# Presentation: 803633

## Direct Photon measurement at RHIC-PHENIX

- Takao Sakaguchi
- Brookhaven National Laboratory
- For the PHENIX Collaboration

## Outline

- What can we learn from photons?
- Photon contribution from various sources
- pQCD, thermal, etc.
- Detector and Analysis
- Result
- Comparison with theoretical calculations
- Summary

## Direct Photon measurement

- Photons come from all the stage from initial hard scattering to final hadronic state.
- Transparent in the strongly interacting medium
- Carry thermodynamical information of the state from that they are emitted
- Temperature
- Degree of freedom
- QGP phase transition can be detected through this direct probe
- Formation time
- Time profile of phase transition

- A Compilation: hep-ph/0410282, Aurenche

## Photon Production

- Leading Order photon production
- Annihilation process
- Compton scattering process
- Next to Leading Order photon production
- Bremsstrahlung
- Next-to-Next-to...
- Jet Fragment photon
- Another Sources?
- Pre-equilibrium photon
- Jet-Photon conversion in presence of hot dense medium
- Scattering of hard scattered parton with thermal partons
- Diagram shown in Drell-Yan process (similar to real photon)

**hep-ph/0201311**

## Photon production (pQCD)

- Produced in initial hard scattering of partons in nucleons
- pT>~3-4GeV to very high pT
- Rumor..
- pQCD calculation describes yields very well.
- How well?
- Typically, 20-30% theoretical uncertainty due to the various choice of scale
- Comparison to experiment: Factor of two difference remains.
- Also dependent of beam energy
- Shooting the factor in experiments
- PHENIX contributes one
- Shown in red points
- Photon is not affected by final state interaction
- Directly comparable with calculation.
- Distinct initial state from final state effect

- Apanasevich, et al., PRD63(2003)014009

- PHENIX p-p data in red points!

## Initial State Effect (kT-smearing)

- Prompt photon production in -Be, p-Be (Tevatron E706)
- Any data don’t agree with simple NLO pQCD calculation.
- Just with primordial kT
- Including additional kT describes data very well.
- Cronin effect!
- Suggesting Cronin effect is not due to recombination
- d-Au high pT hadron spectra showed kT broadening behavior as well
- d-Au photon data should show initial state effect only
- Direct measurement of kT is possible
- Kill or save recombination model

- E706 Collaboration, PRD70(2004)092009

- Center-of-mass energy, and xT region:
- RHIC: *s**NN*=200GeV, xT ~0.04-0.25
- E706: *s**NN*=32-40GeV, xT~0.2-0.5

## Thermal & Hadron Gas~Photon is Difficult!~

- Thermal radiation from QGP state.
- LO, NLO, LPM included
- Resummed 22 process
- exchange partons
- The state of art calculation from different sources shows that radiation from QGP state is dominant in pT=1-3GeV
- pT>3GeV, initial pQCD
- pT<1GeV, Hadron Gas interaction
- ()(), K*K
- Here is where we want to shoot!
- Estimated excess  in 1.5<*pT*<3GeV/c: ~10%
- Systematic error should be less than this.
- Needs precise estimate of background photons from known hadronic sources
- Needs precise knowledge of detector response
- High photon efficiency, and charged and neutral particle rejection

**PRC69(2004)014903**

## Single photons in Heavy Ion collisions ~Before RHIC~

- WA80, WA98 are the dedicated experiments for direct photon search in relativistic heavy Ion collisions.
- WA98 data can be either explained by kT-smearing or higher initial temeprature
- Any data did not see pT>4GeV, where pQCD photons dominate
- No information on kT does not allow us to resolve the issue.
- Recent data points at ~100MeV available from WA98.
- By analysis of correlation strength in interferometry,
**WA98, PRL93(2004)022301**

- WA98 data and theoretical interpretation
**PRC69(2004)014903**

## And RHIC...

- STAR measurement of 0 and photon in Au-Au at ***s**NN*=130GeV
- 0.5<pT<2.5
**PRC70(2004)044902**
- Using photon conversion method
- Systematic Errors
- 0 normalization error: 49%
- 19% additional error on 0-11% centrality data
- pT spectra fit errors: 5-11%
- Basically, no excess is seen.

## PHENIX Detector

- Use of both Lead scintillater and lead glass electro-magnetic calorimeter (EMCal)
- Radiation Length: PbSc: 18X0, PbGl: 16X0
- Coverage:  ||<0.38, = 
- PbSc: Energy Resolution:  8.1/E(GeV)  2.1 %
- 		       Position Resolution:  5.7/E(GeV)  1.6 mm
- PbGl: Energy Resolution :  5.9/E(GeV)  0.8 %
- 		       Position Resolution:  8.4/E(GeV)  0.2 mm
- RICH and Tracking: Energy calibration of the calorimeter
- Using electrons
- Pad chamber and Tracking: Estimate of Charged particle contamination

**PbGl**

**PbSc**

**       **_**Events used**_
_***30M***_ Minimum bias trigger events
_***55M***_ Minbias-equivalent LVL-2 trigger events

## 0’s and ’s 

- 0’s and ’s spectra are measured in Run2 Au-Au Run.
- /0 ratio is obtained:  0.45+/-0.1
- ~95% of background sources are already determined
- Other is less than ~5%.(-> by some estimate)

- prel.

