Medium and High pT Direct Photons from PHENIX at RHIC A.Bazilevsky For the PHENIX Collaboration Winter Workshop on Nuclear Dynamics February 3-10, 2013 Squaw Valley, CA 1

Direct Photons in (polarized) pp Quark-Gluon Compton scattering is the dominant source of high pT photons in pp collisions (q-qbar annihilation – 10-15%) Access to gluon (polarized) distributions through factorized pQCD No fragmentation involved -> access to Sivers function in transversely polarized protons (correlation between proton spin and parton k T ) TMD factorization? Sivers function universality? … Also fragmentation photons See Spin talks by C.McKinney and O.Eyser 2

Direct photons in HIC Initial Condition: hard  QGP: thermal  Jet fragmentation Bremsstrahlung Jet-photon conversion Hadron gas: thermal  Time Photons emitted at all stages of the system evolution With different characteristic p T spectrum Once produced photons leave the medium unmodified Carry dynamical information of the state Initial state Init. temperature, thermalization time Modification of fragmentation (in γ -jet) … Turbide, Rapp, Gale, Phys. Rev. C 69 (014903),

PHENIX detector for photon measurements Fine segmented EMCal for γ +RICH&Tracking&Magnet for e + e - BBC&ZDC for event characterization BBC&RxNP for reaction plane High rate DAQ: 5-7 kHz Trigger MB (collision) with BBC Rare with EMCal ( γ ), EMCal&RICH (e) e+e+ ee   4

Direct γ reconstruction: higher pT   -tagging (pp and dA): Statistical subtraction: p T (GeV/c) 4 pp R AuAu Direct γ = Inclusive γ – Hadron_Decay γ 5

Direct γ reconstruction: lower pT Source of real photons is also a source of virtual photons decaying into e + e - Focus on the mass region where  0 contribution dies out: M ee >m π 0 For M ee <<p T : M<300 MeV/c 2 – qq ->  * contribution is small – Mainly from internal conversion of photons Can be converted to real photon yield using Kroll-Wada formula ( for Dalitz decay spectra) One parameter fit: (1-r)f c + r f d f c : cocktail calc., f d : direct photon calc. q  g q e+e+ e-e- S~1 for M ee << p T PRL 104, (2010) Direct γ = Inclusive γ – Hadron_Decay γ See talk by S.Rolnick 6

pp Measured up to p T =25 GeV/c The pQCD NLO calculation is in good agreement with data Important reference for HI measurements to study hot and cold nuclear effects PRD 86, (2012) The pT range in the previous report. (PRL 2007) 7

pp: scaling Plotting cross-sections in p-p and p-pbar from various experiments against x T = 2p T /√s Hard scattering process should ~scale with x T (if no α s, PDF evolution, etc.) 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 n=4 for pure gluon exchange with no evolution of α s, PDF, etc. PRD 86, (2012) 8

pp: event shape Checked the event shape with an isolation cut Generally the theory calculation agrees with data Deviation from theory at low pT? How reliable is theory at low pT? At high pT most direct photons are isolated Decay photons are not isolated (as expected) E cone < 0.1E  PRD 86, (2012) 9

Application to proton spin structure studies Clean access to ΔG But needs more luminosity and larger acceptance (-> RHIC upgrade, sPHENIX) See talk by C.McKinney See talk by O.Eyser Projected stat. sensitivity 3.0<|η|<3.8 Includes correction for background from π0, η Gives access to Sivers function (correlation between proton spin and parton k T ) 10

AuAu: R AA PRL 109, (2012) L in p+p N col l R AA ~1 : Consistent with binary scaling of p+p. Minimum bias 0-5% 60-92% … Though many mechanisms may affect it (and/or compensate each other): Isospin difference, nPDF, Cronin Suppression of fragmentation photon Jet-photon conversion, Bremsstrahlung in QGP, etc. 11

Also CuCu-200GeV and AuAu-62GeV Cu+Cu at √s NN = 200 GeV R AA ~1 : Consistent with binary scaling of p+p. 12

AuAu: R AA  Initial state effects (IS) include isospin and nuclear PDF, consistent with data  Final state effects (FS) include suppression of jet fragmentation photons and photons from jet-plasma interaction, consistent with data  Another model with both IS and FS disagrees with data JHEP1104, 055 PRC77, arXiv: PLB669, 337 PRL 109, (2012) Need for more precise data 13

dAu R d+Au is consistent with 1. It suggests little or no nuclear effect within our uncertainty. Consistent with theory calculations with CNM effects arXiv: Theory calculations from I.Vitev et al., PLB669, 337 (2008) 14

dAu We have x30 statistics in Run8 compared to published Run3. Will help to constrain the gluon PDF further. 15 arXiv: RHIC range

v2 annihilation Compton scattering Medium induced (inc.energy loss) jet jet fragment photon v 2 > 0 v 2 < 0 Hard scattering photons: v 2 ~0 V2 depends on production process S. Turbide, C. Gale and R.J. Fries, PRL 96, (2006) V2 is sensitive to thermalization time R. Chatterjee and D. K. Srivastava PRC 79, (2009) More details in talk by S.Rolnick 16

v2 At high pT, v2~0. It is consistent with the hard scattering source. At low pT (thermal region), an unexpectedly large v2 is observed! It depends on the formation time Theory predictions are a bit small. PRL 109, (2012) MB (0-92%) 0-20% 20-40% calculation from PRC 79, (R) More discussion on low p T v2 in talk by S.Rolnick 17

PHENIX upgrade for photons 3.1<  <3.8 To be ready for Run-14/15 Initial condition for HI Gluon Distribution in CNM at Low-x Direct photons (h ,  0 suppression + isolation cut) MPCEX A combined charged particle tracker and EM preshower detector – dual gain readout allows sensitivity to MIPs and full energy EM showers. Direct photons  0 reconstruction out to >80GeV Charged track identification MPC MPC-EX More details in talk by S.Cambell 18

