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

QM2006D.Peressounko2 Outline Physics of direct photons at small p t 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

QM2006D.Peressounko3 Direct photons at small p t 1/p t n ptpt ~5 GeV ~3 GeV e -p t /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 d 3 N/dyd 2 p 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 p t photon review.

QM2006D.Peressounko4 Subtraction and Tagging methods Measure photon spectrum and remove those photons which make  0 mass with any other Correct direct photon candidate sample for contribution from  0 decay photons with missing partner and for direct photons with fake partners Subtract 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. N  dir = N  incl - N  decay Subtraction Tagging

QM2006D.Peressounko5 Direct photon spectrum in p+p collisions Submitted to Phys.Rev.Lett., hep-ex/ PbGl PbSc NLO pQCD CTEQ6M PDF µ=p T /2, p T, 2p T (by W. Vogelsang) Tagging Data agrees with pQCD predictions in entire p t range More data from Run-5 are coming (already available for p t >5 GeV) Subtraction

QM2006D.Peressounko6 Direct photon spectrum in d+Au collisions Data agrees with pQCD predictions in entire p t range => No indication for nuclear effects NLO pQCD calculations from W.Vogelsang Run-3 d+Au Subtraction (p t 5 GeV) methods

QM2006D.Peressounko7 Internal conversion: idea N.M.Kroll and W.Wada, Phys. Rev. 98 (1955) 1355 Number of real photons e + e - pair massMass (√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 (M ee >m  ) and thus increase Signal/Background ratio ~10 times

QM2006D.Peressounko8 Internal conversion: implementation Fit measured mass spectrum with function a - absolute normalization b – proportion of direct photons:

QM2006D.Peressounko9 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)

QM2006D.Peressounko10 d+Au collisions: Spectrum Internal conversion extends range of significant points to p t > 2 GeV Data agrees with pQCD predictions in full p t range => No indication for nuclear effects

QM2006D.Peressounko11 Au+Au collisions: External conversion Combining this photon with others measured in EMCAL with loose PID cut, estimate proportion of photons coming from  0 decays Correct for missing  0 decay partners Subtract , , ’ decay photons Calculate ratio N  all /N  decay 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 conversionTagging no pair cut with pair cut Dalitz Conversion Uses very pure photon sample avoid explicit calculation of  0 spectrum =>reduce systematic errors

QM2006D.Peressounko12 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

QM2006D.Peressounko13 Comparison: Au+Au Minimum Bias Direct photon spectrum measured in d+Au collisions and scaled with agrees pretty well with spectrum measured in Au+Au at high p t =>Systematic errors are still too big to extract thermal photons. =>There is room for thermal photons

QM2006D.Peressounko14 Comparison: Au+Au, central collisions Spectra agree within errors in entire p t 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

QM2006D.Peressounko15 Direct photon HBT Only direct photon correlation matters (R  decay ~10 8 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 1/R decay ~ 1 eV q

QM2006D.Peressounko16 Two-photon correlations 11 22 11  2 rec e-e- e+e+ External conversion: No close cluster interference No hadron contamination C 2 calculated in EMCAL and converted+EMCAL agree => both effects are under control

QM2006D.Peressounko17 Conclusions Measurement of direct photon yield at small p t 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…

QM2006D.Peressounko18 Backup slides

QM2006D.Peressounko19 Internal conversion: implementation

QM2006D.Peressounko20 Comparison to Subtraction Au+Au results