# Presentation: 237779

## Enhanced production of direct photons in Au+Au collisions at          =200 GeV

- Y. Akiba (RIKEN/RBRC)
- for
- PHENIX Collaboration
- 2008.04.25

## Thermal Photons from the hot matter

- High energy density matter is formed at RHIC
- If the matter is thermailzed, it should emit “thermal radiation”,
- The temperature of the matter can directly be measured from the spectrum of thermal photon.
- Thermal photons can be the dominant source of direct photon for 1<pT<3 GeV/c at RHIC energies.

## Thermal photons (theory prediciton)

- It is predicted that themal photons from QGP can be the dominant source of direct photons for 1<pT<3 GeV/c
- Higher pT: pQCD photon
- Lower pT: from hadronic phase
- Recently, other sources, such as jet-medium interaction are discussed
- Measurement is difficut since the expected signal is only 1/10 of photons from hadron decays

## Photon measurement in PHENIX

- PHENIX measured direct photons both in p+p and Au+Au
- Good agreement with NLO pQCD and Ncoll scaling at high pT
- Measurement is limited to pT > 4-5 GeV/c

- p+p

- Au+Au

## Alternative method --- meaure virtual photon

- Source of real photon should also be able to emit virtual photon
- If the Q2 (=m2) of virtual photon is sufficiently small, the source strength should be the same
- The ratio of real photon and quasi-real photon can be calculated by QED
-  Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs

## Virtual Photon Measurement

-  Case of Hadrons
-  Obviously *S* = 0 at Mee > Mhadron
-  Case of direct *
-  If **pT****2****>>M****ee****2**
-  Possible to separate hadron decay components from real signal in the proper mass window.

-  Any source of real  can emit * with very low mass.
-  Relation between the * yield and real photon yield is known.

***S *****: Process dependent factor**

- Eq. (1)

## Not a new idea

- The idea of measuring direct photon via low mass lepton pair is not new one. It is as old as the concept of direct photon.
- This method is first tried at CERN ISR in search for direct photon in p+p at 55GeV. They look for e+e- pairs for 200<m<500 MeV, and they set one of the most stringent limit on direct photon production at low pT
- Later, UA1 measured low mass muon pairs and deduced the direct photon cross section.

- /0 = 10%

- J.H.Cobb, et al, PL 78B, 519 (1978)

- /0 = 0.53 ±0.92%
- (2< pT < 3 GeV/c)

- Dalitz

## Measurement of low mass electron pairs

**arXiv: 0706.3034**

**arXiv: 0802.0050**

**Au+Au**

**p+p**

-  Real signal
-  di-electron continuum
-  Background sources
-  Combinatorial background
-  Material conversion pairs
-  Additional correlated background
-  Visible in p+p collisions
-  Cross pairs from decays with 4 electrons in the final state
-  Pairs in same jet or back-to-back jet

## Enhancement of almost real photon

- Kinematic region of e+e- pairs
- m<300 MeV and 1<pT<5 GeV/c
- 	In this kinematic region, the S/B of the continuum is at least 10% (combinatorial BG is not a problem)
- p+p
- Good agreement of p+p data and hadronic decay cocktail
- Small excess in p+p at large mee and high pT
- Au+Au
- Clear enhancement visible above for all pT

**1 < p****T**** < 2 GeV**
**2 < p****T**** < 3 GeV**
**3 < p****T**** < 4 GeV**
**4 < p****T**** < 5 GeV**

## Possible sources of the excess

- Internal conversion of direct photon
- 	A source of real photon should also produce quasi-real virtual photon
- However, presence of virtual photon does not necessarily mean that it is related to real photon
- 	Example:
- 	q+q  e+e- , +-  e+e-
- BUT
- 	if they contribute for m<300MeV, the effective mass quark should be smaller than 150MeV and pion mass should be strongly modified...

## Is excess low mass enhancement?

-  The low mass enhancement decreases with  higher pT
-  We see no significant indication that this low mass enhancement contribute to m<300 MeV and pT>1 GeV/c (see next slide)
- We assume that excess is entirely due to internal conversion of direct 

-  Au+Au
-  p+p

- Normalized by the yield in mee < 100MeV

## Determination of * fraction, r

*r* : direct */inclusive *

- Direct */inclusive * is determined by fitting the following function for each pT bin.

-  Fit in 80-300MeV gives
-  Assuming direct * mass shape
-  2/NDF=13.8/10
-  Assuming  shape instead of direct * shape
-  2/NDF=21.1/10
-  Assumption of direct * is favorable.
-  the mass spectrum follows the    expectation for m>300 MeV
-  No significant contribution from “low mass enhancement”

- Reminder : *f*direct is given by Eq.(1) with *S *= 1.

