Prospective of gamma hadron correlation. study in CMS experiment Yeonju Go (Korea University) for the CMS collaboration 5-6 Dec. 2014 HIM meeting
Contents Physics Motivation Direct gamma-hadron correlation technical methods Theoretical expectation of jet fragmentation function PHENIX measurement ALICE measurement Plan for CMS Measurement Summary 2
Hard probe of QGP Initial hard scattered partons traverse hot and dense medium in heavy-ion collisions and undergo multiple interactions prior to hadronization. We can explore the medium properties by comparing jet shape and yields Direct gamma Direct gamma proton proton Pb Pb in vacuum in hot & dense medium Jet fragmenta-on shape modified by medium Previous <photon-jet correlation study> can teach us jet quenching structure. Now <photon-hadron correlation study> can reveals the jet tomography. 3
Di-hadron vs. gamma-hadron Direct gamma hadron or jet Hadrons Hadrons Photon does not have any color charge. without strong interaction can be a reference for the initial momentum of associated away-side jet hadron(or jet)-hadron correlation some surface bias modified parton p T no surface bias Dis-advantage low signal rate large background Golden channel 4
Signal photon identification Different kinds of photons Direct photon produced from primary vertex. Fragmentation photon radiated after the primary hard scattering. Decay photon decayed from mesons, such as π 0 -> γ+γ major backgrounds which we are interested in! statistical subtraction of the decay photon-hadron per-trigger yields from inclusive photon-hadron per-trigger yields. Identify decay photon by using isolation cut and shower shape techniques 80 % bkg removed Isolated γ Non-isolated γ 5
Hadron yield calculation Pb+Pb S NN = 2.76 TeV Underlying event subtraction flow effect correction associated particles Direct gamma-hadron yield Δ Φ trigger particle 6
Theoretical expectation of FF arxiv:hep-ph/0506218v1 Momentum fraction of jet fragments x = p h /p jet zt = pt h /pt γ To focus on the low x region, high p T low p T e+e- collision experiments results of fragmentation function are consistent with theoretical values in vacuum. medium modification : suppression at high p T, enhancement at low p T 7
PHENIX Measurement PRL 111, 32301 (2013) ~2 GeV low p T high p T Enhancement of low p T hadrons high p T low p T Suppression of high p T hadrons 8
PHENIX Measurement PRL 111, 32301 (2013) To understand Δ Φ dependence, jet photon There is significant variation in observed I AA as a function of the integration range. Enhancement at low pt is predominantly at large angles high p T low p T 9
ALICE Measurement Isolated π 0 arxiv:1211.6620v1 2π/3< Φ<4π/3 <z> = p T π0 /p T parton isolated π 0 low p T high p T Isolated π 0 carry a large fraction of the total jet energy. It increases <z> compared to be expected value <z>=0.5 w/o isolation. The possibility of using the isolated π 0 as a trigger particle : it s not good. ( p T π0 <p T parton ) 10
ALICE Measurement Isolated photons arxiv:1211.6620v1 Purity = λ 0 To extract direct photon results, isolated π 0 x E distributions are scaled with respect to the purity estimate. x E distribution measured in pp 7 TeV is the baseline of that in heavy ion collisions. 11
CMS(Compact Muon Solenoid) eta range HCAL η < 5.2 ECAL η < 2.6 Tracker η < 2.5 Photon Reconstructed in ECAL Full azimuthal range & Large eta coverage Good momentum resolution hadrons (& jet) Reconstructed in Silicon tracker, ECAL and HCAL Tracker : 1% resolution in barrel, 2% resolution in endcap at p T =100 GeV/c 12
Kinematic Range [CMS] CMS kinematic range Pb+Pb S NN = 2.76 TeV or (after 2015 run, ~5 TeV) Gamma : p T > 20 GeV/c, η <1.4, 0<Φ<2π Hadron : p T > 0.5 GeV/c, η <1.6, 0<Φ<2π Photon high p T region can be explored. With other experiment results, whole picture can be drew. Previous other experiments [ALICE] p+p S NN = 7 TeV Gamma - Reconstructed in PHOS & EMCAL - 8<p T <25 GeV/c, η <0.7, Δ Φ<100 - Δ p/p < 5% at E < 100 GeV Hadron - Reconstructed in ITS & TPC & EMCAL - p T >0.2 GeV, η <0.9, 0<Φ<2π [PHENIX] p+p & Au+Au S NN = 200 GeV Gamma : - Reconstructed in EMCAL - 5<p T <9 GeV/c, η <0.35, Δ Φ<π/2 Hadron : - Reconstructed in Drift & Pad chambers - 0.5< p T <7 GeV/c, η <0.35, Δ Φ<π/2 13
Summary Direct gamma-hadron correlation is a powerful tool to explore the properties of hot and dense medium produced in heavy ion collisions. PHENIX experiments show that there are significant modifications in jet fragmentation function in heavy ion collisions compared to pp collisions. CMS are planning to measure the gamma-hadron correlations at high p T photon region with high resolution. Combining with the previous PHENIX and the future ALICE results, CMS experiment which have different kinematic regions from other experiments can make whole picture of jet tomography. 14
BACK UP 29 Sep. 2014 Yeonju Go / Photon Meeting 15
Initial k not exactly back-to-back because of initial kt effects 16
pseudo-rapidity vs. angle 17
Photon sources Compton scattering is the dominant channel of high p T isolated direct photon at LHC. gamma-hadron correlations mainly reflect quark fragmentation (Compton scattering dominent) 18
CMS(Compact Muon Solenoid) 19
CMS(Compact Muon Solenoid) 20
CMS(Compact Muon Solenoid) 21