PHENIX -> sPHENIX A plan to upgrade the detector. Large acceptance with high  /  0 separation. More tracking layers, hope eID is still powerful. Hadron calorimeter for  +Jet analysis. High Rate capability and large acceptance : 10 6 jets per year above 30 GeV/c 10 4 direct  per year above 20 GeV/c More details in talk by D.Morrison 19

Summary p+p reference measurements are on the universal curve of world’s measurements; in agreement with NLO pQCD dAu: R dA ~1; expected nuclear modification within current uncertainites AuAu: No nuclear modification (R AA ~1) in high p T photon (> 5 GeV/c) v2~0 at pT>4 GeV/c: consistent with prompt (hard) photons A challenging probe, both experimentally and theoretically  Need for more precise measurements in wider kin. range with a variety of observables (R AA, v2, v3, …, HBD …)  Short and long term PHENIX upgrade plan γ for Spin By C.McKinney and O.Eyser Low pT and v2 By S.Rolnick γ -h correlation By A.Hanks PHENIX Upgrade By S.Campbell and D.Morrison 20

Backup 21

AuAu Long standing issue of direct photons R AA at high p T 22

AuAu PRL 109, (2012) 23

Low pT -> thermal  arXiv:  fraction = Yield direct / Yield inclusive Lines are NLO pQCD calculation with scales μ= 0.5p T, 1.0p T, 2.0p T Largest excess above pQCD is seen at Au+Au Nuclear effect measured in d+Au does not explain the photons in Au+Au R AA >1 at p T <3 GeV/c, whereas R dA ~1  Medium effects (additional source of direct photons) in AuAu 24

Low pT -> thermal  Au+Au PRL 104, (2010) First information about the temperature of the system averaged over the space- time evolution of the collision From fit to exp + n coll scaled pp: T ave = 221  19 stat  19 syst MeV (MB) Small (if any) centrality dependence  NLO pQCD consistent with p+p down to p T =1 GeV/c  Excess of photons (with 1<p T <3 GeV/c) in Au+Au beyond the N coll scaled p+p yield.  Interpreted as thermal radiation emitted by the medium 25

Initial temperature & formation time T c ~170MeV from lattice QCD PRC 81, (2010) All above the critical temperature T init depends on  0 assumption  Need observable to constrain initial conditions T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c 26

dA 27

Initial k T broadening or recombination? 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) 28

Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) Compton scattering of hard scattered and thermal partons (Jet-photon conversion) 29

Also CuCu, AuAu-62GeV Cu+Cu at √s NN = 200 GeV 30

Au-Au vs Cu-Cu Could d+d collisions help? (One could tag pp, pn, nn collisions!) At 62GeV there is no experimental difficulty The isospin effect should be (almost) independent of centrality Little overlap with Cu+Cu, but the two are consistent within errors 31

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:

STAR STAR, Phys.Rev.C81,064904(2010) Excess in d+Au? –No exponential excess High-p T direct photon results from PHENIX and STAR –d+Au Agree with T AB scaled pQCD consistent with PHENIX and STAR –p+p Agree with pQCD and PHENIX Low-p T direct photon –No publication data at STAR 33

M ee Phys. Rev. Lett. 104, (2010) 34

v2 Consistency between EMCal photons and internal conversion photons (at low pT) 35

v2 High p T (p T >5 GeV/c) – v 2 ~ 0 (independent of centrality) – Consistent with STAR results within large error. Low p T (p T <3 GeV/c) – Inconclusive centrality dependence STAR, arXiv: PHENIX, arXiv: Cent20-40% Cent0-20% Cent10-40% 36

Direct photon v2 PRL 109, (2012) Au+Au √s NN = 200 GeV minimum bias Large v 2 at p T < 4 GeV/c where thermal photons dominate v 2 consistent with 0 at high p T where prompt photons domininate  Very surprising result: large v 2 implies late emission whereas thermal radiation implies early emission  Models have difficulties in reproducing simultaneously yield and v 2 of photons 37

Direct photon v2 p T (GeV/c)  dir. v 2 Large v 2 of low p T thermal photon 200GeV Au+Au 0-20% R. Chatterjee and D. K. Srivastava PRC 79, (R) (2009) PRL96, (2006) Several models have failed in v 2 magnitude with similar shape Thermal photon in quark matter – v 2 >0 at low p T – v 2 ~0 at high p T Thermalization time  0 – Early (smaller v 2 ) – Late (larger v 2 ) Constrain  0 – Measure v 2 at low p T Hydro after  0 38

v Hydro region pQCD region Needs *2-3 smaller errors to become decisive

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… thermal + prim.  van Hees, Gale, Rapp, PRC84, (2011) 40

v2: theory comparison 0-20% Phys. Rev. C 84, (2011) H. van Hees, C. Gale, R. Rapp Important features/differences from hydrodynamic expansion – Larger equilibrium hadronic rates – Hadronic phase includes meson-chemical potentials – Hadronic phase lasts longer (smaller T fo and larger T ch ) – Elliptic flow builds up faster Thermal radiation dominated by hadronic phase Is the QGP window closed? 41

v2 42

FF shape modification  x E = -p T h /p T  cos()~z T  ln x E - MLLA variable  Universal scaling in pp  Enhancement at very low z T  Suppression at large z T -pQCD photons provide a reasonable estimate for the energy of recoil jet (q or g); -Measured medium FF is softer compared to vacuum fragmentation; -Further studies with jets tagged by strangess (gluon / quark jet separation) follow 43