direct photon production in heavy-ion collisions ben-wei zhang t-16, los alamos national laboratory...
DESCRIPTION
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.TRANSCRIPT
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Direct photon production in heavy-ion collisions
Ben-Wei ZhangT-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 God’s 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 doesn’t strongly interact with
the produced medium (s), 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
Ivan Vitev and BWZ, arXiv:0804.3805
Different systems: d+Cu, d+Au, Cu+Cu, Au+Au. 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 .
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Direct photon in pp collisions
q
q
g
g
g
γγ
Direct photon: annihilation, Compton, bremmstrahlung
Bremmstrahlung
Compton
Annihilation
LOLO
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Data VS pQCD Theory (p+p)
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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)
Probability distribution
q
q
g
g
g
γγ
hadrons
ph
parton
E
Gyulassy-Levai-Vitev(GLV) formalismGyulassy, Levai, Vitev, NPB 594(2001)371
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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
Theoretical approaches developed to describe gluon emission cannot be directly generalized to photon radiation.
Gluon radiative amplitude for single scattering of a fast on-shell quark:
Without three-gluon vertex, is photon emission a simple exercise ??
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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, withnon-linear corrections with L. Number of interactions <n> = 32/ qL
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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:
Jet quenching Photon emission Jet conversion
f(t) gives the time dependence of radiative energy loss.
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Cold nuclear effects (I) Initial-state energy loss: partons may also lose
energy in cold nuclei before hard scattering.
I. Vitev, PRC 75(2007)064906due to energy fluctuations
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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EMC effect: use the parametrization by EKS.
Isospin effect: Direct photon cross-sections for p+p, p+n and n+n are different ( p= uud, n= udd ): different electric charges of u and d quark (eq
2). Cronin effect:
Cold nuclear effects (II)
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 d+A collisions
When pT < 6 GeV, Cronin effect is dominant.
When pT > 6 GeV, isospin effect is very important.
Initial-state energy loss contributes substantially.
When pT~15 GeV, nuclear effects suppress direct photon produ. by 20-40%.
Nuclear effects are more pronounced at 62.4GeV.
Big error bars in data don’t give tight constraints on different nuclear effects.
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Direct photon in A+A collisions (I)
Direct photon prod. is dominated by cold nuclear effects and amplified by two large nuclei.
At small pT, RAA> RdA and RAuAu> RCuCu while at high pT, RAA< RdA and RAuAu< RCuCu. Nuclear effects in larger nuclear systems are larger.
Large Cronin enhancement is excluded.
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Direct photon in A+A collisions (II)
Incoherent photon emission is ruled out.
Jet conversion contributes at pT < 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 2GeV< pT< 5GeV, and about ~5% at high pT..
2) Cold-nuclear effects dominate in the whole range. Cronin effect is dominant pT < 6 GeV, and isospin effect is important when pT > 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 don’t consider…
Thermal photon production in the QGP:q + g +q q +q + g
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 hadronic gas:
……
……
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QCD QGPIt 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 Tc=170MeV.
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Data VS pQCD Theory (p+p)
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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)064906due 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.