- PRL **91**,072301 (2003) + high pT triggered data

- 0 data

## Estimate of Background photons

- Fit the measured 0 transverse momentum spectra with a function.
- Move each data points up and down by systematic errors
- Fit the new spectra again
- Iteration of Generating  from 0, and estimating /0 ratio error due to fit.
- Assume Flat Rapidity
- , ’ : Incorporate pT of the fit function by sqrt(pT2+M2-M2)
- Normalization scale obtained by /0 ratio measured (/0=0.450.1)
- Generate photons, and count the number of photons that entered calorimeter.
- Systematic Error on estimated background /0 is _***3-4%***_
- Note that it is the error to Ratio, not to  or 0 individually.

## Real photon measurement

- Efficiency to various particles

- Electron, photon

- ,K,p

- Deposited energy [GeV]

**0         2         4        6         8        10       12**

**1**

**0.1**

**0.01**

- “Photon ID likelihood function consists of “shower dispersion, ratio of energy in several towers”
*f*(*Ecent,Ecore**,****2**, etc.,*)
- Apply threshold to the function
- Efficient Photon ID and hadron rejection
- Photon ID efficiency, hadronic cluster background, and detector response as a function of centrality estimated by event embedding method
- Unfold the real photon spectra from measured cluster energy distribution
- Systematic Errors:
- Non  correction error: ~3%
-  Yield correction (off-vertex, conversion): ~10%
- Energy Scale of  :12-15%
- 0 Yield extraction: 7-10%
- 0 Yield correction (off-vertex, conversion): ~12%
- Total Systematic Error after combining PbGl and PbSc:
- Spectra: ~15-20%
- /0 double ratio: ~12-16%

## Results (/0 double ratio), Au-Au)

- (/0)meas/(/0)estimated
- = (meas/estimated)
- Systematic Error on Data: 12-16% of the ratio
- Gray box shows all errors including systematic uncertainty
- Showing the contribution over the background photons from known hadronic sources
- Suppression of 0 and  contributions enhanced ratios
- Red lines show thickness-scaled NLO pQCD calculation + known background contribution
- Yellow shades show uncertainty from thickness function and theoretical calculation
- Well described by the calculation

- Error Bar: Statistical error

## Results (Direct photon spectra)

- Direct photon spectra over centralities
- Systematic Error: ~15-20%
- Clearly seen that we measured photons over the order of 1027!
- See the scale please..
- Again, Thickness-scaled NLO QCD calculation describes all the spectra very well
- From Central to Peripheral
- No exception within current errors
- Yellow bands show uncertainty on NLO pQCD calculation and thickness function

## Results for p-p

- NLO-pQCD calculation
- CTEQ6M PDF.
- Gluon Compton scattering
-      + fragmentation photon
- Set Renormalization scale
-     and factorization scale pT/2,pT,2pT
- Systematic Error:
- 20(high pT)-45(low pT)%

- The theory calculation shows a good
- agreement with our result.

- (Subtraction)

- Bands represent systematic errors.

- Errors on the backgrounds result in
- enlarged errors on the signal,
- especially at low-pT region.

## Results (RAA)

- Results (RAA)

- Photon RAA is consistent with unity over all the centrality compared to 0 results.
- Clear evidence of that the yield follows thickness-scaled hard scattering
- p-p reference from NLO pQCD Calculation
- 0 RAA decreases to ~0.2 at Npart=320
- Dotted line shows uncertainty of thickness function
- Error bars show total error (systematics + statistical) except thickness function error
- Yellow shows uncertainty on pQCD calculation

- Direct 

- 0

## Comparison with calculations

- Any of pQCD calculations describe data well
- Adding kT broadening makes factor of ~2 difference
- Around same factor as E706
- Calculation suggests that slopes of the spectra at RHIC and E706 are same
- Jet Photon included calculation (***Fries et al., PRL 90, 132301 (2003)***) is also shown
- Fits very well above 4GeV!
- Assuming existence of hot dense medium
- Prompt partons scatter with thermal partons
- The line approaches to simple pQCD calculation in high pT

## Jet Photon overwhelms QGP?

- Break-up of Fries prediction
- Jet Photons overwhelms all the other contributions below 7GeV/c
- Jet production rate calculated by LO pQCD with K factor compensation of 2.5
- pQCD photon calculation from LO with no K factor
- Fitting too good!
- In Peripheral, the calculation should fit the data as well
- RAA and spectra themselves tell you what happens
- Calculation is assuming existence of hot dense medium, which is not the case in peripheral!

## State-of-Art; Zooming into low pT

- Most realistic calculation
- Including all the contributions
- We may be able to see QGP contribution in 1-3GeV/c in Run4!

**Thanks to Ralf Rapp for providing theoretical calculations!**

**PRC69(2004)014903**

## Next Target?

**?**

## Summary

- First high pT direct photon observed in heavy ion collisions.
- Yield scaled by thickness function
- Thickess-scaled NLO pQCD calculation describing initial hard scattering process fits well.
- Any Model fits to data in the current errors.
- Jet Photon model is quite unlikely given the fact that the spectra scales to thickness function
- Needs Improvement of systematic errors to scope lower pT region
- Run4 data will extract more precise information in the intermediate pT region

## Backup Slides