Direct photon measurement in heavy ion collisions

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Direct photon measurement in heavy ion collisions

• Takao Sakaguchi
• Brookhaven National Laboratory

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Outline

• Direct photons and their origins
• History of direct photon measurements in high
  energy nucleon and heavy ion collisions
• Latest photon data from RHIC
• Current landscape of photon production
• Future exploration of heavy ion collisions using
  direct photons

I apologize in advance that Im heavily biased by
PHENIX
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Direct shine
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Direct photon basics

• Emitted at all stages then surviving unscathed
  strongly (a ltlt as, almost transparent to medium)
• Production Process
• Compton and annihilation (LO, direct)
• Fragmentation (NLO)
• Often carry thermodynamical information of the
  state
• Temperature, Degrees of freedom

Small Rate Yield ? aas
Gordon and Vogelsang, PRD48, 3136 (1993)
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Direct photon production in pp collisions

• Started back in 1979-1982 at SPS. R806,
  R110(?sNN63GeV), WA70 (?sNN24GeV). Later, UA2,
  UA6 reached at ?sNN630GeV

R110, ?sNN63GeV, pp collisions.
dir. g/p0 ratio
Invariant cross-section
CMOR, NPB 327(1989)541
A compilation Vogelsang and Whalley, J. Phys.
G23 (1997)A1
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Continued at CDF, and..

• ppbar ?sNN1.8TeV. Prompt (isolated) photons had
  been measured
• Data from many experiments are compiled and
  compared with pQCD

Aurenche et al., PRD73, 094007(2007)
D0, PRL77, 5011 (1996)
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State-of-art at 200GeV

• Extended up to 25GeV/c
• Systematic error 20
• Baseline for RHIC heavy ion results

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Even fragment/prompt components are decomposed

• Two Methods in pp 200GeV
• Fraction of isolated/all photons, which is
  calculable from NLO pQCD. Isolation cut ( 0.1E?
  gt Econe(R0.5) )
• Looking at angular correlation between leading
  hadrons and photons

g(Isolated)/g(all direct)
PHENIX, PRL98, 012002 (2007)
PHENIX, J. Phys. G35 (2008)104121
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Direct photon production in heavy ion collisions

• Purpose is to look for thermal radiation from
  possibly produced hot and dense matter in
  collisions
• Measurements in heavy ion collisions
• HELIOS (Z. Phys. C46(1009)369), CERES (Z. Phys.
  C71(1996)571), null result..
• WA80, WA98 at CERN
• PHENIX and STAR at RHIC
• Measurements complementing heavy ion data
• Many pp reference data
• Some pA data (E706 produced nice results)
• PHENIX at RHIC

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Landscape before RHIC begins
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Only processes admitted until 2001

• pQCD
• Thermal radiation from QGP (1ltpTlt3GeV)
• S/B is 10
• Spectrum is exponential. One can extract
  temperature, dof, etc..
• Hadron-gas interaction (pTlt1GeV/c) ??(?) ?
  ??(?), ?K ? K?

Also, many seminal works by Sinha, Alam, Nayak,
Srivastava, Fries, Rapp et al.
A compilation on photons, PRC 69(2004)014903
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WA80 results

• SAu at 200GeV/u
• ?sNN20GeV
• g/p0 observed is compared to the one estimated
  from hadronic background calculation
• Onset for pTgt2.5GeV/c?
• Systematic Error 5!
• Most interesting part is hidden by statistics

WA80, PRL76, 3506(1996)
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WA98 results

• A dedicated experiments for direct photon search
  in relativistic heavy Ion collisions. (PRL 85
  (2000) 3595)
• ?sNN17.3GeV
• Data can be either explained by kT-smearing or
  higher initial temperature
• Any data did not see pTgt4GeV, where pQCD photons
  dominate
• No information on kT does not allow us to resolve
  the issue.

WA98, J. Phys. G35 (2008)104123
WA98 data and theoretical interpretation PRC69(20
04)014903
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Then, RHIC changed the landscape..
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Large suppression of high pT hadrons

• A big discovery from RHIC Year-1
• Energy loss of partons in medium
• RAA now extended to 20GeV/c
• Integrated RAA proved that the energy loss is
  consistent with fractional (increases with pT)

PHENIX, PRL, in press
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Another big discovery

• Suppression of ?0 and ? reduced background, and
  highlighted direct photons
• Ratio increases as centrality increases
• Hard photons, whose yield is well explained by
  NLO pQCD
• Initial state condition is understood.
• NO direct photon suppression (initial state), and
  large ?0 suppression (final state)

?measured / ?background
Nuclear Modification factor
Direct photons
?0
g/p0measured / g/p0background
gmeasured/gbackground
S.S.Adler, et. al. (PHENIX Collaboration), PRL
94, 232301(2005)
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Highlights from high pT hadron results

• Run1, Run2, Run4 AuAu high pT p0
• Run2 AuAu 200GeV high pT direct photons
• Run3 pp 200GeV high pT direct photons
• Run5 pp high pT p0 200GeV
• Run5 CuCu 22, 62 and 200GeV high pT p0

PRC75, 024909(2008) preliminary QM05
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E-loss of partons did job on photons?
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Yes, new production mechanism has been introduced

• 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.
• Bremsstrahlung from hard scattered partons in
  medium

Both are thermally
Turbide et al., PRC72, 014906 (2005) R. Fries et
al., PRC72, 041902 (2005) Turbide et al.,
arXiv0712.0732 Liu et al., arXiv0712.3619, etc..
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New contents..

