# Presentation: 572381

## Phenomenology of Prompt       Photon Production in                 Heavy Ion Collisions 

**Prof. Brian A. Cole.**
**Columbia University**

**Caveat Emptor:** I am in no way, shape, or form an expert on thermal photon production (why isn’t someone else giving this talk?)
**I have relied heavily on papers by Gelis, work by Gale**

## Prompt Photons: Perspective

- Historically:
-  Most attention focussed on prompt “thermal” photons
- Direct signature and probe of deconfined phase
- pQCD photon production considered a background
- Insensitive to final-state modifications
- Much of work devoted to hadronic vs QGP yields
- More modern view:
- Photon production affected by final-state medium over     large range of photon energies (e.g. up to 10 GeV @ RHIC)
- Thermal – “soft” and “hard”
- Modification of jet fragmentation photon yield
- Jet conversion photons
- Major advances in thermal field theoretic techniques
- Problem is no longer “academic” ****** data!**

## Prompt Photon Production: Time History

- Pre-equilibrium typically considered unimportant
- Because initial particle production mostly  gluons
- But, recent CGC calculations suggest significant initial quark production
-  Non-negligible pre-equilibrium  yield? (evaluate!)
- And naively:
- rapid equilibration  more photon production(?)
- Clean separation of time-scales? (Boyanovsky)

_**F. Gelis:**_
**Nucl. Phys. A715 329 (2003)** **hep-ph/0209072**

## Prompt  Contributions: Low Energy

- “Sweet spot” for QGP photons for **1 < E < 3 GeV**
- But, beware pQCD uncertainties
**And medium modifications of jet-associated ******** yield**

- From **Turbide, Rapp, Gale:**
**Int. J. Mod. Phys. A19, 5351 (2004)**** **
**hep-ph/0408119**

## Prompt  Contributions: High Energy

- Extremely rich mixture of physics contributing to the photon spectrum in ~ 4-10 GeV/c range.
- Medium contributions > 10% out to 15 GeV/c!
- Many contributions at same order of magnitude @ 4 GeV/c

**Turbide *****et al***
**Phys. Rev. C72, 014906 (2005)**
**hep-ph/0502248**

## Hard Photon Production in pQCD

- @ LO in pQCD, photon production is simple.
- Two contributions:
- “partonic” photons: direct from hard scattering
- “Fragmentation” photons – from fragmentation of jet(s)

- But, @ NLO things are much more complicated
- Distinction between partonic & fragmentation contributions becomes ambiguous.
- In principle, “isolation” cuts possible – but matching those cuts with pQCD is difficult (infrared sensitivity).

## Start by Measuring in p-p Collisions

- PHENIX Preliminary  Run-3 p-p prompt 
- Background removed via combination of:
- (Jet) isolation cuts
- 0 decay tag
- Statistical subtraction
- Spectrum and yield well-described by NLO pQCD (w/ threshold & soft gluon resummation)
- ~ 15% scale uncertainty above 5 GeV/c

## A different NLO pQCD Calculation

_**INCNLO (v1.4): J.Ph. Guillet, M. Werlen **__***et al***_

- NLO pQCD calculation
- With calculated  frag. function.
- Compares well with HERA, CDF, D0 data
- Fragmentation ~ 25%  at high pT
- Large increase in fragmentation yield below 5 GeV.
- Mostly from NLL contributions to  FF
**Right where we want       to measure thermal**

## PHENIX Comparison to INCNLO

**PHENIX Comparison to INCNLO**

- No K factors, no fudge factors, absolute comp.
- Completely independent calculation.
- Good control over pQCD prompt photon calculation @ RHIC.

- Points: PHENIX
- Curve: INCNLO

## Jet Conversion Photons

- There is a new source of “hard” photons in QGP
- High pT quarks/gluons **convert** into **photons**** in medium**

- This extra contribution must be present
- @ large enough *t*, incident jet sees unscreened partons
- What about at low-*t *?
- In principle, pole in the t channel produces “large”
- But medium screens @ low-t & regulates pole.
- Jet-conversion **** rate sensitive to screening mass.
- And potentially also to quasi-particle masses(?)

## From S. Turbide et al

- Inclusion of extra QGP contributions improves agreement with data ??
- Too soon for conclusion – but _real motivation!_

## Bremsstrahlung in Heavy Ion Collisions

- Bremsstrahlung contribution only!
- Potential increase in bremsstrahlung yield  in medium.
- More important:
- Energy & kT  spectrum will directly reflect medium properties.

