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Direct photons Basis for characterizing heavy ion collisions

Direct photons Basis for characterizing heavy ion collisions. Takao Sakaguchi Brookhaven National Laboratory. Photon measurement is “astronomy”. We’ve seen lights from universe ever since people started dreaming, hoping dreams come true. Jantar Mantar (Observatory).

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Direct photons Basis for characterizing heavy ion collisions

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  1. Direct photonsBasis for characterizing heavy ion collisions Takao Sakaguchi Brookhaven National Laboratory T. Sakaguchi for QM08, Jaipur, India

  2. Photon measurement is “astronomy” We’ve seen lights from universe ever since people started dreaming, hoping dreams come true Jantar Mantar (Observatory) And, dreams came true in heavy ion collisions. First Heavy ion collisions in universe (SN1987) T. Sakaguchi for QM08, Jaipur, India

  3. Outline • Current landscape of photons • “Much simpler, cleaner and precision measurement” to “much complicated, uncleaner and challenging measurement” • High pT to Low pT • High Energy to Nuclear physics • Future exploration using direct photons T. Sakaguchi for QM08, Jaipur, India

  4. Direct photons are emitted at all stages then surviving unscathed strongly ( a << as, almost transparent to medium) Direct photon history hard scatt Cartoon only: sources of g, mean pT vs time (dashed: hadrons) pT (GeV) jet Brems. jet-thermal jet fragmentation sQGP hadron gas hadron decays log t 1 10 107 Good Write-up to read: nucl-ex/0611009 (fm/c) Cartoon from G. David, Hard Probe 2006 T. Sakaguchi for QM08, Jaipur, India

  5. direct fragment Direct photon basics Small Rate: Yield aas • Production Process • Compton and annihilation (LO, direct) • Fragmentation (NLO) • Often carry thermodynamical information of the state • Temperature, Degrees of freedom Gordon and Vogelsang, PRD48, 3136 (1993) T. Sakaguchi for QM08, Jaipur, India

  6. Blue line: Ncoll scaled p+p cross-section High pT – how well it is described • Finding of hard photons, whose yield is well explained by NLO pQCD, opened precision measurements • Initial state condition is understood • Also holds in Au+Au collisions Aurenche et al., PRD73, 094007(2007) T. Sakaguchi for QM08, Jaipur, India

  7. Two Methods in p+p 200GeV Fraction of isolated/all photons, which is calculable from NLO pQCD. Isolation cut: ( 0.1*E > Econe(R=0.5) ) Looking at angular correlation between leading hadrons and photons Even fragment/prompt components are decomposed Look at associated photons Triggering leading hadron g(fragment) / g(inclusive) R Eg Near side g(Isolated)/g(all direct) PHENIX, PRL98, 012002 (2007) See: Nguyen, Session XV T. Sakaguchi for QM08, Jaipur, India

  8. hadron Run4/5 result, presented at QM06 Application of “the probe” -jet correlation • Idea: (Direct  pT) = (away side jet pT) • Momentum loss of jets can directly be measured • PHENIX: subtract p0-h, h-h from (incl g)-h • STAR: subtract p0-h from (enriched g)-h so that near-side associated yield become zero • It would be nice to check the absolute yield by a spectrum measurement See: Nguyen, Hamed, Session XV Many theoretical effort both on prediction and interpretation: X.N. Wang, et al., PRC55, 3047 (1997), F. Arleo, JHEP 0609 (2006) 015, T. Pietrycki et al., arXiv:0706.3442, etc.. T. Sakaguchi for QM08, Jaipur, India

  9. Hard scattered partons interact with thermal partons in matter Sources of Radiation in A+A (interaction of jet and medium) • Compton scattering of hard scattered and thermal partons (Jet-photon conversion) • A recent calculation predicted yields for radiative and collisional E-loss case • This itself probes the matter on similar way as jets do. New way to look at photons? • Bremsstrahlung from hard scattered partons in medium Turbide et al., PRC72, 014906 (2005) R. Fries et al., PRC72, 041902 (2005) Turbide et al., arXiv:0712.0732 Liu et al., arXiv:0712.3619, etc.. C. Gale, NPA774(2006)335 T. Sakaguchi for QM08, Jaipur, India

