# Presentation: 631851

## Direct photon production in heavy-ion collisions

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

-      Collaborator: Ivan Vitev

##      
        Motivations

-         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

-     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

- Direct photon:  annihilation,
-                       Compton,
-                      bremmstrahlung

**Bremmstrahlung**

- LO

## 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

- Gyulassy-Levai-Vitev(GLV) formalism
- Gyulassy, 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.

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

**Notes:**
For medium-induced gluon radiation in a single scattering, we have three Feynman diagrams. For photon radiation,
One may expect that without three-gluon vertex, photon emission will be just a simple degeneration of gluon emission.
Is this expectation true? Let’s us see the calculations.

##         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, with
- non-linear corrections with L.
- Number of interactions <n> =

## 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:

##   Cold nuclear effects (I)

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

- I. Vitev, PRC 75(2007)064906

- due 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 (eq2).
Cronin effect:

- 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 (eq2).
- 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.

-    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:

- 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                  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 T****c****=170MeV.**

## Data  VS pQCD Theory (p+p)

**Notes:**
Here we show the comparisons of experimental data with pQCD theory on direct photon production in pp collision. In this figure the solid points are the data from phenix at the center of mass energy 200GeV, and three curves are the theoretical results of NLO pQCD calculation with different choices of hard scale. We can see NLO pQCD calculaition gives a good description of the measurements. In this small figure the derivation between theory and data are elaborated. It is shown that the derivations are small. On the left panel we illustrate the comparison of our LO calculaitions with experimental data.  K  factor.   The insert shows the fraction of fragmentation photons to all direct photons in pp collision. At low pt ~40%, at high pt~25%.   Agree with the previous theoretical discussion that the contribution of Frag. Photon is the same order to those from Anni. and Compton. Even though it is smaller.   

## All orders in opacity

-     Photon bremsstrahlung contributions vanish beyond second order in opacity.

**Notes:**
To derive the results at order n in opacity expansion, we will construct the relationship between amplitudes at order (n-1) of opacity expansion and order n of opacity expansion. Here we use A i1,i2..in-1 to denote the amplitude at order n-1 of opacity expansion. One we construct the probability Pn as this. The reaction operator Rn descibes the effect of one additional correlated in-medium scattering.  In the amplitude lever, one additional scattering means we have one addition gluon leg attached, and in probability lever, or cross section lever, it means we have two additional gluons legs. Therefore we can draw Rn as this. There are three different cuts: central, left and right.  

##    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

**Notes:**
The reason that final-state energy loss has been found to play a dominant role in AA collisions in the large density of the QGP co-moving with the jet. Moreover, the effect of final-state energy loss is amplified by the steeply falling spectra of outgoing partons with an inverse power of pT.

##   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.