- In my opinion: “Holy Grail” of energy-loss physics
- Can “see” the radiation itself.
-  Photon bremsstrahlung calculation is much less model dependent than the gluon radiation calculations.
**Will not be easy to measure but it’s worth trying ...**

- No quenching

- w/ quenching

_**Zakharov (**__hep-ph/0405101__**)**_

## Thermal Photons: Ideally

- Do thermal photons provide direct insight on medium (QGP) properties?
- From thermal field theory
- e.g. Gale and Kapusta,   Nucl. Phys. B357 65 (1991)
- Emission rate depends on
- Medium boson dist. function
- Photon self-energy
- Answer: **definitively, yes**
- Validity of formula:
- Leading order in ****
- All order in ******s**

- Thermal emission rate

- Thermal distribution function

- In-medium photon self-energy function (polarization tensor)

## Thermal Photons: Ideally (2)

- Why the photon self-energy?
- Unitarity!
- Decay rate  Im self-energy
- “Cut” of self-energy graphs gives thermal  rates

**e.g. ****P. Stankus,** **Ann. Rev.** **Nucl. Part. Sci. 55:517, 2005**

- Leading terms: **O(******** ********s****)**

- Sub-leading terms: **O(******** ********s****2****)**

## Thermal Photons: The Reality

- Calculation of “thermal”  production non-trivial
- Cancellation between real   and virtual emission.
- Divergence when  is   collinear with quark.
- Regulated by in-medium masses (e.g. Hard Thermal Loop resummation).
- Sub-leading corrections comparable to leading terms due to collinear enhancement.
- LPM interference important
- Long  formation time in low  Q2 processes  interference.

**Leading contribution O(****** ******s****)**

**“****Corrections” O(****** ******s****2****)**

- LPM: formation time vs MFP

## Some Results: Gelis et al

- Significant corrections to leading (22) terms
- Beware: unrealistically high initial temperatures!
- LPM suppresses yield at E ~ T

## Next Step: Embed in Hydro

- Calculate above rates/volume and evaluate over time history of heavy ion collision using (e.g.) hydrodynamic calculation.
- e.g. calculation by d’Enterria and Perressounko below
- Conclude: **complicated hydrodynamical evolution does not destroy the utility of prompt ********’s as “thermal” probe.**

## Is “Hydro” Approach Correct?

- Is it appropriate to use static thermal calculations of emission rates in evolving medium?
- How to correctly handle build-up of virtual photon cloud around partons that appear at t=0.
- These issues & others addressed by Boyanovsky *et al*
- Using **“real-time”** formalism vs **“S-Matrix”** formalism

- Emisson Rate

- t-t0 (fm)

- t-t0 (fm)

## Evaluating Boyanovsky Results

- Fraga, Gelis, Schiff: **Phys. Rev. D71, 085015 (2005)**
- Boyanovsky approach has a number of problems
- Boyanovsky formula for photon production rate is infinite due to unphysical vacuum terms, *ad hoc* subtraction procedure unjustified.
- Thermal photon spectrum too hard, would lead to infinite total energy in emitted photons.
- Use of finite initial, final times unphysical.
- But, **Serreau:** **JHEP 0405:078 (2004)**, **hep-ph/0310051**
- There may be corrections needed to the static thermal emission rates when system evolves rapidly relative to photon formation time.
- Use new formalism to handle out-of-equilibrium problem
- 2-particle irreducible action

## Summary

- Physics of prompt photon production is complicated
- At moderately high pT (< 10-15 GeV/c @ RHIC)
- Jet conversion photons
- Jet-associated photons (fragmentation, bremsstrahlung)
- May contaminate high-end of “thermal” spectra
- Physics of thermal photon production also complicated
- Huge advances in last 5 years in thermal field theory
**Static equilibrium rate calculations much more robust**
- But, pre-equilibrium contribution uncertain
- Maybe more quarks than originally expected
- Whatever causes rapid equilibration must produce ’s (?)
**What about strongly coupled plasma?**
**Scale hierarchy & HTL out the window ...**
- Hawking-Unruh production of prompt photons?