  10. Direct photons in 200GeV Au+Au • 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? • Remember the extended “highlight plots” from PHENIX • Consistent with old published result up to ~ 12GeV/c • Direct photons suppressed at very high pT? T. Sakaguchi for QM08, Jaipur, India

  11. 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 T. Sakaguchi for QM08, Jaipur, India TS, INPC07, arXiv.org:0708.4265

  12. Both are reasonable! The test: 62GeV Au+Au direct photons • 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 ~18GeV/c@200GeV Also see: Miki, Session XV T. Sakaguchi for QM08, Jaipur, India

  13. Direct photons in 200GeV Cu+Cu See: Miki, Session XV T. Sakaguchi for QM08, Jaipur, India

  14. photons dileptons fB: Bose dist. em: photon self energy Low pT is interesting, and difficult • Thermal radiation from QGP (1<pT<3GeV) • S/B is ~10% • Spectrum is exponential. One can extract temperature, dof, etc.. • Hadron-gas interaction (pT<1GeV/c): ()  (), K*  K Also, many seminal works by Sinha, Alam, Nayak, Srivastava, Fries, Rapp et al. A compilation on photons, PRC 69(2004)014903 Dilepton to Photon ratio gives T: Nayak et al., arXiv:0705.1591 T. Sakaguchi for QM08, Jaipur, India

  15. Low pT photons at SPS • WA98 data can be either explained by kT-smearing or higher initial temperature • Could not see pT>4GeV, where pQCD photons dominate • Lack of information of kT prevents us from resolving the issue. WA98 data and theoretical interpretation, PRC69(2004)014903 p+Pb, p+C data from WA98 have come! See: Baumann, Session XV T. Sakaguchi for QM08, Jaipur, India

  16. 0-30 90-140 200-300 140-200 Rdata ÷ ÷ e+ Compton e- g* ÷ q g q Low pT photons at RHIC • 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 p0 contribution is small  S/B increases T. Sakaguchi for QM08, Jaipur, India

  17. Low pT photons in Au+Au and p+p Low pT photons in Au+Au and p+p are measured using internal conversion method p+p agrees with NLO pQCD well, and Au+Au has excess over the calculation Theoretical fit results are: - 0.15 fm/c, 590 MeV (d’Enterria, Peressounko) - 0.2 fm/c, 450-660 MeV (Srivastava et al.) - 0.5 fm/c, 300 MeV (Alam et al.) - 0.17 fm/c, 580 MeV (Rasanen et al.) - 0.33 fm/c, 370 MeV (Turbide et al.) Lines are NLO pQCD Shorter 0 increases total yield and temperature Result also strongly depends on EOS. See: Dahms, Session XV T. Sakaguchi for QM08, Jaipur, India

  18. jet fragment photon annihilation compton scattering jet v2 > 0 Bremsstrahlung (energy loss) v2 < 0 Direct photon v2~a photon source detector~ • Depending the process of photon production, angular distributions of direct photons vary • Jet fragmentation (v2>0), Jet-photon conversion, in-medium bremsstrahlung (v2<0), Turbide, et al., PRL96, 032303(2006), etc.. • Thermal photon v2 ~ Quark v2 (>0), (based on hydro calculation) • Chatterjee, et al., PRL 96, 202302(2006), Kopeliovich et al., arXiv:0712.2829, etc.. Turbide et al., arXiv:0712.0732 T. Sakaguchi for QM08, Jaipur, India

  19. Direct photon v2 in Au+Au collisions • Hadron decay photon subtracted from inclusive photon v2. • Reached up to ~8GeV/c, no significance for pT<3GeV/c • Tends to be positive? • Uncertainty of gmeas/gbkgd dominates error here. • It is not due to v2 measurement of inclusive photons or 0 • Could be improved by internal conversion method? 20-40% result is similar. FYI, WA98 Result, EPCJ41(2005)287, no significance See: Miki for PHENIX result, Session XV, Raniwala for STAR result, Session XII T. Sakaguchi for QM08, Jaipur, India

