DP photons1

Direct photons at low pt measured in PHENIX: 

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

Outline: 

Outline Physics of direct photons at small pt p+p collisions Tagging Subtraction d+Au collisions Tagging Subtraction Internal conversion Au+Au collisions Subtraction External conversion Direct photon HBT Comparison of binary scaled d+Au and Au+Au Conclusions

Direct photons at small pt: 

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/dyd2pt 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: 

Subtraction and Tagging methods Measure photon spectrum and remove those photons which make p0 mass with any other Correct direct photon candidate sample for contribution from p0 decay photons with missing partner and for direct photons with fake partners Subtract contribution from h, w etc. decays Measure yield of inclusive photons corrected for hadron contaminations, conversion, efficiency and acceptance. Measure spectrum of p0, h, w etc. Calculate yield of decay photons and subtract it from the inclusive yield. Ngdir = Ngincl - Ngdecay Subtraction Tagging

Direct photon spectrum in p+p collisions: 

Direct photon spectrum in p+p collisions Submitted to Phys.Rev.Lett., hep-ex/0609031 PbGl PbSc NLO pQCD CTEQ6M PDF µ=pT/2, pT, 2pT (by W. Vogelsang) 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: 

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: 

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. p0 contribution is reduced (Mee>mp) and thus increase Signal/Background ratio ~10 times

Internal conversion: implementation: 

Internal conversion: implementation Fit measured mass spectrum with function a - absolute normalization b – proportion of direct photons:

d+Au collisions: Double ratio: 

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: 

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: 

Au+Au collisions: External conversion Combining this photon with others measured in EMCAL with loose PID cut, estimate proportion of photons coming from p0 decays Correct for missing p0 decay partners Subtract h, w, h’ decay photons Calculate ratio Ngall/Ngdecay 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 cut with pair cut Dalitz Conversion Uses very pure photon sample avoid explicit calculation of p0 spectrum =>reduce systematic errors

Au+Au collisions, different methods: 

Au+Au collisions, different methods All three methods produce consistent results and comparable systematic errors. (QM05) See posters (#87) of Hijiang Gong and (#88) of Takao Sakagutchi

Comparison: Au+Au Minimum Bias: 

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: 

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: 

Direct photon HBT Only direct photon correlation matters (Rgdecay~108 fm) Size of emitting region can be extracted Proportion of direct photons can be evaluated from the correlation strength parameter: Distortion of two-photon correlation function: Apparatus effects; Contamination by (correlated?) hadrons Contamination by photon conversion on detector material Background photon correlations See poster (#111) of Dmitri Peressounko for details 1 3/2 l 1/Rdecay ~ 1 eV q

Two-photon correlations: 

Two-photon correlations g1 g2 g1 g2rec 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: 

Conclusions Measurement of direct photon yield at small pt is very interesting but extremely complicated task PHENIX has developed a variety of methods for direct photon extraction in p+p, d+Au and Au+Au collisions producing consistent results Because 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: 

Backup slides

Internal conversion: implementation: 

Internal conversion: implementation

Comparison to Subtraction Au+Au results: 

Comparison to Subtraction Au+Au results