## PHENIX Au+Au Prompt Photon

- Ratios of total photon to decay photon yields
- Observe large prompt (non-decay) yield, pT >4

## Photon - Jet Measurements: Status

- PHENIX has clear, statistically significant pQCD prompt photon measurement in Au+Au (Run 2).
- ~ 10x statistics in Run 4.
- High-pT suppression reduces decay bkgd.
- In principle, possible to remove part of remaining background:
- Direct tagging of 0 decays
- “Jet” isolation cuts – require that photon not in jet
- But these cuts are problematic in central Au-Au
- Frequently satisfied by background.
- What about non-central?
- For now, I am pessimistic: background drops slowly
- Will need to do statistical bkgd subtraction
-  Stay tuned.

## Jet Quenching: Photon Bremstrahlung

**Jet Quenching: Photon Bremstrahlung**

- For light quarks (and gluons??), in-medium energy loss dominated by radiation.
- Interference between vacuum & induced radiation.
- For large parton pT (> ~10 GeV/c) **coherence** crucial.
- Unfortunately, we can’t measure the gluons.
- But we could measure photon bremstrahlung!
**Direct measurement of medium properties.**

## How to Measure Frag. & Brem. ? (2)

- But, we can measure prompt photons produced in associate with hadrons of given pT.
- e.g. for hadron pT > 3 GeV, with kT<1 GeV
- confusion from IS radiation, jet intermingling, inter-jet radiation strongly reduced

- Need statistical subtraction of      decay photons.
- 0  “tagging” will help significantly.
- In p-p can obtain ~     60% 0 rejection.
- Pythia study:
- E > 4 GeV
- pT hadron > 3 GeV/c.

****

- dN/d**** (arb. Norm.)

******-hadron pairs**

## Bremsstrahlung Measurements

- Real opportunity for qualitatively new insight on the physics of in-medium parton scattering.
- Let’s be clear – this measurements won’t be easy.
- In worst case, need to dig out bremsstrahlung out from under x10 larger decay signal.
- Jet quenching no longer helps when you require the photon to be in a jet !!
- But, if the bremsstrahlung is enhanced, angular distribution is broadened, then life is better.
- Observation by Axel: “trigger bias effect”
- Will be an issue.
- But potentially controllable by using opposite-side “jet”/high-pT hadron requirement.
- Guidance from complete in-medium interaction calculation like AMY would be a big help.

## How to Measure Jet Conversion  ?

- Brute force:
- Measure p-p accurately enough to provide a baseline with ~ 10% accuracy.
- Measure the Au-Au, Cu-Cu yield vs pT well enough to see >30% effects.

- Might work if the jet-conversion yield is as large as has been predicted.
- But, Cronin ???
- Bremsstrahlung? (measure it – but enough of total yield ??)

## One Hope: Reaction Plane Dependence

- Both Bremsstrahlung and jet-conversion photons could contribute to prompt photon v2.
- Observation of prompt photon v2  one or both mechanisms are present (high priority!)
- If we observe prompt photon v2, then we need to unravel the contributions.

- Jet conversion will produce more photons out-of-plane than in-plane
-  Negative v2 for these “jet quenching” photons

## Prompt Photon v2 @ Moderately High pT

- Turbide, Gale, Fries: **PRL 96, 032303 (2006)**
- Explicit analysis of photon v2 taking into account different contributions
- Top includes hadron decay photons including recombination source of .

## Prompt Photon v2 – How ?

- Do  vs 

- More specifically, measure inclusive photon, 0,  yield vs **** extract prompt yield vs ****

## Summary

- There is surprisingly rich physics in the hard photon sector.
- So much so, that understanding it & unraveling the contributions will take some time.
- Another example of the importance of penetrating probes
- Bremsstrahlung photons should provide cleaner insight on parton energy loss physics.
- Jet-conversion photons are the extreme case of radiation from parton interactions in medium.
- I personally have real hope for the medium generated bremsstrahlung measurement
- but it will take time.

## Summary (2)

- We are still in the infancy of pQCD physics program RHIC.
- Only a few relevant measurements in p-p
- And some (e.g. prompt photons) subject to non-trivial ambiguities.
- Many opportunities to study effects like those discussed by Werner.
- Huge final-state effects in Au-Au
- So big that understanding the observed effects may be difficult (e.g. di-jet)
- I have high hopes for Cu-Cu data
- See effects “turn on”
- Need more differential “jet” probes.

## Bremsstrahlung: kT distribution

- Pythia (p-p) kT distribution reflects
- Non-perturbative jet fragmentation scale (jT)
- “Hard” tail due to accumulated radiation effects
- Medium bremsstrahlung should have completely different kT scale (> 1 GeV !?)

- kT

- Pythia photon-hadron kT dist.

**E****** > 4 GeV
- pT hadron > 3 GeV/c