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Direct photon production in p+p and d+Au collisions at s=200GeV from STAR

Direct photon production in p+p and d+Au collisions at s=200GeV from STAR. outline: motivation experiment and dataset analysis technique results in d+Au 2003 systematic errors first results / outlook p+p 2005 conclusions. M.J. Russcher , Utrecht University / NIKHEF.

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Direct photon production in p+p and d+Au collisions at s=200GeV from STAR

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  1. Direct photon production in p+p and d+Au collisions at s=200GeV from STAR outline: • motivation • experiment and dataset • analysis technique • results in d+Au 2003 • systematic errors • first results / outlook p+p 2005 • conclusions M.J. Russcher, Utrecht University / NIKHEF

  2. motivation direct photons are produced in processes like quark-anti-quark annihilation and quark-gluon Compton scattering q + q   + g q + g   + q • Au+Au • low pT: thermal component • high pT: tagged dir - jet events, a well-calibrated energy loss study • in general • no interaction with the medium • directly produced and for prompt  no fragmentation uncertainty • p+p • testing pQCD • gluon density • d+Au • initial state effects • nuclear kT , CGC • with p+p  HI baseline result P3.2: S. Chattopadhyay,  - charged in p+p and d+Au M.J. Russcher for the STAR collaboration

  3. BEMC TPC ZDC ZDC BBC BBC BEMC -1 <  < 1 experimental setup • BEMC • 4800 PbSc towers • -1<<1, 0<<2,  = 0.05 x 0.05 • TPC • charged particle veto • -1.6 <  < 1.6 • BBC • minbias trigger p+p • -3.4 <  < 5.0 • ZDC • minbias d+Au from ZDC-Au M.J. Russcher for the STAR collaboration

  4. experimental setup (2) • BSMD • wire-proportional counter •  = 0.007 x 0.007 • 0 reconstruction at high pT • high ET trigger • d+Au 2003: 1720 b-1 (4.5 GeV) • p+p 2005: 2830 nb-1 (3.5 GeV) • both datasets have 50% of BEMC installed and triggering M.J. Russcher for the STAR collaboration

  5. analysis method dir subject to a large background of decay photons, such as: 0 (~80%),  • measure all photon candidates in BEMC • subtract neutral hadron contamination  incl • 0 spectrum in same dataset, mT scaling for other mesons • simulate decay photons  decay • dir= incl– decay double ratio: • R = (incl/0)data  (/0)decay = 1 + dir /decay • advantage: cancellation of systematics • dir  (1 – R-1)  incl M.J. Russcher for the STAR collaboration

  6. d+Au results M.J. Russcher for the STAR collaboration

  7. 0 < y < 1 EPJ C43, 311 0 in d+Au • 0   • ~1000 b-1 after event cuts • reasonable agreement with pQCD calculation folded with nuclear density functions • 20-30% uncertainty due to 5% error on absolute energy scale dn/dM calculations by W. Vogelsang • error on yield extraction is 5-10% from uncertainty on background in peak region • no cancellation in double ratio pT (GeV/c) M.J. Russcher for the STAR collaboration

  8. double ratio in d+Au R = 1 + dir / decay • incl in same dataset as 0 • signal consistent with pQCD NLO calculation • further reduction of error bars needed for dir calculations by W. Vogelsang both data and calculation include contribution from fragmentation photons sizeable fragmentation contribution M.J. Russcher for the STAR collaboration

  9. systematic errors • beam background events contaminate high ET datasample • characterized by high energy in BEMC and no associated tracks reconstructed in TPC • cut on EBEMC/pT,TPC to reject events • cut has been varied to estimate the systematic error work in progress: pattern recognition tool to better identify beam background events M.J. Russcher for the STAR collaboration

  10. systematic errors errors on double ratio d+Au: dominant errors on R: • yield extraction • beam background • efficiency correction factors • BSMD gain energy scale almost cancels reduction needed for spectrum • more stats for efficiency correction • in situ calibration of BSMD • study effect of jet-structure M.J. Russcher for the STAR collaboration

  11. first results and outlook for p+p M.J. Russcher for the STAR collaboration

  12. 0in p+p • preliminary, subset of data • Lsampled = 0.4 pb-1 • consistent with pQCD+KKP • disfavors Kretzer FFs • agrees well with published PHENIX Run-2 result • PRL91, 241803 0 < y < 1 M.J. Russcher for the STAR collaboration

  13. minv [GeV/c2] minv [GeV/c2] minv [GeV/c2] minv [GeV/c2]  meson in p+p • clear signal in subset of data shown for 4 < pT < 12 GeV/c • reconstructed from  • more challenging than 0 because of BR ~= 40%, less abundant and acceptance loss • spectrum in progress • /0 = 0.45  0.05assumed for d+Au M.J. Russcher for the STAR collaboration

  14. outlook to direct photons in p+p • total statistical error on double ratio shown with pQCD + scale uncertainty • large improvement compared to d+Au • p+p result dominated by systematics • better calorimeter performance expected to lead to smaller systematic errors not data, statistical errors only calculations by W. Vogelsang M.J. Russcher for the STAR collaboration

  15. summary and conclusions • first BEMC direct photon result by STAR • d+Au double ratio R = 1 + dir/decayconsistent with pQCD • investigating possible reduction of systematic errors for dirspectrum • analysis in p+p well on its way • 0 shows agreement with pQCD + KKP and PHENIX Run-2 data • clear signal of  meson, spectrum in the pipeline • systematic error evaluation for double ratio in progress • access to nuclear effects for pions and photons • provides baseline for analysis in Au+Au. M.J. Russcher for the STAR collaboration

  16. backup slides M.J. Russcher for the STAR collaboration

  17. 100% 0 trigger efficiency p+p and d+Au dataset • BEMC trigger • high ET deposition in a single calorimeter cell • d+Au: • 1720 b-1 (ET > 4.5 GeV) • p+p: • 2830 nb-1 (ET > 3.5 GeV) • acceptance • 50% of BEMC (0 <  < 1) • 5-10% dead towers • 20% dead strips M.J. Russcher for the STAR collaboration

  18. kT / world data M.J. Russcher for the STAR collaboration

  19. eta to pion ratio • PHENIX PRL96 202301 •  /   15%, so 0.45 +/- 0.05 means < +/-2% for simulated decay background M.J. Russcher for the STAR collaboration

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