Photons at RHIC Yorito Yamaguchi CNS, University of Tokyo 1/14 - PowerPoint PPT Presentation

Photons at RHIC Yorito Yamaguchi CNS, University of Tokyo

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction → Provide key inputs (T init & τ 0 ) to describe evolution of the matter � Their p T are characterized by their origin. S. Turbide et al., PRL77,024909(2008)

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction → Provide key inputs (T init & τ 0 ) to describe g q evolution of the matter γ � Their p T are characterized by their origin. q � High p T : Initial hard scatterings S. Turbide et al., PRL77,024909(2008)

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction → Provide key inputs (T init & τ 0 ) to describe g q evolution of the matter Jet ‐ QGP γ � Their p T are characterized by their origin. photons q � High p T : Initial hard scatterings � Mid p T : Jet ‐ Medium interactions S. Turbide et al., PRL77,024909(2008)

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction → Provide key inputs (T init & τ 0 ) to describe π π g q evolution of the matter Thermal Jet ‐ QGP γ � Their p T are characterized by their origin. photons photons q ρ γ � High p T : Initial hard scatterings � Mid p T : Jet ‐ Medium interactions � Low p T : Thermal radiations from QGP and Hadron Gas S. Turbide et al., PRL77,024909(2008)

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction → Provide key inputs (T init & τ 0 ) to describe π π g q evolution of the matter Thermal Jet ‐ QGP γ � Their p T are characterized by their origin. photons photons q ρ γ � High p T : Initial hard scatterings � Mid p T : Jet ‐ Medium interactions � Low p T : Thermal radiations from QGP and Hadron Gas � Elliptic flow, v 2 of direct photons can disentangle their production processes. S. Turbide et al., PRL77,024909(2008)

1/14 Direct photons � Sensitive and direct probe for all stages of a collision � Generated in every stage of the collision � Leave a medium without a strong interaction Hadron decay → Provide key inputs (T init & τ 0 ) to describe photons π π g q evolution of the matter Thermal Jet ‐ QGP γ � Their p T are characterized by their origin. photons photons q ρ γ � High p T : Initial hard scatterings � Mid p T : Jet ‐ Medium interactions � Low p T : Thermal radiations from QGP and Hadron Gas � Elliptic flow, v 2 of direct photons can disentangle their production processes. � Direct photon measurements are very challenging due to a large background from hadron decays. S. Turbide et al., PRL77,024909(2008)

2/14 Hard photon measurements � Statistical subtraction method by EMCals γ = γ − γ Direct All Hadronic � Firstly measure π 0 , η � Strong suppression of high p T hadrons helps to measure direct γ for Au+Au → Identify remaining γ after subtraction of hadron decay γ as direct γ . d+Au d+Au p+p B.I. Abelev et al., PRC81,064904(2010) � p+p : Consistent with NLO pQCD calculation → Works for pQCD test � d+Au : Also consistent with N coll ‐ scaled NLO pQCD → Little nuclear effects

3/14 Hard photons in Au+Au � Spectra : Also follows N coll ‐ scaled p+p for different centrality bins � R AA : Suppression at high p T (p T >14GeV/c) due to isospin effect? or due to initial state energy loss? → Experimentally challenging due to merging effect for decay photons.

4/14 Different collision energy Au+Au : 62.4GeV Pb+Pb : 2.76TeV � 62.4GeV : Isospin effect would be at lower p T → Consistent NLO pQCD � 2.76TeV : CMS measured isolated photons → No suppression → Inconsistent with 200GeV Au+Au, but efforts to finalize the 200GeV Au+Au result are ongoing.

5/14 How to measure low pT photons � Hard to measure by EMCals due to a finite energy resolution → Alternative method has been developed : “Virtual photon method” e + Virtual photon method q e - � Basic idea : Any source of γ can emit γ *, convert to low mass e + e ‐ γ ∗ � How to identify direct γ * → e + e ‐ : g q Relation between γ and associated γ * → e + e ‐ emission rates Process dependent factor � Direct γ * : If p T 2 »m ee 2 , S(m ee )~1 � Dalitz decay : → Extraction of direct γ * → e + e ‐ can be made by utilizing m ee shape difference between direct γ * and hadrons.

