Direct photon production in heavy-ion collisions

Ben-Wei ZhangT-16, Los Alamos National

Laboratory Collaborator: Ivan Vitev

Motivations

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.

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??

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 .

Direct photon in pp collisions

q

qq

q

g

g

g

γγ

Direct photon: annihilation, Compton, bremmstrahlung

Bremmstrahlung

Compton

Annihilation

LOLO

Data VS pQCD Theory (p+p)

Direct photon in AA: Jet quenching

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

Probability distribution

q

qq

q

g

g

g

γγ

hadrons

ph

parton

E

Gyulassy-Levai-Vitev(GLV) formalismGyulassy, Levai, Vitev, NPB 594(2001)371

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.

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 ??

Photon emission

Photon bremsstrahlung contributions vanish beyond second order in opacity.

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

Photon emission: numerical results

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)

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.

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.

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.

Numerical results

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.

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.

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.

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.

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!

Backup Slides

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:

……

……

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.

Data VS pQCD Theory (p+p)

All orders in opacity

Photon bremsstrahlung contributions vanish beyond second order in opacity.

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

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.