  20. Where do we go? T. Sakaguchi for QM08, Jaipur, India

  21. Something we may have missed.. Initial multiple scattering effect should exist…? : kT broadening, p0: kT broadening + recombination. Detail study of p+A or d+A should help Also, structure function of Au? PHENIX, d+Au, RAA STAR, d+Au, gmeas/gbkgd Russcher (STAR), QM06 Peressounko (PHENIX), QM06 T. Sakaguchi for QM08, Jaipur, India

  22. Direct photons in LHC era? • Hard scattering cross-section even goes higher • -Jet correlation analysis is the primary target • Key issue of the analysis is how clean tagged photon samples can be Input and Reconstructed fragmentation function from direct photon tagged events (0-10% Pb+Pb) Discrimination power of photons to hadrons See: Morsch (ALICE), Session XXIV See: Loizides (CMS), Session XXIV T. Sakaguchi for QM08, Jaipur, India

  23. Jet-photon conversion LHC Thermal pQCD ~6GeV? ~15GeV? My LHC favorite • A calculation tells that even in low pT region(pT~2GeV/c), jet-photon conversion significantly contributes to total • What do we expect naively? (or guessively?) • Jet-Photon conversions  Ncoll  Npart  (s1/2)8 f(xT), “8” is xT-scaling power • Thermal Photons  Npart  (equilibrium duration)  f( (s1/2)1/4 ) • Bet: LHC sees huge Jet-photon conversion contribution over thermal? • Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons Turbide et al., arXiv:0712.0732 T. Sakaguchi for QM08, Jaipur, India

  24. T. Renk, PRC71, 064905(2005) See: Kistenev, Poster 24 More future interest • Determining Time evolution scenario by looking at forward direct photons • Landau expansion will make ~ one order reduction at pT=4GeV/c at y=2 compared to y=0 • Nose-cone calorimeter upgrade in PHENIX (Covering 1<|y|<3) • Forward Photon Detector in STAR (Covering -3.7<y<-2.3) T. Sakaguchi for QM08, Jaipur, India

  25. Summary • Direct photons become indispensable probe for characterizing matter • Partons interacting with matter would produce photons with aas. Cleaner probes, but harder observable • High pT direct photons are well calibrated, and become basis on evaluating all interactions of partons with medium • g-jet correlation, etc. • Jet interacting with medium produces photons, and would be a useful tool for parton E-loss mechanism study • The process will be much useful in LHC era. • Isospin effect may have been observed in Au+Au. • low pT photon emission suggests a thermalized state. • Photon production at high rapidity would help determining the system expansion scenario T. Sakaguchi for QM08, Jaipur, India

  26. And… I would like to express my highest respect to those who devoted their efforts on founding direct photon measurements. Where there is an interaction, there is a photon T. Sakaguchi for QM08, Jaipur, India

  27. Backup T. Sakaguchi for QM08, Jaipur, India

  28. arXiv:0712.0732 T. Sakaguchi for QM08, Jaipur, India

  29. Comparison with some models • Turbide et al. (Phys. Rev. C72 (2005) 014906 + Private communication.) • AMY formalism for jet-quenching effect for fragmentation photons. • Systematically data points are below theoretical prediction. • F. Arleo (JHEP 0609 (2006) 015) • High-pT suppression due to isospin effect, in addition to jet-quenching and shadowing. • BDMPS for jet-quenching. • Medium induced jet-photon is not taken into account. • The suppression of very high-pT photon is well reproduced. T. Sakaguchi for QM08, Jaipur, India

  30. Direct Fragment Can’t we yet tell something more from it? • NLO pQCD tells that there are direct photons and jet-fragment photons • Direct photons should not be suppressed • Fragment photons should be suppressed if jet is suppressed • There should be an additional source! T. Sakaguchi for QM08, Jaipur, India