6/14 Determination of direct γ fraction ( ) ( ) ( ) ( ) = 1 − ⋅ + ⋅ r : direct γ /inclusive γ f m r f m r f m data ee c ee dir ee Direct γ * Hadrons � Determination of direct γ fractions in 0.1 ‐ 0.3GeV/c 2 for p T >1GeV/c � Negligible contribution of π 0 → γ e + e ‐ 2 → S(m ee )~1 � Satisfy an important assumption of p T 2 »m ee � No contribution of π + π ‐ → e + e ‐ � Enhanced e + e ‐ yield over known hadron contributions is clearly seen due to direct γ * → e + e ‐ . � Extended fit result can also describe the data well in m ee >0.3GeV/c 2 . A. Adare et al., PRL104,132301(2010)

7/14 Direct γ fractions ⎛ ⎞ σ σ σ NLO NLO hadron d d d ⎜ ⎟ γ γ γ + NLO pQCD expectations are calculated as : ⎜ ⎟ dp dp dp ⎝ ⎠ T T T μ = 0.5p T μ = 1.0p T μ = 2.0p T � Direct γ fractions from virtual photon method plays an important role on determination of direct photon v 2 in low p T region.

8/14 Low p T direct photon results p+p vs d+Au p+p vs Au+Au A. Adare et al., PRL104,132301(2010) � p+p vs d+Au : Consistent → Little nuclear effects � p+p vs Au+Au : Observation of a clear excess in p T <3GeV/c → Exponential fit gives inverse slope of T = 221 ± 19 stat ± 19 syst MeV (Central).

9/14 T init & τ 0 T AuAu (fit) ~ 220 MeV T C from Lattice QCD ~ 170 MeV A. Adare et al., PRC81,034911(2010) � Hydrodynamic models agree with the data within a factor of 2 � Uncertainty on T init (300 ‐ 600MeV) is still large. � Depending on thermalization time τ 0 (0.1 ‐ 0.6fm/c) → Need sensitive observable to further constrain T init

10/14 Further constraint of T init & τ 0 by v 2 � Different direct photon production v 2 < 0 Thermal radiation Bremsstrahlung processes have different behavior of v 2 . Fragmentation Jet-Photon � Initial hard scattering → v 2 =0 conversion v 2 > 0 � Thermal radiation & Fragmentation → v 2 >0 � Bremsstrahlung & JPC → v 2 <0 → Helps to disentangle compositions of R. Chatterjee & D. K. Srivastava, PRC 79 , 021901 (2009) direct photon spectrum � Direct photon v 2 is also sensitive to Hydro after τ 0 thermalization time τ 0 . � Early thermalization → Small thermal photon v 2 � Late thermalization → Large thermal photon v 2

11/14 How to obtain direct photon v 2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 − inclusive hadron r v v γ = direct 2 2 v − 2 Au+Au@200 GeV r 1 γ minimum bias Measure inclusive γ v 2 using preliminary 1. EMCals inclusive photon v 2

11/14 How to obtain direct photon v 2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 − inclusive hadron r v v γ = direct 2 2 v − 2 Au+Au@200 GeV r 1 γ minimum bias Measure inclusive γ v 2 using preliminary 1. EMCals � Confirm NO charged hadron contamination by external conversion method inclusive photon v 2

11/14 How to obtain direct photon v 2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 − inclusive hadron r v v γ = direct 2 2 v − 2 Au+Au@200 GeV r 1 γ minimum bias Measure inclusive γ v 2 using preliminary 1. π 0 v 2 EMCals � Confirm NO charged hadron contamination by external conversion method Measure π 0 v 2 , and then evaluate 2. inclusive photon v 2 other hadron v 2 ( η , ω , …) using MC calculation π 0 v 2 looks similar to inclusive γ v 2 �

11/14 How to obtain direct photon v 2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 − inclusive hadron r v v γ = direct 2 2 v − 2 Au+Au@200 GeV r 1 γ minimum bias Measure inclusive γ v 2 using preliminary 1. π 0 v 2 EMCals � Confirm NO charged hadron contamination by external conversion method Measure π 0 v 2 , and then evaluate 2. inclusive photon v 2 other hadron v 2 ( η , ω , …) using MC calculation π 0 v 2 looks similar to inclusive γ v 2 � 3. Subtract hadron v 2 from inclusive γ v 2 with direct γ fractions from virtual photon method

12/14 Direct photon v 2 A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV � Surprisingly, a large direct minimum bias photon v 2 is observed in Direct photon v 2 p T <3GeV/c � Direct photon v 2 → 0 indicates preliminary prompt photons from initial hard scatterings are dominant in p T >5GeV/c