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Medium and High pT Direct Photons from PHENIX at RHIC

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. Direct Photons in (polarized) pp.

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Medium and High pT Direct Photons from PHENIX at RHIC

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  1. 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

  2. Direct Photons in (polarized) pp • Quark-Gluon Compton scattering is the dominant source of high pT photons in ppcollisions (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 partonkT) • TMD factorization? • Sivers function universality? • … Also fragmentation photons See Spin talks by C.McKinney and O.Eyser

  3. Direct photons in HIC Time Turbide, Rapp, Gale, Phys. Rev. C 69 (014903), 2004 Initial Condition:hard  QGP: thermal  Jet fragmentation Bremsstrahlung Jet-photon conversion Hadron gas: thermal  • Photons emitted at all stages of the system evolution • With different characteristic pTspectrum • Once produced photons leave the medium unmodifiedCarry dynamical information of the state • Initial state • Init.temperature, thermalization time • Modification of fragmentation (inγ-jet) • …

  4. 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) p g e+ e-

  5. Direct γreconstruction: higher pT Direct γ = Inclusive γ – Hadron_Decayγ R AuAu Statistical subtraction: 12 pT(GeV/c) 0 4 8 pp p0-tagging (pp and dA):

  6. Direct γ reconstruction: lower pT e+ Direct γ = Inclusive γ – Hadron_Decayγ See talk by S.Rolnick q e- g* PRL 104,132301 (2010) • Source of real photons is also a source of virtual photons decaying into e+e- • Focus on the mass region where p0 contribution dies out: Mee >mπ0 • For Mee<<pT : M<300 MeV/c2 • 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) g q One parameter fit: (1-r)fc + rfd fc: cocktail calc., fd: direct photon calc. S~1 for Mee << pT

  7. pp PRD 86, 072008 (2012)  Measured up to pT=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 The pT range in the previous report. (PRL 2007)

  8. pp: scaling PRD 86, 072008 (2012)  • Plotting cross-sections in p-p and p-pbar from various experiments against xT = 2pT/√s • Hard scattering process should ~scale with xT(if no αs, PDF evolution, etc.) • Scale yields by (√s)n • PHENIX data includes virtual photon results in low pT(1<pT<5GeV/c) • n=4.5 makes a universal line • n=4 for pure gluon exchange with no evolution of αs, PDF, etc.

  9. pp: event shape PRD 86, 072008 (2012)  Econe < 0.1E • 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)

  10. Application to proton spin structure studies See talk by C.McKinney • Clean access to ΔG • But needs more luminosity and larger acceptance (-> RHIC upgrade, sPHENIX) 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 partonkT)

  11. AuAu: RAA PRL 109, 152302 (2012) Minimum bias 0-5% 60-92% L in p+p Ncoll RAA~1 : Consistent with binary scaling of p+p. • … 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.

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

  13. AuAu: RAA PRL 109, 152302 (2012) • Initial state effects (IS) include isospinand 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, 024909 arXiv:0904.2184 • PLB669, 337 Need for more precise data

  14. dAu Theory calculations from I.Vitev et al., PLB669, 337 (2008) arXiv:1208.1234 Rd+Au is consistent with 1. It suggests little or no nuclear effect within our uncertainty. Consistent with theory calculations with CNM effects

  15. dAu We have x30 statistics in Run8 compared to published Run3. Will help to constrain the gluon PDF further. arXiv:1012.1178 RHIC range

  16. jet fragment photon v2 > 0 annihilation Compton scattering jet v2 > 0 Medium induced (inc.energy loss) v2 < 0 v2 Hard scattering photons: v2~0 • V2 depends on production process • S. Turbide, C. Gale and R.J. Fries, PRL 96, 032303 (2006) • V2 is sensitive to thermalization time • R. Chatterjee and D. K. Srivastava PRC 79, 021901 (2009) More details in talk by S.Rolnick

  17. v2 PRL 109, 122302 (2012) calculation from PRC 79, 021901(R) 20-40% 0-20% MB (0-92%) At high pT, v2~0. It is consistent with the hard scattering source. At low pT (thermal region), an unexpectedly large v2 is observed! More discussion on low pT v2 in talk by S.Rolnick It depends on the formation time Theory predictions are a bit small.

