Direct photon production in heavyion collisions

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Direct photon production in heavy-ion collisions

• Ben-Wei Zhang
• T-16, Los Alamos National Laboratory

Collaborator Ivan Vitev
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• Motivations

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Hard Probes initial-state VS final-state

• QGP signatures help to tell whether a new kind of
  matter is produced in heavy-ion collisions.
• From SPS to RHIC, and to LHC, the colliding
  energy is larger and larger, hard probes will
  become more and more important jet quenching,
  J/psi suppression,
• Applications of hard probes asymptotic freedom,
  factorization.
• We need observables to constrain the
  initial-state nuclear effects in order to enjoy
  the power of hard probes.

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Photon Production

• Gods answer God Said, Let there be light. And
  there was light. God saw that the light was good,
  
• ------ From HOLY BIBLE
• In physicists eyes
• 1) Photon doesnt strongly interact with the
  produced medium (a ltlt as), so direct photon is a
  good tool to study cold nuclear matter effect
  (Cronin, shadowing)
• 2) Large enhancement due to photon production in
  the QGP medium-induced photon emission in the
  QGP, jet-photon conversion in the QGP??

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• A systematic study of
• direct photon Production
• in heavy ion collisions

• Different systems dCu, dAu, CuCu, AuAu.
• Different center of mass energies 62.4GeV,
  200GeV.
• Different hot nuclear medium effects jet
  quenching, photon emission, jet-photon
  conversion.
• Different cold nuclear effects Cronin effect,
  shadowing effect, cold nuclear energy loss,
  isospin effect .

Ivan Vitev and BWZ, arXiv0804.3805
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Direct photon in pp collisions
Direct photon annihilation,
Compton, bremmstrahlung

LO
Bremmstrahlung
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Data VS pQCD Theory (pp)
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Direct photon in AA Jet quenching

• Parton energy loss(FS) in the QGP will
  effectively modify the parton fragmentation func.
  (PFF)

Gyulassy-Levai-Vitev(GLV) formalism Gyulassy,
Levai, Vitev, NPB 594(2001)371
Probability distribution
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Medium-induced photon emission

• An energetic parton propagating in hot medium may
  radiate photons as well as gluons another source
  of photon production

Induced gluons
Induced photons
Zakharov, JETP Lett. 80(2004)1.

• It has been argued that medium-induced photon
  emission may give large enhancement to photon
  production.

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Gluon versus Photon

• Without three-gluon vertex, is photon emission a
  simple exercise ??

• Theoretical approaches developed to describe
  gluon emission cannot be directly generalized to
  photon radiation.

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Photon emission

• Photon bremsstrahlung contributions vanish
  beyond second order in opacity.

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photon emission analytic results

• Two limits interference is important.
• Leading contribution is L-dependence, with
• non-linear corrections with L.
• Number of interactions ltngt

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Photon emission numerical results
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Jet-photon conversion in QGP

• High-energy photon could be produced by
  conversion of a jet passing through the QGP due
  to jet-thermal interaction.

R. Fries et al., PRL90,132301(2003)
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Medium modified FF

• Effective fragmentation functions for obtaining
  photons from partons are

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Cold nuclear effects (I)

• Initial-state energy loss partons may also lose
  energy in cold nuclei before hard scattering.

due to energy fluctuations
I. Vitev, PRC 75(2007)064906

• Shadowing effect is calculated from the coherent
  final-state parton interactions.

Qiu, Vitev, PRL 93(2004)262301 Qiu, Vitev, PLB
632(2006)507.
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Cold nuclear effects (II)

• EMC effect use the parametrization by EKS.
• Isospin effect Direct photon cross-sections for
  pp, pn and nn are different ( p uud, n udd
  ) different electric charges of u and d quark
  (? ? ?eq2).
• Cronin effect

Eskola, Kolhinen, Salgado, EPJC 9(1999)61.
I. Vitev, PLB 562(2003)36.
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Numerical results
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Direct photon in dA collisions

• When pT lt 6 GeV, Cronin effect is dominant.
• When pT gt 6 GeV, isospin effect is very
  important.
• Initial-state energy loss contributes
  substantially.
• When pT15 GeV, nuclear effects suppress direct
  photon produ. by 20-40.
• Nuclear effects are more pronounced at 62.4GeV.
• Big error bars in data dont give tight
  constraints on different nuclear effects.

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Direct photon in AA collisions (I)

• Direct photon prod. is dominated by cold nuclear
  effects and amplified by two large nuclei.
• At small pT, RAAgt RdA and RAuAugt RCuCu while at
  high pT, RAAlt RdA and RAuAult RCuCu. Nuclear
  effects in larger nuclear systems are larger.
• Large Cronin enhancement is excluded.

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Direct photon in AA collisions (II)

• Incoherent photon emission is ruled out.
• Jet conversion contributes at pT lt 5 GeV, 25.
• Medium-induced photon is limited to 10.
• At high pT region, total enhancement contribution
  is found to be 5.
• Reduction of fragment. photons contributes at
  large pT .
• No large enhancement of direct photon production
  due to medium-induced photon emission and
  jet-photon conversion.

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Summary

• We derived the medium-induced photon production
  in GLV formalism coherent interference will
  strongly suppress medium-induced photon
  bremsstrahlung.
• We study direct photon production systematically
  in different nuclear sizes with different
  colliding energies by including many different
  nuclear effects consistently
• 1) Contributions of photons created via
  final-state interactions is limited to 35 for
  2GeVlt pTlt 5GeV, and about 5 at high pT..
• 2) Cold-nuclear effects dominate in the whole
  range. Cronin effect is dominant pT lt 6 GeV, and
  isospin effect is important when pT gt 6 GeV as
  well as initial-energy loss.

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• In the abode of light are the
• origins of truth, and from the
• source of darkness are the
• origins of error.

From the Dead Sea Scrolls
Thank you!
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Backup Slides
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We dont consider

• We focus on direct photon production with
• large pT, and neglect thermal photon
  production, which gives contribution only to
  photon production at low pT.

• Thermal photon production in the QGP

• Thermal photon production in hadronic gas

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QCD QGP
It would be interesting to explore new phenomena
by distributing high energy or high nuclear
density over a relatively large volume. T. D. Lee
Lattice QCD predicts phase of thermal QCD matter
with sharp rise in number of degrees of freedom
near Tc170MeV.
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Data VS pQCD Theory (pp)
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All orders in opacity

• Photon bremsstrahlung contributions vanish
  beyond second order in opacity.

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Initial-state energy loss

• Partons may also lose energy by interacting with
  other partons in cold nuclei before hard
  scattering.

I. Vitev, PRC 75(2007)064906
due to energy fluctuations
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Shadowing effect

• Shadowing effect is calculated from the coherent
  final-state parton interactions.

Qiu, Vitev, PRL 93(2004)262301 Qiu, Vitev, PLB
632(2006)507.