PHENIX measurement on direct photon production

Jiamin JinJournal Club07/12/2006

2023.04.19 J. Jin - JClub 2

Outline

motivation direct photon in pp & AuAu (RUN2)

-- hep-ex/0502006, nucl-ex/0503003 direct photon in AuAu (RUN4)

extend to higher pT two analyses to improve on mid pT range

using stachastic cuts (David et al) photon internal conversion (Y. Akiba)

-- various ANs

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What are direct photons? @LO level, 3 channels:

Compton scatting g+q+q Annihilation q+qbar+g

The latter is suppressed for p+p, because qbar is less probable than g in the proton

The parton-parton scattering with the scattered quark or gluon fragmenting to a

@NLO level: Bremstrahlung emission of g from the

quarks undergoing hard scattering

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photons not from hadronic decays Experimental challenge to differentiate

direct & decay

cocktail subtraction method 0 tagging method Isolation cuts

Direct photons != isolated photons Fragmentation and bremstralung

processes will produce photons with jets

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Why direct photons? p+p

Test on QCD explore PDF of the gluon inside the proton Reduce uncertainty on pQCD photons in AA

A+A Photons don’t strong interact with medium Hard photons

Allow test of Ncoll scaling for hard processes Interpret high-pt hadron suppression at RHIC

Thermal photons Carry information of early stage of collisions Detect QGP via thermal photon radiation

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Direct photons in p+p

1st step: determine direct-photon-candidate hits reject a EMCal cluster if:

form an invariant mass in the 0 or range with other clusters

asymmetry cut combinatorial background studied by

embedding (small effect in p+p) a factor of 2 rejection achieved for

cluster pT>5GeV/c

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Formula of differential cross section

N: # of direct-photon-candidates in a pT&y bin Creco: acceptance and efficiency correction (MC sim.) Cconv: correction for photon conversions (<7.4%) Closs: correction for the loss of true direct photons due to c

ombinatorial bkgd (<2%) f = 0.75(+/-0.02): MinBias trigger efficiency L: integrated luminosity (~41nb-1 for RUN2 p+p)

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2nd step: deal with the remaining hadronic decay photons mostly from missing the partner photon also from other decay sources (,’,…) plot from data feed p0 spectrum into MC, simulate get double gamma ratio R

mT scaling for other decay sources

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Results

Ratio of direct-photon-candidates to the 0 spectrum (data & MC)

Direct cross section small but significant sig

nal observed agree with NLO pQCD

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Other methods I

0 tagging method:

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Other methods II

Isolation cuts photon energy < 10% of cone energy cone size: 0.5 around the candidate photon cone energy: sum of track momentum in DC & E

MCal cluster energy correct for limited PHENIX acceptance, which c

auses fake isolated photons Emphasize the Compton process

direct access to gluon distribution

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Results (RUN3 p+p)

direct photon production spectrum up to 16GeV/c well described by NLO pQCD

for pT>5GeV/c

ratio of isolated photons to all direct photons large systematics in low pT

agree with pQCD(lines) for pT

>7GeV/c

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Direct photons in Au+Au

What’s new in Au+Au? centrality determination by BBC&ZDC high pT hadron suppression in central Au+A

u, measure R directly:

R > 1, indicates direct photon signal Direct photon spectra extracted as:

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Results

double gamma ratio plot advantage: many systema

tics cancel an excess over unity is obs

erved at high pT

magnitude increases with centrality due to hadron suppression

expectation for Ncoll scaling of direct photons holds for all centrality bins

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spectra shown 9 centrality bins and M

B comparison to NLO at low to mid-pT where

additional production of thermal photons appears, we have large error bars

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Improvement on mid-pT thermal photon region

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goal: improve upon mid-pT region systematic errors

using most stable subset of RUN4 data improving various corrections

p0 peak extraction energy scale/linearity energy smearing fitting function to p0 spectrum

using stochastic cuts to extract photons

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Going to low pT: No apparent excess

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Excess above the scaled pQCD?

new points agree with, but consistently higher than the old ones ~4GeV/c

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A new approach:very low-mass di-lepton pairs

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The idea: Dalitz decay

mass distribution of e+e- pairs (Knoll-Wada formula):

0

0

e+

e-

1) pure QED part, universal to all Dalitz decays2) phase factor part, depends on M of parent hadron

3) hadronic form factor

at low mass, both the second and third part become 1at high mass, suppressed by phase factor

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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 mee<< pT

photon

Compton scattering

q

g q

e+

e-

90-3

00

MeV

0-3

0

Rdata

÷

We measure Rdata:N(90<Mee<300)/ N(0<Mee<30)

We can calculate:R0, R, Rdirect

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Rdata excess over calculated expected ratio from hadronic decays: virtual & real direct photon signal

N

Nd irect

a ll

0 30

0 30

True in very low mass window!

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0-20 %

Results: Rdata

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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*direct/*inclusive

Significant 10% excess of very-low-mass virtual direct photons

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

0-20 %

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Centrality Dependence

Indication for centrality dependence

more peripheral

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Comparison to conventional results

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

( + 1 )

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direct

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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The Spectrum

Compare to the published Run2 results (black arrow)

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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The Spectrum

• excess above pQCDCompare to NLO pQCD

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• excess above pQCDCompare to NLO pQCD

Compare to thermal model

• data above thermal at high pT

The Spectrum

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Compare to thermal + pQCD

• data consistent with thermal + pQCD

• excess above pQCDCompare to NLO pQCD

Compare to thermal model

• data above thermal at high pT

The Spectrum

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Backup

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Schematic Photon Spectrum in Au+Au

Decay photons

nT

1

phard:

/ E Tethermal: