Direct photons Basis for characterizing heavy ion collisions

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Direct photonsBasis for characterizing heavy
ion collisions

• Takao Sakaguchi
• Brookhaven National Laboratory

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Photon measurement is astronomy

• Weve 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)

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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

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Direct photon history

• Direct photons are emitted at all stages then
  surviving unscathed strongly ( a ltlt as, almost
  transparent to medium)

Good Write-up to read nucl-ex/0611009
(fm/c)
Cartoon from G. David, Hard Probe 2006
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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)
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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 AuAu collisions

Aurenche et al., PRD73, 094007(2007)
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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)
See Nguyen, Session XV
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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., arXiv0706.3442, etc..
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Sources of Radiation in AA (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.,
arXiv0712.0732 Liu et al., arXiv0712.3619, etc..
C. Gale, NPA774(2006)335
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Direct photons in 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.
• Jet-photon conversion is not taken into account
• Low pT region is underestimated because of lack
  of jet-photon conversion?

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Is it (only) an isospin effect?

• 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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The 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
Also see Miki, Session XV
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Direct photons in 200GeV CuCu
See Miki, Session XV
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Low pT is interesting, and difficult

• 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
Dilepton to Photon ratio gives T Nayak et al.,
arXiv0705.1591
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Low pT photons at SPS

• WA98 data can be either explained by kT-smearing
  or higher initial temperature
• Could not see pTgt4GeV, where pQCD photons
  dominate
• Lack of information of kT prevents us from
  resolving the issue.

WA98 data and theoretical interpretation,
PRC69(2004)014903
pPb, pC data from WA98 have come!
See Baumann, Session XV
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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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Low pT photons in AuAu and pp

• Low pT photons in AuAu and pp are measured
  using internal conversion method
• pp agrees with NLO pQCD well, and AuAu has
  excess over the calculation

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.)
Lines are NLO pQCD
Shorter ?0 increases total yield and temperature
Result also strongly depends on EOS.
See Dahms, Session XV
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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 See Miki for PHENIX result, Session
XV, Raniwala for STAR result, Session XII
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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

PHENIX, dAu, RAA
STAR, dAu, gmeas/gbkgd
Russcher (STAR), QM06
Peressounko (PHENIX), QM06
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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
See Morsch (ALICE), Session XXIV
See Loizides (CMS), Session XXIV
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My LHC favorite

• A calculation tells that even in low pT
  region(pT2GeV/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., 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
• 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 AuAu.
• low pT photon emission suggests a thermalized
  state.
• Photon production at high rapidity would help
  determining the system expansion scenario

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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
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Backup
28

• arXiv0712.0732

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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.

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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 suppressed
• Fragment photons should be suppressed if jet is
  suppressed
• There should be an additional source!

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What is expected from structure function?
gq -gt?q is main contribution
AuAu minimum bias
Eskola,Kolhinen,Ruuskanen Nucl. Phys.
B535(1998)351
Structure function ratios drop by 20 from x0.1
to 0.2?
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New! 62GeV AuAu Direct photons

• Direct photon over NLO pQCD is consistent with
  unity at 62.4GeV
• pp reference can not be well-defined. Needs a
  measurement at RHIC

T. Sakaguchi, INPC07
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Direct photon RAA in 200GeV AuAu

• Used pp data is the denominator
• NLO pQCD as denominator is shown as well for a
  reference
• For pTlt10GeV, RAA is consistent with Ncoll scaled
  pp reference.
• RAA seems to decrease at very high-pT (especially
  for central)
• Difference of NLO pQCD calculation and pp data
  affects quite a bit.

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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 pTgt4GeV, 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(20
04)014903
Hope to hear new result from Christoph Bermann.
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Direct photons in pp collisions

• pp cross-section at ?s200GeV is well
  established with PHENIX Year-5 data set.
• Statistically improved from the published Run3
  result
• Reference for AuAu collisions
• Data parameterized by a fitting function to
  interpolate to the pT of AuAu points
• Measured pp yield is higher than NLO pQCD
  calculation by more than 20.

Data/fit
Isobe et al., J.Phys.G34, S1015(2007).
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200GeV? A big discovery!

• Suppression of ?0 and ? reduced background, and
  highlighted direct photons
• Ratio increases as centrality increases
• Direct photon yield for pTgt6GeV/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/p0measured / g/p0background
gmeasured/gbackground
S.S.Adler, et. al. (PHENIX Collaboration), PRL
94, 232301(2005)
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Other recent Theoretical interpretation
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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
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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.
• Recent data points at 100MeV available from
  WA98.
• By analysis of correlation strength in
  interferometry,
• PRL93(2004)022301

WA98 data and theoretical interpretation PRC69(20
04)014903
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