# Presentation: 122278

## Direct Photon Production at RHIC

**Stefan Bathe****                                                            ****UC  Riverside**

**University of Frankfurt, May 31, 2007**

**Notes:**
Changed title 

## Introduction

- Photons don’t interact strongly
- Two great promises:
- Initial temperature, transition temperature via thermal photons
- Jet energy scale, precise energy loss via ****-jet correlations
- Problems
- richer probe than originally thought (medium-induced photons)
- Old idea of unfolding photon sources (starting from the clearly understood  high pT spectra and moving down in pT) seriously challenged

## Photon Sources in N+N

**Photon Sources in N+N**

- Experimenter’s definition of direct photons
- not from hadronic decays
- Elementary processes
- compton:
- annihilation:
- fragmentation = bremsstrahlung

## Photon Sources in A+A

**Photon Sources in A+A**

**Photons in A+A**

**Direct Photons**

**Decay Photons**

## Disentangling Sources

*log t*

*p****T***

- 1

- 10

- 10**7**

*(GeV)*

*(fm/c)*

- hadron
- decays

- hadron
- gas

- sQGP

- jet-thermal

- hard
- scatt.

- jet
- brems.

- <p**T**> vs. creation time
- in principle:   working one’s way down
-    from the highest p**T** (and excluding hadron decays)
-    one might be able to disentangle different sources

**Disentangling Sources**

## Possible Scenario

- some chance of disentangling sources
- (using v2 and isolation, more later)

**Possible Scenario**

## PHENIX Experiment

**PHENIX Experiment**

- EMCal
- 0 via 0 
- |**| < 0.35
-  = 0.01 x 0.01
- Lead scintillator (PbSc)
- Lead glass (PbGl)
- DC, PC
- Charged particles
- RICH
- e**±** ID
- Muon arms (not shown, forward rapidity)

## 1. High pT inclusive

## Photon Run-2 Result—The T-Shirt Plot

******0**’s, ****’s suppressed

- direct photons not suppressed

## New p+p Run-5

## Parameterization of p+p Data

- For RAA parameterization of p+p reference used
- While consistent, data tends to be higher than pQCD by at least 20%, increasing to low and high pT

## New Au+Au, Run-4

**New Au+Au, Run-4**

**Phys. Rev. Lett. 94, (2005) 232301**

## Direct Photon RAA

**First direct photon R****AA**** using p+p data as reference**

## Updated T-Shirt Plot

**Updated T-Shirt Plot**

**Direct ******** R****AA**** with measured **
**p+p reference data**

**η**** ****    ****π****0**

- Direct photons and 0 touch at highest pT
- Still consistent with
- final state effect for 0 and initial state effect for direct photons
- structure functions different for the two

## Comparison to Theory

- Turbide et al.
- Jet photons + pQCD + thermal
- AMY formalism for jet-quenching of fragmentation photons
- Data systematically below theory
- Phys. Rev. C72 (2005) 014906 + private communication

- F. Arleo
- pQCD photons only
- High-pT suppression due to isospin effect, shadowing, and energy loss
- BDMPS for jet-quenching
- JHEP 0609 (2006) 015

## Further Comparison

- Bremsstrahlung photons through bulk matter
- B.G. Zakharov, JETP Lett. 80 (2004) 1

## Experimental Handles

**Experimental Handles**

- suppression seen in pT region where sensitive to detector bias (cluster merging)
- Use different detectors
- So far only PbSc analyzed
- PbGl better spatial resolution
- Less sensitive to merging
- xT scaling
- If suppression due to modified structure function, it should depend on xT, not pT
- At lower √s, lower pT for same xT
- shower merging smaller at lower pT
- But other sources come into play here

## 2. Elliptic Flow

## Direct Photon Elliptic Flow

- Jet-photons and induced bremsstrahlung photons : negative v2 for direct photons
- Jet-quenching  less fragmentation photons: positive v2 for direct photons

## Theoretical Predictions

- R. Chatterjee et al., PRL 96, 202302 (2006)

- Thermal photons produced throughout expansion history and reflect quark anisotropy: net effect positive v2

- Jet photons, fragmentation photons:
- Net effect negative v2

**Theoretical Predictions**

## Measure inclusive and hadron v2 by reaction plane method
Calculate direct photon v2, using double ratio R, as:

- Calculate direct photon v2, using double ratio R, as:

- R * v2inc – v2BG

- R = N inc / NBG

- v2dir =

- R – 1

**Measurement**

## Result

- v2 consistent with zero

- Systematic errors mostly from R:                       large background of decay photons at low pT

- 7

**Result**

## Disentangling++

-   Contribution		softer	v2	isol.
- 1 Compton, annihi.		0	yes
- 2 jet-thermal			<0	yes
- 3 Fragmentation		>0	no
- 4 induced. brems.		<0	no

- NPA 774, 335 (2006)

- NPA 785, 93c (2007)

## 3. -jet

## Photon-tagged Jets

- 

- Hadrons

- Observing jets and dijets through leading hadrons biases toward high fragmentation *z*, and also toward sources at the periphery.
- Tagging jets opposite isolated direct photon measures jet *p*T, and does not bias fragmentation or location of jet production.
- “Clean” measurement of medium effects on hadronization.