  31. 100 xT What is expected from structure function? gq ->q is main contribution Au+Au minimum bias Eskola,Kolhinen,Ruuskanen Nucl. Phys. B535(1998)351 Structure function ratios drop by ~20% from x=0.1 to 0.2? T. Sakaguchi for QM08, Jaipur, India

  32. New! ~62GeV Au+Au Direct photons~ • Direct photon over NLO pQCD is consistent with unity at 62.4GeV • p+p reference can not be well-defined. Needs a measurement at RHIC T. Sakaguchi, INPC07 T. Sakaguchi for QM08, Jaipur, India

  33. RAA with pQCD RAA with p+p data Direct photon RAA in 200GeV Au+Au • Used p+p data is the denominator • NLO pQCD as denominator is shown as well for a reference • For pT<10GeV, RAA is consistent with Ncoll scaled p+p reference. • RAA seems to decrease at very high-pT (especially for central) • Difference of NLO pQCD calculation and p+p data affects quite a bit. T. Sakaguchi for QM08, Jaipur, India

  34. WA98, PRL93(2004)022301 Are we in discovery? Single photons in Heavy Ion collisions~Before RHIC~ HELIOS (Z. Phys. C46(1009)369), CERES (Z. Phys. C71(1996)571), null result.. • WA80, WA98 are the dedicated experiments for direct photon search in relativistic heavy Ion collisions. • WA98 data can be either explained by kT-smearing or higher initial temeprature • Any data did not see pT>4GeV, where pQCD photons dominate • No information on kT does not allow us to resolve the issue. • Recent data points at ~100MeV available from WA98. • By analysis of correlation strength in interferometery WA98 data and theoretical interpretation PRC69(2004)014903 Hope to hear new result from Christoph Bermann. T. Sakaguchi for QM08, Jaipur, India

  35. Direct photons in p+p collisions • p+p cross-section at s=200GeV is well established with PHENIX Year-5 data set. • Statistically improved from the published Run3 result • Reference for Au+Au collisions • Data parameterized by a fitting function to interpolate to the pT of Au+Au points • Measured p+p yield is higher than NLO pQCD calculation by more than 20%. Data/fit Isobe et al., J.Phys.G34, S1015(2007). T. Sakaguchi for QM08, Jaipur, India

  36. 200GeV?~A big discovery!~ • Suppression of 0 and  reduced background, and highlighted direct photons • Ratio increases as centrality increases • Direct photon yield for pT>6GeV/c is well described by NLO pQCD calculation • NO direct photon suppression (initial state), and large 0 suppression (final state) measured / background Nuclear Modification factor Direct photons 0 [g/p0]measured / [g/p0]background= gmeasured/gbackground S.S.Adler, et. al. (PHENIX Collaboration), PRL 94, 232301(2005) T. Sakaguchi for QM08, Jaipur, India

  37. Other recent Theoretical interpretation T. Sakaguchi for QM08, Jaipur, India

  38. High Mass, High pT dilepton • High Mass, High pT dileptons are very interesting! • Contribution from virtual photons, Drell-Yan, jet-photon conversion, etc. • Can be used as an alternative measure of direct photons • Process: • Annihilation q and qbar • Conversion of real photons • Advantage • Combinatorial backgrounds can be absolutely normalized, and subtracted from measured invariant mass spectra Turbide et al., hep-ph/0601042 T. Sakaguchi for QM08, Jaipur, India

  39. WA98 results • A dedicated experiments for direct photon search in relativistic heavy Ion collisions. (PRL 85 (2000) 3595) • sNN=17.3GeV • Data can be either explained by kT-smearing or higher initial temperature • Any data did not see pT>4GeV, where pQCD photons dominate • No information on kT does not allow us to resolve the issue. • Recent data points at ~100MeV available from WA98. • By analysis of correlation strength in interferometry, • PRL93(2004)022301 WA98 data and theoretical interpretation PRC69(2004)014903 T. Sakaguchi for QM08, Jaipur, India

  40. T. Sakaguchi for QM08, Jaipur, India

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