Direct photons at low p t measured in PHENIX D.Peressounko RRC “Kurchatov institute” for the PHENIX collaboration

Direct photons at low pt

measured in PHENIX

D.Peressounko RRC “Kurchatov institute”

for the PHENIX collaboration

QM2006 D.Peressounko 2

Outline

Physics of direct photons at small pt

p+p collisionsTaggingSubtraction

d+Au collisionsTagging Subtraction Internal conversion

Au+Au collisionsSubtractionExternal conversionDirect photon HBT

Comparison of binary scaled d+Au and Au+Au Conclusions


Direct photons at small pt

1/ptn

pt~5 GeV~3 GeV

e-pt/T

Prompt photons: control the initial state of the collision, modification of structure functions etc.

Jet-matter interaction: test matter density

Thermal photons: temperature and equation of state of the hot matter

d3N

/dyd

2p

t

Goal: extract thermal photon spectrum in A+A collisions and measure (effective) temperature.

Need a baseline from p+p and p+A collisions.

Jet-matter

See talk of TadaAki Isobe (Parallel session 3.3) for high pt photon review.


Subtraction and Tagging methods

Measure photon spectrum and remove those photons which make 0 mass with any otherCorrect direct photon candidate sample for contribution from 0 decay photons with missing partner and for direct photons with fake partnersSubtract contribution from , etc. decays

Measure yield of inclusive photons corrected for hadron contaminations, conversion, efficiency and acceptance.Measure spectrum of 0, , etc.Calculate yield of decay photons and subtract it from the inclusive yield.

Ndir = N

incl - Ndecay

Subtraction Tagging


Direct photon spectrum in p+p collisions

Submitted to Phys.Rev.Lett., hep-ex/0609031

PbGlPbScNLO pQCD CTEQ6M PDF µ=pT/2, pT, 2pT

(by W. Vogelsang)

GeV 200s at pp

Tagging

Data agrees with pQCD predictions in entire pt range

More data from Run-5 are coming (already available for pt>5 GeV)

Subtraction


Direct photon spectrum in d+Au collisions

Data agrees with pQCD predictions in entire pt range

=> No indication for nuclear effects

NLO pQCD calculations from W.Vogelsang

Run-3 d+Au

Subtraction (pt<5 GeV) and tagging (pt>5 GeV) methods


Internal conversion: idea

N.M.Kroll and W.Wada, Phys. Rev. 98 (1955) 1355

Number of real photons e+e- pair mass Mass (√s) of the

emitting system

Formfactor (~1)

Part common for all processes

Phase space

Due to phase-space factor we can measure direct photons in the region where e.g. 0 contribution is reduced (Mee>m) and thus increase Signal/Background ratio ~10 times


Internal conversion: implementation

Fit measured mass spectrum with function

a - absolute normalization

b – proportion of direct photons:

decay

dir

N

Nb


d+Au collisions: Double ratio

• Internal conversion method provides smaller systematic errors • But not as small as in the case of Au+Au collisions in Run-4 (Large background of external conversion on MVD detector in Run-3)


d+Au collisions: Spectrum

Internal conversion extends range of significant points to pt > 2 GeV

Data agrees with pQCD predictions in full pt range

=> No indication for nuclear effects


Au+Au collisions: External conversion

Combining this photon with others measured in EMCAL with loose PID cut, estimate proportion of photons coming from 0 decaysCorrect for missing 0 decay partnersSubtract , , ’ decay photonsCalculate ratio N

all/Ndecay

Use very tight PID cut on EMCAL photons to produce very clean photon sample

Use external photon conversion in beam pipe to produce very clean photon sample

External conversion Tagging

no pair cutwith pair cut

Dalitz

Conversion

Uses very pure photon sample

avoid explicit calculation of 0 spectrum=>reduce systematic errors


Au+Au collisions, different methods

All three methods produce consistent resultsand comparable systematic errors.

(QM05)

See posters (#87) of Hijiang Gong and (#88) of TakaoSakagutchi


Comparison: Au+Au Minimum Bias

Direct photon spectrum measured in d+Au collisions and scaled with <Ncoll> agrees pretty well with spectrum measured in Au+Au at high pt

=>Systematic errors are still too big to extract thermal photons.=>There is room for thermal photons


Comparison: Au+Au, central collisions

Spectra agree within errors in entire pt range

Systematic errors are still too large to extract thermal photons.There is room for thermal photons

This is consistent with excess above pQCD reported previously


Direct photon HBT

Only direct photon correlation matters (R

decay~108 fm)Size of emitting region can

be extractedProportion of direct photons can be evaluated from the correlation strength parameter:

Distortion of two-photon correlation function:Apparatus effects;Contamination by (correlated?) hadronsContamination by photon conversion on detector materialBackground photon correlations

See poster (#111) of Dmitri Peressounko for details

2

2

1

all

dir

N

N

1

3/2

1/Rdecay ~ 1 eV

q


Two-photon correlations

1

2

1

2rec

e-

e+

External conversion:• No close cluster interference• No hadron contamination

C2 calculated in EMCAL and converted+EMCAL agree =>both effects are under control


Conclusions

Measurement of direct photon yield at small pt is very interesting but extremely complicated taskPHENIX has developed a variety of methods for direct photon extraction in p+p, d+Au and Au+Au collisions producing consistent resultsBecause of the large systematic errors comparison of binary scaled d+Au spectrum with Au+Au does not allow to make a statement on the origin of the excess above pQCD observed in Au+Au. More precise data are coming…


Backup slides


Internal conversion: implementation


Comparison to Subtraction Au+Au results

Documents

Direct photons at low p t measured in PHENIX D.Peressounko RRC “Kurchatov institute” for the PHENIX collaboration