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Direct g ‘s & Hard Scattering at SPS

Direct g ‘s & Hard Scattering at SPS. Direct Photons in Pb+Pb Analysis Details Systematic Errors Results Comparison to pA and Theory Back-to-Back g-g Correlations ‘‘High p T ’’ p 0 ’s Conclusions. Ecole Polytechnique September 4-7,2001.

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Direct g ‘s & Hard Scattering at SPS

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  1. Direct g ‘s & Hard Scattering at SPS • Direct Photons in Pb+Pb • Analysis Details • Systematic Errors • Results • Comparison to pA and Theory • Back-to-Back g-g Correlations • ‘‘High pT’’ p0’s • Conclusions • Ecole Polytechnique • September 4-7,2001 T.C.Awes, ORNL, QGP Workshop

  2. Evolution of Relativistic Heavy-ion Collision T.C.Awes, ORNL, QGP Workshop

  3. The WA98 Experiment T.C.Awes, ORNL, QGP Workshop

  4. Event Selection • 10% most central = QGP search sample. • 20% most peripheral = Reference sample - pQCD prompt photons. T.C.Awes, ORNL, QGP Workshop

  5. Effect of Shower Overlap on Efficiency T.C.Awes, ORNL, QGP Workshop

  6. Direct Photon Analysis Overview • Statistical Basis: Compare measured g spectrum to calculated decay g spectrum. • Decay g’s calculated from measured p0’s and h0’s • Use GEANT “test” particles to determine efficiency (overlap effects) • Vary g identification criteria to investigate systematical errors. T.C.Awes, ORNL, QGP Workshop

  7. Charged-Hadron Background • Use Charged-Particle Veto to tag charged showers. • Correct for g conversions, CPV efficiency, random association. • Case: • Narrow Showers = g Candidates • 5% charged-hadron • contamination. T.C.Awes, ORNL, QGP Workshop

  8. Final g pT Spectra • Fit to power-law form: • Corrected for charged and neutral • background, efficiency, and • acceptance. • ( • )n • p0 • p0 + pT • Inverse slope: • T= p0/n + pT/n • TCentral= .198 + pT/37 GeV • TPeripheral= .147 + pT/22 GeV T.C.Awes, ORNL, QGP Workshop

  9. Systematical Error on g Yield • Check consistency of final result with different g identification criteria. • Sensitivity to charged-hadron and neutron backgrounds vary by factors of 2-4. • Identification efficiency varies by 2-3 for different methods for central case. • Systematical error on g yield ~3% T.C.Awes, ORNL, QGP Workshop

  10. Decay Background g Sources • Direct g‘s extracted from difference between measured g‘s and calculated decay g‘s. • ~97% decay g‘s from measuredp0 and h0. • Assume mT-scaling for h0 and others. T.C.Awes, ORNL, QGP Workshop

  11. Extraction of p0 Yield • Combinatorial gg Background Problem: Use mixed gg events to determine shape of background. T.C.Awes, ORNL, QGP Workshop

  12. Shower Overlap Effects Again: p0 T.C.Awes, ORNL, QGP Workshop

  13. Final p0 pT Spectra • Fit to power-law form: • Corrected for efficiency, and • acceptance. • )n • ( • p0 • p0 + pT • Inverse slope: • T= p0/n + pT/n • TCentral= .250 + pT/80 GeV • TPeripheral= .216+ pT/45 GeV T.C.Awes, ORNL, QGP Workshop

  14. Extraction of h0 Yield • HUGE gg combinatorial background. • At limit of statistics for central, no signal for peripheral. • Goal: check mT-scaling assumption for central. T.C.Awes, ORNL, QGP Workshop

  15. mT-scaling and h0/p0 Ratio • Compilation: • h0/p0(pT ) = 0.55±0.02 • WA98 mT-scaling of h0/p0 : • h0/p0 =0.55 • WA98 Central: • h0/p0(pT ) = 0.49±0.08±0.1 T.C.Awes, ORNL, QGP Workshop

  16. Direct g search: Peripheral Pb+Pb • First, consider g/p0 ratio and compare to calculated decay background g/p0 ratio. (many errors cancel) • Ng and Np measured at each pT for “all showers” condition (largest corrections - errors) • No significant g excess within statisticalerrors (low pT though) T.C.Awes, ORNL, QGP Workshop

  17. Direct g search: Central Pb+Pb • First, consider g/p0 ratio and compare to calculated decay background g/p0 ratio. (many errors cancel) • Ng and Np measured at each pT for “all showers” condition (largest corrections - errors) • Clear g excess beyond statistical errors - must consider systematical errors. T.C.Awes, ORNL, QGP Workshop

  18. Comparison of g Excess to Errors • Compare measured g yield to calculated background g. • Total systematicerrors include those of (g/p0)Meas (g/p0)Bkgd and the p0 fit uncertainty. • Significant excess g beyond sstat + ssyst for central collisions at high pT. T.C.Awes, ORNL, QGP Workshop

  19. Central Pb+Pb Direct g pT Spectrum • Compare to proton-induced prompt g results: • Assume hard process - scale with the number of binary collisions (=660 for central). • Assume invariant yield has form f(xT)/s2 where xT=2pT/s½ for s½-scaling. • Factor ~2 variation in p-induced results. • Similar g spectral shape for Pb case, but factor ~2-3 enhanced yield. • WA98 nucl-ex/0006007, PRL 85 (2000) 3595. T.C.Awes, ORNL, QGP Workshop