## Fit pp 1.0<pT<1.5 GeV/c

- Fit range: 0-300, 30-300, 50-300, 80-300, 100-300, 120-300

- There is little direct photon component in this pT bin.
- For the last three fit ranges, chi**2/DOF ~ 1
- Variation of the fit results is included in sys. error in r

## Fit pp 3.0-4.0 GeV/c

- For higher pT, small direct photon contribution is revealed

## Fit AuAu MB. 1.0<pT<1.5

- Au+Au data has much larger excess

## Systematic uncertainties

- Function fc(m) and fdir(m) are both normalized to the data for m<30 MeV so the sys. error. in the absolute normalization cancels
- Sources of the systematic uncertainties are
- Particle composition in the hadronic background coktail
- The subtraction of background
- Combinatorial background
- Correlated background (Jet pairs and cross pairs)
- Distortion of the mass spectrum due to dead area of the detector
- Efficiency correction
- We evaluate the distortion of mass spectrum due to these sources, and then evaluated their effect on the direct photon fraction r.

## Sys. error. Cocktail hadron Ratio

-  mee spectra with upper/lower value of particle ratio normalized to mee<30MeV
-  ratio: upper/nominal, lower/nominal
-  in the next slide we’ll show these ratios for different pT bins
-  fit is repeated with the distorted cocktail fc(m). The change in the fraction r is taken as the systematic error.

- upper

- lower

## Fraction of direct photons

- Compared to direct photons from pQCD
- p+p
- Consistent with NLO pQCD
- favors small μ
- Au+Au
- Clear excess above pQCD

- μ = 0.5pT
- μ = 1.0pT
- μ = 2.0pT

- p+p

- Au+Au (MB)

- NLO pQCD calculation is provided by Werner Vogelsang

## Inclusive photon

- To convert the direct photon fraction r to direct photon yield, we need the invariant yield of inclusive photon.
- We measure inclusive photon yield from the yield of low mass electron pairs at Dalitz peak (m<30 MeV)
- We remind:
- For small Mee, the process dependent factor S becomes unity. This means that electron pair yield in Dalitz peak is proportional to inclusive photon yield.
- C(Mmax) is the same within a few % for any photon source for Mmax = 30 MeV

## Inclusive photon (continued)

- The sys. uncertainty of the cocktail does not contribute to the sys. error in the inclusive photon.

## Direct photon spectra

- p+p

- Au+Au (MB)

- Direct photon yield is determined as

## Fit to the p+p spectrum

- To characterize the p+p data, a modifed power-law function is fit to the spectrum
- The fit is repeated for the upper/lower systematic error of the spectrum (mostly common-mode) to determine the systematic uncertainty of the fit.
- A simple power-law gives worse 2/DOF

- Fit function:

- PHENIX EMCal
- (PRL98, 012202)

- This analysis

## Fit to Au+Au

- To characterize the excess of Au+Au spectrum over the TAA scaled p+p spectrum, exponentail + scaled pp is fit to the Au+Au data

- p+p spectrum scaled
- by TAA

- scaled pp

- exponential

- sys. error
- of pp fit

- Au+Au (MB)

## Direct  via  * for p+p, Au+Au

- Direct  via  * for p+p, Au+Au

- New p+p result with * method agrees with NLO pQCD predictions, and with the measurement by the calorimeter
- For Au+Au there is a significant low pT excess above p+p expectations
- The excess above TAA scaled p+p spectrum is characterized by the exponential fit explained in the previous slides. The inverse slope and the yield of the exponential is determined.

## Direct  via  * for p+p, Au+Au

- Direct  via  * for p+p, Au+Au

- New p+p result with * method agrees with NLO pQCD predictions, and with the measurement by the calorimeter
- For Au+Au there is a significant low pT excess above p+p expectations
- The excess above TAA scaled p+p spectrum is characterized by the exponential fit explained in the previous slides. The inverse slope and the yield of the exponential is determined.

- NLO pQCD (W. Vogelsang)

- Fit to pp

- exp + TAA scaled pp

## Summary of the fit

- Significant yield of the exponential component (excess over the scaled p+p)
- The inverse slope is ~220MeV. (If power-law is used for the pp component, the value of T would increase by ~24MeV)

## Theory comparison

- Hydrodynamical models are compared with the data
- 	D.d’Enterria &D.Peressounko
- 	   T=590MeV, 0=0.15fm/c
- 	S. Rasanen et al.
- 	   T=580MeV, 0=0.17fm/c
- 	D. K. Srivastava
- 	   T=450-600MeV, 0=0.2fm/c
- 	S. Turbide et al.
- 	   T=370MeV, 0=0.33fm/c
- 	J. Alam et al.
- 	   T=300MeV, 0=0.5fm/c
- Hydrodynamical models are in qualitative agreement with the data

- Thery compilation by D. d’Enterria and D. Peressounko
- EPJC46, 451 (2006)

## Summary and conclusion

- We have measured e+e- pairs for m<300MeV and 1<pT<5 GeV/c
- Excess above hadronic background is observed
- Excess is much greater in Au+Au than in p+p
- Treating the excess as internal conversion of direct photons, the yield of direct photon is dedued.
- Direct photon yield in pp is consistent with a NLO pQCD
- Direct photon yield in Au+Au is much larger.
- Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±23(stat)±18(sys) MeV
- Hydrodynamical models with Tinit=300-600MeV at 0=0.6-0.15 fm/c are in qualitative agreement with the data.