmentation contribution:,3. Thermal
10、contribution,4. Jet-photon conversion,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.C79:014905,2009,14,来自RHIC实验的经验,热光子会使低横动量区直接光子产额明显增加能量损失会压低高横动量
11、区碎裂光子的产额,The tendency for the data to be above (below) the theory for lower (large) pt 高能pp碰撞中可能产生了热密物质2)采用流体力学方法来处理高能pp碰撞中次级碰撞 14TeV 50 AuAu 200GeV,对pp碰撞的启示:,15,Plasma evolution in pp (EPOS),16,Plasma contribution to direct photon,Effect from secondary scattering:,光子探测器 Pt range: 100Mev100GeV, detectable!,preliminary,17,Conclusion,1. 流体力学可否应用于高能pp碰撞? LHC的直接光子测量将提供参考答案2. 高能PP碰撞可否产生QGP？ 可能：高能、对心（大 事件）,18,Thank you!,