  20. Direct g: Comparison to pQCD Calculation • NLO pQCD calculations factor of 2-5 below s½ =19.4 GeV p-induced prompt g results. • But p-induced can be reproduced by effective NLO (K-factor introduced) if intrinsic kT is included. • Same calculation at s½=17.3 GeV reproduces p-induced result scaled to s½ =17.3 GeV • Similar g spectrum shape for Pb case, but factor ~2-3 enhanced yield. • WA98 nucl-ex/0006007, PRL 85 (2000) 3595. T.C.Awes, ORNL, QGP Workshop

  21. Photons - kT Broadening • pQCD-calculations • Fit intrinsic kT in pp (E704) (Q2 = (2pT)2) • kT - broadening in Pb+Pb • Magnitude “consistent“ with expectations from pA • Hard processes cannot explain excess at low pT • Dumitru et al., hep-ph/0103203. T.C.Awes, ORNL, QGP Workshop

  22. Direct g: Comparison to Model Calculations • Low pT upper limits are important • Imply number of d.o.f. > p’s (as WA80 S+Au g upper limit) • Many sources of theoretical uncertainty • intrinsic kT, pT broadening • preequilibrium • QM g rates: convergence • HM g rates: in-medium masses • Hydro evolution: flow • Need further experimental constraints: • dileptons (CERES,NA50) • pA g results (WA98) • Hadron spectra • Results from RHIC • Steffan & Thoma PLB 510 (2001) 98. T.C.Awes, ORNL, QGP Workshop

  23. Direct g: Expectations for RHIC & LHC • Photon production grows faster than average particle multiplicity • QM contribution dominates for pT>2-3 GeV/c • kT effects will be less important • Optimistic prospects • Preliminary look from PHENIX “No huge excess” • Steffan & Thoma PLB 510 (2001) 98. T.C.Awes, ORNL, QGP Workshop

  24. 1(1,pT1) 0 0  M=(pT1+pT2)  2(2,pT2) 00-Correlations • search for 00-correlations • 0 reconstruction only on statistical basis • measured by • correlation of decay  • all possible combinations • pair characterisation: , pseudo-mass M=pT1+pT2 • combinatorial background of uncorrelated pairs: event mixing T.C.Awes, ORNL, QGP Workshop

  25. 0 other decays “Signal“ -pairs Hijing p+C Real Mix real/mix: correlated pairs signal from back-to- back correlations 0.6< pT1+pT2 <1.2 GeV • Additional correlations from radiative decays • mainly0 • ,,‘,... • “signal“ = different parents • smaller opening angle with higher pT • pseudo-mass M=pT1+pT2  Real Mix 1.2< pT1+pT2 <1.8 GeV  T.C.Awes, ORNL, QGP Workshop

  26. Real Mix 0.<M<0.6 GeV 1.2<M<1.8 GeV 2.4 GeV < M p-C   Correlations in the Data • Strong correlation at =180º • gaussian shape • exponential increase with M • stronger effect in p+C T.C.Awes, ORNL, QGP Workshop

  27. HIJING   VENUS   0.6< M <1.2 GeV 1.8< M <2.4 GeV Comparison: VENUS & HIJING Real Mix p-C Real Mix No Hard- Scattering in Venus! T.C.Awes, ORNL, QGP Workshop

  28. Pair Correlations in Pb+Pb •  pair correlation • Single particle azimuthal Correlation w.r.t R.Plane • Pair Correlation function Real/Mix preliminary  • Extract v2 • No reaction plane used • No correction factors needed • Elliptic flow or Hard Scattering? T.C.Awes, ORNL, QGP Workshop

  29. Flow or Hard Scattering? reaction plane • Reaction plane from plastic ball detector • Correlation relative to reaction plane • 2ndg in-plane irregardless of 1stg • In-plane elliptic flow dominates Real/Mix • 1stg in-plane -> 2ndg in-plane  • 1stg out-plane -> 2ndg in-plane Real/Mix  T.C.Awes, ORNL, QGP Workshop

  30. Particle Production at High pT • Central reactions explained by • Hydrodynamical fits • pQCD Calculations • Expectations for centrality dependence: • Nuclear Enhancement • Cronin effect WA98, PRL 81 (1998) 4087 T.C.Awes, ORNL, QGP Workshop

  31. p+p  (Pb+Pb)per • Cronin effect • (Pb+Pb)med-cent  (Pb+Pb)cent • increase weaker than NColl • decreasing with pT • Different from HIJING • “Jet-quenching“ or Thermalization? • Compare to RHIC results p0 Scaling with NColl T.C.Awes, ORNL, QGP Workshop

  32. Summary • Direct photons in Pb+Pb • First observation in RHI • PRL 85 (2000) 3595, nucl-ex/0006008 • Photon excess compared to p+p • Thermal and/or pQCD origin? • Particle production at high pT • Azimuthal  correlations • Strong back-to-back correlation in p+A • Increasing strength with increasing pseudo-mass • Correlations seen in Pb+Pb, v2 increases with M • Elliptic flow dominates - Hard scattering also? • Centrality dependence of 0 production • Modification of Cronin effect • Implications for kT broadening and Energy loss T.C.Awes, ORNL, QGP Workshop

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