Direct Photons: Flow, Thermal Yield and High p T R AA Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

Direct photons basics Production Process – Compton and annihilation (LO, direct) – Fragmentation (NLO) – Escape the system unscathed Carry dynamical information of the state Temperature, Degrees of freedom – Immune from hadronization (fragmentation) process at leading order – Initial state nuclear effect Cronin effect (k T broardening) T. Sakaguchi, Rico 2 Photon Production: Yield   s   e+e+ e-e-

PHENIX detector 2 central arms: electrons, photons, hadrons – charmonium J/ ,  ’ -> e + e - – vector meson r, w,  -> e + e - – high p T p o, p +, p - – direct photons – open charm – hadron physics Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee   Designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance T. Sakaguchi, Rico

T. Sakaguchi, Rico 4 High pT  dir in p+p – (p)QCD test NLO pQCD calculation of  dir yield is tested with p+p collisions The calculation works very well Aurenche et al., PRD73, (2007)

Scaling in p+p direct photons Plotting cross-sections from various experiments against x T = 2p T /√s Hard scattering process should scale with x T Scale yields by (√s) n PHENIX data includes virtual photon results in low p T (1<p T <5GeV/c) n=4.5 makes a universal line – Consistent with expectation from NLO pQCD. – See similar analysis arXiv: T. Sakaguchi, Rico 5

T. Sakaguchi, Rico 6 A theory: F. Arleo (JHEP 0609 (2006) 015) Isospin effect, in addition to jet-quenching(BDMPS) and shadowing. Jet-photon conversion is not taken into account Low pT region is underestimated because of lack of jet-photon conversion? Direct photons in 200GeV Au+Au Direct photons suppressed at very high pT? – Final result is coming

T. Sakaguchi, Rico 7 (fm/c) log t hadron decays sQGP hard scatt Rich sources of photons in QGP jet Brems. jet-thermal parton-medium interaction hadron gas EE Rate Hadron Gas sQGP Jet-Thermal Jet Brems. Hard Scatt See e.g., Turbide, Gale, Jeon and Moore, PRC 72, (2005)

Adding virtuality in photon measurement T. Sakaguchi, Rico 8 (fm/c) log t hadron decays hadron gas sQGP hard scatt Mass (GeV/c 2 )  *  e+e- virtuality jet Brems. jet-thermal parton-medium interaction By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c 2 )

T. Sakaguchi, Rico 9 Compton q  g q e+e+ e-e- Internal conv. One parameter fit: (1-r)f c + r f d f c : cocktail calc., f d : direct photon calc. Focus on the mass region where  0 contribution dies out For M<<p T and M<300MeV/c 2 – qq ->  * contribution is small – Mainly from internal conversion of photons Can be converted to real photon yield using Kroll-Wada formula – Known as the formula for Dalitz decay spectra PRL104,132301(2010), arXiv: Low p T photons with very small mass

System size dependence of  fraction  fraction = Yield direct / Yield inclusive Lines are NLO pQCD calculation with mass scales (pT=0.5, 1.0, 2.0) Largest excess above pQCD is seen at Au+Au. – Moderately in Cu+Cu T. Sakaguchi, Rico 10 No excess in d+Au (no medium) Excess also in Cu+Cu

d+Au Min. Bias Low p T photons in Au+Au (thermal?) 11 PRL104,132301(2010), arXiv: T. Sakaguchi, Rico Inclusive photon ×  dir /  inc Fitted the spectra with p+p fit + exponential function – T ave = 221  19 stat  19 syst MeV (Minimum Bias) Nuclear effect measured in d+Au does not explain the photons in Au+Au Au+Au

Initial k T broadening or recombination? T. Sakaguchi, Rico 12 Recombination model claims that the Cronin effect in hadron production is built up by recombination – e.g. R. Hwa, Eur.Phys.J.C43:233(2005) – Cronin effect in direct photon production should be smaller than one in  0 Within quoted errors, the effect is same for  0 and photon production   RAA in d+Au at 200GeV. PRL91, (2003)

Comparison with a model calculation Nuclear Effect is slightly seen T. Sakaguchi, Rico 13

Direct photon v T. Sakaguchi, Rico 14

T. Sakaguchi, Rico 15 Photon source detector ~v2~ annihilation compton scattering Bremsstrahlung (energy loss) jet jet fragment photon v 2 > 0 v 2 < 0 Depending the process of photon production, path length dependence of direct photon yield varies – v 2 of the direct photons will become a source detector – Later thermalization gives larger v 2 For prompt photons: v 2 ~0 Turbide et al., PRC77, (2008)

Measuring direct photon v T. Sakaguchi, Rico 16 Calculation of direct photon v 2 = inclusive photon v 2 - background photon v 2 (    etc) PHENIX, arXiv:

Centrality dependence of direct photon v 2 Very large flow in low pT v 2 goes to 0 at high p T – Hard scattered photons dominate T. Sakaguchi, Rico 17 PHENIX, arXiv:

Comparison with models. No success.. Later thermalization gives larger v 2 (QGP photons) Large photon flow is not explained by partonic flow models T. Sakaguchi, Rico 18 Curves: Holopainen, Räsänen, Eskola., arXiv: v1 thermal diluted by prompt Chatterjee, Srivastava PRC79, (2009) Hydro after  0

This fits to data well, but.. Large flow can not be produced in partonic phase, but could be in hadron gas phase This model changed ingredients of photon spectra drastically! – We realized the importance of the data… T. Sakaguchi, Rico 19 thermal + prim.  van Hees, Gale, Rapp, PRC84, (2011)

Summary T. Sakaguchi, Rico 20 Primordial hard scattering process is investigated with direct photons in p+p collisions – x T scaling parameter n=4.5 makes a universal curve Low p T photons in Au+Au show thermal characteristics (exponential slope) Low p T photons in d+Au exhibit initial state nuclear effect – The effect is very small. Direct photon v 2 has been measured for the first time – Powerful source detector – Unexpectedly large flow was seen. Not explainable by models except for the one assuming long hadron gas phase.

Backup T. Sakaguchi, Rico 21

T. Sakaguchi, Rico 22 New production mechanism introduced Jet in-medium bremsstrahlung Jet-photon conversion Both are “thermal  hard” Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) Compton scattering of hard scattered and thermal partons (Jet-photon conversion) Turbide et al., PRC72, (2005) R. Fries et al., PRC72, (2005) Turbide et al., PRC77, (2008) Liu et al., arXiv: , etc..

T. Sakaguchi, Rico 23 A plate ~After cooking up ingredients~

T. Sakaguchi, Rico 24 Is it (only) an isospin effect? Taking for example, the isospin effect: Direct photon cross-sections for p+p, p+n and n+n are different because of different charge contents (  e q 2 ) Effect can be estimated from NLO pQCD calclation of p+p, p+n and n+n – In low pT, quarks are from gluon split  no difference between n and p – At high pT, contribution of constituent quarks manifests Minimum bias Au+Au can be calculated by: (  AA /N coll )/  pp vs pT (  AA /N coll )/  pp vs xT Same suppression will be seen in lower pT at  s NN =62.4GeV TS, INPC07, arXiv.org:

T. Sakaguchi, Rico 25 Looks like there is an isospin effect (and/or PDF effect) Question: p+p is a right reference to take? – Isospin effect is electric charge dependent, which affects to photons;  0 is color charge dependent – Therefore, e-loss models so far are still valid Also see: Miki, Session XV The test: 62GeV Au+Au direct photons Both are reasonable!

Calculations reasonably agree with data Factors of two to be worked on.. Correlation between T and   T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c 26 T. Sakaguchi, Rico % Au+Au PRL104,132301(2010), arXiv:

T. Sakaguchi, Rico 27 CentdN/dy (pT>1GeV) Slope (MeV)  2 /D OF 0-20%1.50±0.23± ±19± / %0.65±0.08± ±18± /3 MinBias0.49±0.05± ±14± /4 Inclusive photon ×  dir /  inc Fitted the spectra with p+p fit + exponential function Barely dependent of centrality PRL104,132301(2010), arXiv: Direct photons through dileptons

Reconstruct Mass and pT of e+e- – Same as real photons – Identify conversion photons in beam pipe using their orientation w.r.t. the magnetic field Reject them –   e+e- at r≠0 have m≠0 (artifact of PHENIX tracking: no tracking before the field) T. Sakaguchi, Rico 28 Dilepton analysis (I) z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay Compton q  g q e+e+ e-e- e+e+ e-e- External conv. Internal conv.

PHENIX applied internal conversion technique – Real photons can convert to virtual photons – Inv. mass shapes for Dalitz decay of mesons are calculable using Kroll-Wada formula If M<<pT, the ratio of observed inv. mass to expected is proportional to direct photon excess ratio Take ratio where  0 contribution is small  S/B increases T. Sakaguchi, Rico 29 Review low-mid pT photons ÷ ÷ ÷ R data Compton q  g q e+e+ e-e-

T. Sakaguchi, Rico 30 Hard scattering  dir in Au+Au (high p T ) Blue line: N coll scaled p+p cross-section Au+Au = p+p x T AB holds – pQCD factorization works NLO pQCD works.  Non-pert. QCD may work in Au+Au system

T. Sakaguchi, Rico Outcome from Au+Au collisions Comparing with various sources of electron pairs Cocktail of the sources are calculated based on  0 /  spectra measured in PHENIX Huge excess over cocktail calculation is seen in GeV/c 2 PRC81, (2010), arXiv:

Thermal radiation from QGP (1<pT<3GeV) – S/B is ~5-10% – Spectrum is exponential. One can extract temperature, dof, etc.. Hadron-gas interaction (pT<1GeV/c):  (  )   (  ),  K*  K  T. Sakaguchi, Rico 32 Difficult objects! Photons from QGP ~big challenge~ f B : Bose dist.  em : photon self energy photons dileptons Interesting, but S/B is small S/B ratio

Reconstruct Mass and pT of e+e- – Same as real photons – Identify conversion photons in beam pipe using and reject them Subtract combinatorial background Apply efficiency correction Subtract additional correlated background: – Back-to-back jet contribution – well understood from MC Compare with known hadronic sources T. Sakaguchi, Rico 33 Dilepton Analysis π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+