  18. More details in talk by S.Cambell PHENIX upgrade for photons MPC-EX MPCEX 3.1<h<3.8 To be ready for Run-14/15 • A combined charged particle tracker and EM preshower detector – dual gain readout allows sensitivity to MIPs and full energy EM showers. • Direct photons • p0 reconstruction out to >80GeV • Charged track identification MPC Initial condition for HI Gluon Distribution in CNM at Low-x Direct photons (h, 0 suppression + isolation cut)

  19. More details in talk by D.Morrison 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 : • 106 jets per year above 30 GeV/c • 104 direct  per year above 20 GeV/c

  20. Summary • p+p reference measurements are on the universal curve of world’s measurements; in agreement with NLO pQCD • dAu: RdA ~1; expected nuclear modification within current uncertainites • AuAu: No nuclear modification (RAA~1) in high pT 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 (RAA, v2, v3, …, HBD …) Short and long term PHENIX upgrade plan PHENIX Upgrade By S.Campbelland D.Morrison γfor Spin By C.McKinneyand O.Eyser Low pT and v2 By S.Rolnick γ-h correlation By A.Hanks

  21. Backup

  22. AuAu Long standing issue of direct photons RAA at high pT

  23. AuAu PRL 109,152302 (2012)

  24. Low pT -> thermal  arXiv:1208.1234 • g fraction = Yielddirect / Yieldinclusive • Lines are NLO pQCD calculation with scales μ= 0.5pT, 1.0pT, 2.0pT • Largest excess above pQCD is seen at Au+Au • Nuclear effect measured in d+Au does not explain the photons in Au+Au RAA>1 at pT<3 GeV/c, whereas RdA~1  Medium effects (additional source of direct photons) in AuAu

  25. Low pT -> thermal  PRL 104,132301 (2010) Au+Au • NLO pQCD consistent withp+p down to pT=1 GeV/c • Excess of photons (with 1<pT<3 GeV/c) in Au+Aubeyond the Ncollscaledp+pyield. • Interpreted as thermal radiation emitted by the medium First information about the temperature of the system averaged over the space-time evolution of the collision From fit to exp + ncoll scaled pp: Tave= 221  19stat  19syst MeV (MB) Small (if any) centrality dependence

  26. Initial temperature & formation time PRC 81, 034911 (2010) Tc~170MeV from lattice QCD All above the critical temperature Tinit depends on 0 assumption Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c  Need observable to constrain initial conditions

  27. dA

  28. Initial kT 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 p0 • Within quoted errors, the effect is same for p0 and photon production p0 RAA in d+Au at 200GeV. PRL91, 072303 (2003)

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

  30. Also CuCu, AuAu-62GeV Cu+Cu at √sNN = 200 GeV

  31. Au-Au vs Cu-Cu 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 Could d+d collisions help? (One could tag pp, pn, nn collisions!)

  32. 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 (  eq2) • 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: (sAA/Ncoll)/spp vs pT (sAA/Ncoll)/spp vs xT Same suppression will be seen in lower pT at sNN=62.4GeV TS, INPC07, arXiv.org:0708.4265

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

  34. Mee Phys. Rev. Lett. 104, 132301 (2010)

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

  36. v2 • High pT (pT>5 GeV/c) • v2 ~ 0 (independent of centrality) • Consistent with STAR results within large error. • Low pT (pT<3 GeV/c) • Inconclusive centrality dependence Cent0-20% Cent20-40% PHENIX, arXiv:1105.4126 Cent10-40% STAR, arXiv:1008.4894

  37. Direct photon v2 PRL 109, 122302 (2012) • Large v2 at pT < 4 GeV/c where thermal photons dominate • v2 consistent with 0 at high pT where prompt photons domininate Au+Au √sNN = 200 GeV minimum bias • Very surprising result: large v2 implies late emission whereas thermal radiation implies early emission • Models have difficulties in reproducing simultaneously yield and v2 of photons

  38. Direct photon v2 R. Chatterjee and D. K. Srivastava PRC 79, 021901(R) (2009) PRL96, 202302 (2006) Hydro after t0 dir. v2 • Thermal photon in quark matter • v2>0 at low pT • v2~0 at high pT • Thermalization time t0 • Early (smaller v2) • Late (larger v2) • Constrain t0 • Measure v2 at low pT 200GeV Au+Au 0-20% pT (GeV/c) Large v2 of low pT thermal photon Several models have failed in v2 magnitude with similar shape

  39. v2 Hydro region 0904.2184 Needs *2-3 smaller errors to become decisive pQCD region 39

  40. 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. g van Hees, Gale, Rapp, PRC84, 054906 (2011)

  41. v2: theory comparison H. van Hees, C. Gale, R. Rapp 0-20% Phys. Rev. C 84, 054906 (2011) • Important features/differences from hydrodynamic expansion • Larger equilibrium hadronic rates • Hadronic phase includes meson-chemical potentials • Hadronic phase lasts longer (smaller Tfo and larger Tch) • Elliptic flow builds up faster • Thermal radiation dominated by hadronic phase • Is the QGP window closed?

  42. v2

  43. FF shape modification • xE = -pTh/pTgcos(Df)~zT • x = -lnxE - MLLA variable • Universal scaling in pp • Enhancement at very low zT • Suppression at large zT • 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

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