# Presentation: 002162

## Direct Photons in PHENIX

**Stefan Bathe****                                                            ****UC  Riverside**
**for the PHENIX Collaboration**

**Hot Quarks, May 18, 2006**

**Notes:**
Changed title 

## Why direct photons?

- Hadrons affected by medium created in A+A collision
- Carry information about final state
- Photons don’t strongly interact
- Exit medium unscathed
- Carry information about initial state
- Provide calibrated baseline

## Direct Photons

**Direct Photons**

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

## Measurement of Direct Photons

- Get clean inclusive-photon sample
- e.g. subtraction of charged particle background
- Measure *p*T spectrum of  and  mesons with high accuracy
- Calculate number of decay photons per 
- Usually with Monte-Carlo
*m*T scaling for ’, , ...
- Finally:Subtract decay background from inclusive photon spectrum

## PHENIX Setup

**PHENIX Setup**

- Central spectrometer arms |**| < 0.35
-  0 via 0 
- EMCal
- Lead scintillator calorimeter (PbSc)
- Lead glass calorimeter (PbGl)
- Charged particles
- DC, PC
- e**±** ID
- RICH
- Muon arms (not shown, forward rapidity)

## Direct Photons in p+p

**Direct Photons in p+p**

- Test of QCD
- direct participant in partonic interaction
- Less dependent on FF than hadron production
- Reduce uncertainty on pQCD  photons in A+A
**good agreement with NLO pQCD**
**Important baseline for Au+Au**

## Direct  in d+Au

**Direct ******** in *****d*****+Au**

**consistent with 1**
**No indication for initial-state effects**
**But large uncertainties**

*d*+Au:
-  initial-state nuclear effects
- no final-state effects      (no medium produced)
- Study initial-state effects

*p*+*p* and *d*+Au spectra compared to NLO pQCD

- ratio to NLO pQCD

## Hard Direct Photons in Au+Au

- Expectation for *N*coll scaling of direct photons

- holds for all centrality classes

- Photons don’t strongly interact with medium
- Allow test of *N***coll** scaling for hard processes
- Important for interpretation of observed high-*p***T** hadron suppression

**Large direct photon excess**

## Direct Photon RAA

**Direct Photon *****R*****AA**

- 0’s suppressed

**direct photons not suppressed**

******0**** suppression caused by created medium**

**PRL 94, 232301**

## Other Modifications of Ncoll scaling?

- For instance:  fragmentation photons?

**fragmentation contribution (%)**

- fragmentation contribution substantial in *p*+*p*
- parton energy loss in QGP reduces fragmentation contribution in Au+Au
- compensated by induced photon bremsstrahlung in QGP

- Effects cancel?
- Potentially many more modifications of direct photon production

- Bremsstrahlung

## Going to intermediate pT

- Au+Au

## Expected photon sources

- Window for thermal photons from QGP in this calculation:*p*T = 1 - 3 GeV/*c*

- Turbide, Rapp, Gale, Phys. Rev. C 69 (014903), 2004

## Limitation of Calorimeter Measurement

**Limitation of Calorimeter Measurement**

- No significant excess at low *p**T*

**PRL 94, 232301**

## 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 17

## Slide 18

## 

********

**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

## Conclusions

*p*+*p*
- Established baseline measurement of hard photons
- More precise measurement about to be published
*d*+Au
- Investigated initial-state effects
- Needs higher-statistics run
- Au+Au, high *p*T
- Confirmed binary scaling of *p*+*p* baseline
- Established hadron suppression as medium effect

- Au+Au, intermediate *p*T
- Promising new technique for measuring direct photons in HI collisions through internal conversion
- Excess of direct photons from * above decay-photon background
- Data consistent with thermal + pQCD
- However
- pQCD calculations questionable at low *p*T
- Needs confirmation from *p*+*p* measurement with same technique
- Direct photon sources other than thermal might contribute as well

## Slide 31

- Gale QM05