Direct photons Basis for characterizing heavy ion collisions

Direct photons

Basis for characterizing heavy ion collisionsTakao SakaguchiBrookhaven National Laboratory

T. Sakaguchi for QM08, Jaipur, India

Photon measurement is astronomyWeve seen lights from universe ever since people started dreaming, hoping dreams come trueAnd, dreams came true in heavy ion collisions.First Heavy ion collisions in universe (SN1987)Jantar Mantar (Observatory)


OutlineCurrent landscape of photons

Much simpler, cleaner and precision measurement to much complicated, uncleaner and challenging measurementHigh pT to Low pTHigh Energy to Nuclear physics

Future exploration using direct photons


(fm/c)Direct photon historyDirect photons are emitted at all stages then surviving unscathed strongly ( a

Direct photon basicsProduction ProcessCompton and annihilation (LO, direct)Fragmentation (NLO)

Often carry thermodynamical information of the stateTemperature, Degrees of freedomGordon and Vogelsang, PRD48, 3136 (1993) Small Rate: Yield aas


High pT how well it is describedFinding of hard photons, whose yield is well explained by NLO pQCD, opened precision measurementsInitial state condition is understoodAlso holds in Au+Au collisionsAurenche et al., PRD73, 094007(2007)


Two Methods in p+p 200GeVFraction 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 photonsEven fragment/prompt components are decomposedPHENIX, PRL98, 012002 (2007)g(Isolated)/g(all direct)See: Nguyen, Session XV


Application of the probe -jet correlationIdea: (Direct pT) = (away side jet pT) Momentum loss of jets can directly be measured

PHENIX: subtract p0-h, h-h from (incl g)-hSTAR: subtract p0-h from (enriched g)-h so that near-side associated yield become zeroIt would be nice to check the absolute yield by a spectrum measurementMany 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..See: Nguyen, Hamed, Session XV


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 caseThis itself probes the matter on similar way as jets do. New way to look at photons?

Bremsstrahlung from hard scattered partons in mediumC. Gale, NPA774(2006)335Turbide et al., PRC72, 014906 (2005)R. Fries et al., PRC72, 041902 (2005)Turbide et al., arXiv:0712.0732Liu et al., arXiv:0712.3619, etc..


Direct photons in 200GeV Au+AuRemember the extended highlight plots from PHENIXConsistent with old published result up to ~ 12GeV/c

Direct photons suppressed at very high pT?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 accountLow pT region is underestimated because of lack of jet-photon conversion?


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+nIn low pT, quarks are from gluon split no difference between n and pAt high pT, contribution of constituent quarks manifests

Minimum bias Au+Au can be calculated by:(sAA/Ncoll)/spp vs pT(sAA/Ncoll)/spp vs xTSame suppression will be seen in lower pT at sNN=62.4GeVTS, INPC07, arXiv.org:0708.4265


The test: 62GeV Au+Au direct photonsLooks 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 dependentTherefore, e-loss models so far are still validAlso see: Miki, Session XV~18GeV/c@200GeV


Direct photons in 200GeV Cu+CuSee: Miki, Session XV


Low pT is interesting, and difficult

Thermal radiation from QGP (1

Low pT photons at SPSWA98 data can be either explained by kT-smearing or higher initial temperatureCould not see pT>4GeV, where pQCD photons dominateLack of information of kT prevents us from resolving the issue.WA98 data and theoretical interpretation, PRC69(2004)014903See: Baumann, Session XVp+Pb, p+C data from WA98 have come!


Low pT photons at RHICPHENIX applied internal conversion techniqueReal photons can convert to virtual photonsInv. mass shapes for Dalitz decay of mesons are calculable using Kroll-Wada formula

If M

Theoretical fit results are:

- 0.15 fm/c, 590 MeV (dEnterria, 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.)Low pT photons in Au+Au and p+pSee: Dahms, Session XVLow 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 Shorter 0 increases total yield and temperature Result also strongly depends on EOS.Lines are NLO pQCD


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 (v20), (based on hydro calculation)Chatterjee, et al., PRL 96, 202302(2006), Kopeliovich et al., arXiv:0712.2829, etc..Turbide et al., arXiv:0712.0732


Direct photon v2 in Au+Au collisionsHadron decay photon subtracted from inclusive photon v2.

Reached up to ~8GeV/c, no significance for pT

Where do we go?


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 helpRusscher (STAR), QM06STAR, d+Au, gmeas/gbkgdPHENIX, d+Au, RAAPeressounko (PHENIX), QM06


Direct photons in LHC era?Hard scattering cross-section even goes higher-Jet correlation analysis is the primary targetKey issue of the analysis is how clean tagged photon samples can beSee: Loizides (CMS), Session XXIVInput and Reconstructed fragmentation function from direct photon tagged events (0-10% Pb+Pb)See: Morsch (ALICE), Session XXIVDiscrimination power of photons to hadrons


My LHC favoriteA 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 powerThermal 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 partonsTurbide et al.,arXiv:0712.0732


More future interestDetermining Time evolution scenario by looking at forward direct photonsLandau 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

SummaryDirect photons become indispensable probe for characterizing matterPartons 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 mediumg-jet correlation, etc.

Jet interacting with medium produces photons, and would be a useful tool for parton E-loss mechanism studyThe 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


AndI 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


Backup


arXiv:0712.0732


Comparison with some modelsTurbide 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.


Cant we yet tell something more from it?NLO pQCD tells that there are direct photons and jet-fragment photons

Direct photons should not be suppressedFragment photons should be suppressed if jet is suppressed

There should be an additional source!


What is expected from structure function?Au+Au minimum biasStructure function ratios drop by ~20% from x=0.1 to 0.2?Eskola,Kolhinen,Ruuskanen Nucl. Phys. B535(1998)351gq ->q is main contribution


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


Direct photon RAA in 200GeV Au+AuUsed p+p data is the denominatorNLO pQCD as denominator is shown as well for a reference

For pT

Are we in discovery? Single photons in Heavy Ion collisions ~Before RHIC~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 temepratureAny data did not see pT>4GeV, where pQCD photons dominateNo 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 interferometeryWA98 data and theoretical interpretation PRC69(2004)014903HELIOS (Z. Phys. C46(1009)369), CERES (Z. Phys. C71(1996)571), null result..Hope to hear new result from Christoph Bermann.


Direct photons in p+p collisionsp+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/fitIsobe et al., J.Phys.G34, S1015(2007).


200GeV? ~A big discovery!~Suppression of 0 and reduced background, and highlighted direct photonsRatio 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)[g/p0]measured / [g/p0]background = gmeasured/gbackgroundS.S.Adler, et. al. (PHENIX Collaboration), PRL 94, 232301(2005)measured / backgroundNuclear Modification factor0Direct photons


Other recent Theoretical interpretation


High Mass, High pT dileptonHigh 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 qbarConversion of real photons

AdvantageCombinatorial backgrounds can be absolutely normalized, and subtracted from measured invariant mass spectraTurbide et al., hep-ph/0601042


WA98 resultsA 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 temperatureAny data did not see pT>4GeV, where pQCD photons dominateNo 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)022301WA98 data and theoretical interpretation PRC69(2004)014903


Direct photons Basis for characterizing heavy ion collisions

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