## jet E-scale in p+p at CERN-ISR

- Angelis et al.
- Nucl. Phys. B327 (1989) 541

- isolated photon in back-to-back ****-jet
- pair good measure of total jet energy
- (modulo initial k**T**)

**R110, 62 GeV**

**jet E-scale in p+p at CERN-ISR**

- disappearance of ****-side partners
- with p**T**>1.0 GeV→
- bremsstrahlung small

## Correlation Function

**Correlation Function**

****

- Trigger

- Assoc

- Mixed events – correct for
- PHENIX pair acceptance

## -Jet Correlations

- p+p collisions at 200 GeV

## Comparison to Pythia

## -Jet Correlations in AuAu

******-Jet Correlations in AuAu**

## PHENIX Cu+Cu – jet E-scale

**systematic from R******
**systematic from subtraction method**

**PHENIX Cu+Cu – jet E-scale**

## 3. Low pT

## A new technique:  opening up the phase space

*M***inv**

*p***T**

- 0

## The Idea

**phase space factor**

**form factor**

**invariant mass of virtual photon**

**invariant mass of Dalitz pair**

**phase space factor**

**form factor**

**invariant mass of Dalitz pair**

**invariant mass of virtual photon**

**The Idea**

- Start from Dalitz decay
- Calculate invariant mass distribution of Dalitz pairs

- Now direct photons
- Any  source of real **** produces virtual **** with very low mass
- Rate and mass distribution given by same formula
- No phase space factor for *m***ee**<< *p***T **photon

**Notes:**
What are Feynman graphs for virtual photons?

## In Practice

- If no direct photons: ratios correspond to Dalitz decays
- If excess:  direct photons

**In Practice**

- ÷

- ÷

- ÷

**0-30**

**90-140**

**140-200 MeV**

**200-300**

***R*****data**

- Material conversion pairs removed by analysis cut
- Combinatorics removed by mixed events

## Slide 36

## Slide 37

## 

********

**S/B=~1**

## 

********

**S/B=~1**

- calculated from
- Dalitz formula

- measured

## 

********

**S/B=~1**

- calculated from
- Dalitz formula

- measured

## 

********

**S/B=~1**

- calculated from
- Dalitz formula

- measured

## Comparison to Conventional result

**Comparison to Conventional result**

## The Spectrum

**The Spectrum**

- Compare to published Run2 result: PRL94 232301

## direct

******direct**

**preliminary direct photon spectrum at 1-5 GeV/c from ***********

## The Spectrum

**The Spectrum**

- Compare to NLO pQCD

- L.E.Gordon and W. Vogelsang
- Phys. Rev. D48, 3136 (1993)

- above (questionable) pQCD

## The Spectrum

**The Spectrum**

- Compare to thermal model

- 2+1 hydro
- T**0****ave**=360 MeV(T**0****max**=570 MeV)
******0**=0.15 fm/c

- D. d’Enterria, D. Perresounko
- nucl-th/0503054

- Compare to NLO pQCD

- L.E.Gordon and W. Vogelsang
- Phys. Rev. D48, 3136 (1993)

- data above thermal at high pT

- above (questionable) pQCD

## The Spectrum

**The Spectrum**

- Compare to thermal + pQCD

- Compare to NLO pQCD

- L.E.Gordon and W. Vogelsang
- Phys. Rev. D48, 3136 (1993)

- data consistent with thermal + pQCD

- above (questionable) pQCD

- Compare to thermal model

- 2+1 hydro
- T**0****ave**=360 MeV(T**0****max**=570 MeV)
******0**=0.15 fm/c

- D. d’Enterria, D. Perresounko
- nucl-th/0503054

- data above thermal at high pT

## The Spectrum

**The Spectrum**

- Compare to thermal + pQCD

- Compare to NLO pQCD

- L.E.Gordon and W. Vogelsang
- Phys. Rev. D48, 3136 (1993)

- data consistent with thermal + pQCD

- above (questionable) pQCD

- Compare to thermal model

- 2+1 hydro
- T**0****ave**=360 MeV(T**0****max**=570 MeV)
******0**=0.15 fm/c

- D. d’Enterria, D. Perresounko
- nucl-th/0503054

- data above thermal at high pT

- Needs confirmation from *p*+*p* measurement

## More Comparisons

**More Comparisons**

## Low pT  new conversion method

**Low p****T ******** new conversion method**

- Direct Photons, Au+Au at 200 GeV

**External conversions**

**Internal Conversions**

**o New measurement with**
**   ****external conversions**
**o Improved systematics**
**o Confirmation of**
**   ****previous results**

## Internal conversion: d+Au

- Fit measured mass spectrum with function:

***a***** - absolute normalization  **
***b***** – fraction of direct photons:**

## Comparison: d+Au--Au+Au

- Ncoll-scaled d+Au agrees with Au+Au

**=>Systematic errors too large to extract thermal photons**
**=>but consistent with thermal photons**

## Outlook

- Current (2007) and future (2008) higher statistics (x4, x10) runs
-  will improve inclusive, v2, and -jet measurement
- Addition of RxNP detector (2007)
- will improve v2 measurement (x2 resolution)
- thereby facilitate disentangling of sources
- Isolation cut (possible at high pT with high statistics)
- will improve disentangling of sources
- give independent systematics for high pT inclusive and -jet
- Future high-statistics p+A run
- will allow precision measurement of structure function in nucleus
- thereby eliminate ambiguity as to initial vs. final state modifications  of spectrum
- Low pT measurement in p+p
- Will provide reference for Au+Au measurement
- New techniques (e.g.  HBT)
- Will give independent systemeatics for low pT measurement