• Energy loss of high pT hadrons will reduce
  background
• S/B increases. Effectively Signal is enhanced
• Photon produced in fragmentation of quarks and
  gluons will also be reduced
• Signal reduced
• Photons produced in the interaction of
  quarks/gluons with medium
• Same process as initial hard scattering, but the
  target is medium
• Signal increased
• Summary sheet tells everything is balanced?

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Discovery of new source? 200GeV AuAu

• Remember the extended highlight plots from
  PHENIX
• Consistent 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.

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Something we have missed before..

• Taking for example, the isospin effect Direct
  photon cross-sections for
• pp, pn and nn are different because of
  different charge contents (? ? ?eq2)
• Effect can be estimated from NLO pQCD calclation
  of pp, pn and nn
• In low pT, quarks are from gluon split ? no
  difference between n and p
• At high pT, contribution of constituent quarks
  manifests
• Minimum bias AuAu can be calculated by

(sAA/Ncoll)/spp vs pT
(sAA/Ncoll)/spp vs xT
Same suppression will be seen in lower pT at
?sNN62.4GeV
TS, INPC07, arXiv.org0708.4265
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A test 62GeV AuAu direct photons

• Looks like there is an isospin effect (and/or PDF
  effect)
• Question pp 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_at_200GeV
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Landscape expanded richly after RHIC
(fm/c)
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Theory also become rich..

• I personally have not seen such a drastic
  progress of theoretical works like now.

Vitev and Zhang, PLB669(2008)337
Liu et al., arXiv0811.0666
C. Gale, NPA774(2006)335
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Direct photon v2 a photon source detector

• Depending the process of photon production,
  angular distributions
• of direct photons vary
• Jet fragmentation (v2gt0), Jet-photon conversion,
  in-medium bremsstrahlung (v2lt0), Turbide, et al.,
  PRL96, 032303(2006), etc..
• Thermal photon v2 Quark v2 (gt0), (based on
  hydro calculation)
• Chatterjee, et al., PRL 96, 202302(2006),
  Kopeliovich et al., arXiv0712.2829, etc..

Turbide et al., arXiv0712.0732
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Direct photon v2 in AuAu collisions

• Hadron decay photon subtracted from inclusive
  photon v2.
• Reached up to 8GeV/c, no significance for
  pTlt3GeV/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
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Be frontier to a new degree of freedom
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g measurement from RHIC

• pT spectra as a function of invariant mass
• Known hadron decay components (cocktail) in lines
• Caveat cocktail calculation here does not
  include contribution from internal conversion of
  direct photons

2 single exponential fits Low-pT 0ltmTlt1
GeV High-pT 1ltmTlt2 GeV
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Landscape after merging with Dileptons
hard scatt isospin effect
Cartoon only sources of g, mean pT vs time
(dashed hadrons)
pT
(GeV/c)
jet Brems.
jet-thermal
jet fragmentation
sQGP
hadron gas
hadron decays
qg -gt qg meets qqbar -gtg
log t
Mass (GeV/c2)
1
10
107
1
(fm/c)
Conventional dilepton contribution becomes
smaller in low mass and high pT
2
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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 MltltpT, 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

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Thermal Photons?

• The result came out.

Fit results Cent. dN/dy(pTgt1GeV/c) T(MeV)
?2/DOF 0-20 1.10?0.20?0.30 221?23?18
3.6/4 20-40 0.52?0.08?0.14 215?20?15
5.2/3 MB 0.33?0.04 ?0.09 224?16?19
0.9/4
PHENIX, arXiv0804.4168
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Where do we go?
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Something we may have missed..

• Initial multiple scattering effect should exist?
• ? kT broadening, p0 kT broadening
  recombination.
• Detail study of pA or dA should help
• RHIC Run-8 dAu collision data are coming

PHENIX, dAu, RAA
STAR, dAu, gmeas/gbkgd
Russcher (STAR), QM06
Peressounko (PHENIX), QM06
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g-Jet correlation

• Subtract p0-h, h-h from (incl g)-h
• If the result is true, the jet-emission is not
  always tangential.
• ? is not necessarily the direct. Fragment,
  bremsstrahlung, Jet-conversion photons could
  significantly contribute
• similar to h-h correlation? Isolation of direct
  photons would be needed
• Ridge structure, same RAA and IAA

• Red points integrated RAA from single p0
  measurement (pTgt5GeV/c)
• Blue points Same (integrated away side yield
  IAA) from g-h measurement (Assoc. pTgt5GeV/c)

Df
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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 PbPb)
Discrimination power of photons to hadrons
Morsch (ALICE), QM08
Loizides (CMS), QM08
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What do we expect at LHC?

• A calculation tells that even in low pT
  region(pT2GeV/c), jet-photon conversion
  significantly contributes to total
• What do we expect naively?
• Jet-Photon conversions ? Ncoll ? Npart ? (s1/2)8
  ? f(xT), 8 is xT-scaling power
• Thermal Photons ? Npart ? (equilibrium duration)
  ? (s1/2)1/4 2
• LHC would see 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., arXiv0712.0732
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More future interest

• Determining Time evolution scenario by looking at
  forward direct photons
• Landau expansion will make one order reduction
  at pT4GeV/c at y2 compared to y0
• Nose-cone calorimeter upgrade in PHENIX (Covering
  1ltylt3)
• Forward Photon Detector in STAR (Covering
  -3.7ltylt-2.3)

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Summary

• Direct photons become indispensable probe for
  characterizing matter
• Starting from 1979, they have been a probe of the
  initial condition the h-h and heavy-heavy
  collisions.
• RHIC has developed many new sources of direct
  photons
• 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 AuAu.
• low pT photon emission suggests a thermalized
  state.
• Photon production at high rapidity would help
  determining the system expansion scenario
• And.

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This is the place to live!
Thank you!
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