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

Y. Akiba (RIKEN/RBRC)

for

PHENIX Collaboration

2008.04.25

NNs

Thermal Photons from the hot matter

Decay photons

nT

1

phard:

/ E Tethermal:

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

S.Turbide et al PRC 69 014903 S.Turbide et al PRC 69 014903

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

Extended in RUN5 data

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 pT2>>Mee2

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.

SdNMM

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M

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dM

Nd

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ee

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ee

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S : Process dependent factor

3

2

222 1

hadron

eeee M

MMFS

1S

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: 0706.3034arXiv: 0802.0050

Au+Au

p+p

Real signal di-electron continuum

Background sources1. Combinatorial background2. Material conversion pairs3. 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

pp Au+Au (MB)

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 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…

0 < pT < 8 GeV/c

0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c

1.5 < pT < 8 GeV/cPHENIX Preliminary

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.

eedirecteecocktaileedata mfrmfrmf 1

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 : fdirect 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, 120-300There is little direct photon component in this pT bin.For the last three fit ranges, chi**2/DOF ~ 1Variation 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 are1) Particle composition in the hadronic background coktail

2) The subtraction of backgrounda) Combinatorial background

b) Correlated background (Jet pairs and cross pairs)

3) Distortion of the mass spectrum due to dead area of the detector

4) 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

• 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

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

TMax

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dN Max

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cocktailPhenixideal

cocktailee

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The source of the systematic error is the error in the acceptance correction , which is estimated to be 14%The sys. uncertainty of the cocktail does not contribute to the sys. error in the inclusive photon.

Inclusive photon (continued)

Direct photon spectra

p+p Au+Au (MB)

Direct photon yield is determined as

)()( Tinclusive

Tdirect pdNrpdN

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

nTpp bpA )/1( 2Fit 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 scaledby TAA

nTppAAT bpATTpA )/1()/exp( 2

scaled ppexponential

sys. errorof pp fit

Au+Au (MB)

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

• 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. PeressounkoEPJC46, 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.