A reanalysis of single photon data at CERN SPS

1. A reanalysis of single photon data at CERN SPS Charles Gale McGill University, Montreal in collaboration with RupaChatterjee, D. K. Srivastava, & S. Jeon

2. Electromagnetic Probes Penetrating probes are emitted at all stages then survive unscathed (ae <<as). “Historians” of the heavy ion collision: encode all sub-processes at all times A jet passing through QGP Different processes: different characteristic spectra

3. Direct PhotonsDifferent Sources - Different Slopes Rate Photons are result of convolutions of the emissions from the entire history of the nuclear collision, so we need rates & a model for evolution. Hadron Gas Thermal Tf QGP Thermal Ti “Pre-Equilibrium”? • Hydrodynamics. • Cascades. • Fire-balls. • Cascade+Hydro. “New” Jet Re-interaction √(Tix√s) pQCD Prompt x√s Eg

4. Upper Limit of Single Photons, WA80 Pb+Pb@SPS Ruled out hadronic gas with limited hadrons: p, r, w, & h. Srivastavaand Sinha, PRL 73 (1994) 2421; Dumitruet al., PRC 51 (1995) 2166. Sollfrank et al., Lee & Brown, Arbex et al., . Cleymans, Redlich, & Srivastava, PRC 55 (1997) 1431. However, nhad >> 2-3 /fm3 ! For No Phase Transition.

5. Direct photon production in 158 A GeV Pb+Pb collisions at CERN SPS •  First observation of single photons in relativistic heavy ion collisions by WA98. •  Important milestone in our search for the quark-hadron phase transition and a testimony to the grit and determination of the chairman of this conference. •  Photon invariant yield or upper limit as a function of pT in the interval 0.5 < pT< 4.0 GeV/c presented. • Significant direct photon excess observed at pT > 1.5 GeV/c in central collisions. Srivastava & Sinha, PRC 64, 034902 (2001). Alamet al., PRC 63, 021901 (2001) Huovinenet al., PLB 535, 109 (2002). Turbideet al., PRC 69, 014903 (2004). M. M. Aggarwalet al., WA98 Collaboration, PRL 85, 3595 (2000).

6. Prompt photons using NLO pQCD and comparison with experimental data • Prompt photons for pp collisions at s= 19.4 GeV. • E704 experiment results for pp collisions and those estimated from p+12C collisions by E629 and NA3 experiments are also given for a comparison. • The factorization, renormalization, and fragmentation scales all equal to pT/2. Effect of parton shadowing and iso-spin on prompt photon production Iso-spin corrected prompt photons at different collider energies as function of xT. Iso-spin as well as shadowing corrected prompt photons at different energies. Shadowing and iso-spin corrected prompt photons using NLO pQCD at sNN = 17.3 GeV, which corr-esponds to the nucleon nucleon centre of mass energy for the WA98 experiment.

7. Central collision. t0from 0.2 to 1.0 fm/c. Flow patterns Results differ marginally beyond 1 fm/c. Time evolution of temperature Total entropy & net baryon fixed for all t0. Energy density Time evolution of average effective temperature Radial flow velocity Effective temperature :

8. Sources: Electromagnetic radiation from QCD First approaches: McLerran, Toimela (1986); Kajantie, Kapusta, McLerran, Mekjian (1986) Baier, Pire, Schiff (1988); Altherr, Ruuskanen (1992) Rates diverge: HTL resummation

9. AMY, Arnold, Moore, and Yaffe, JHEP 12, 009 (2001); JHEP 11, 057 (2001): incorporates LPM; photon rates complete to leading order in αs Going to two loops: Aurenche, Kobes, Gelis, Petitgirard (1996) Aurenche, Gelis, Kobes, Zaraket (1998) Co-linear singularities: Can be expressed in terms of the solution to a linear integral equation Hadrons from the confined sector: a similar treatment

10. t0 = 0.2 fm/c t0 = 1.0 fm/c • Single photon, • Pb+Pb@SPS • 0-10% most • central. • WA98 • Centrality dependent • Hydrodynamics with well tested EOS. • Quantitative explanation of the data by multiplying prompt yield with a K factor (Cronin effect ?) • t0 (fm) K • 0.2 2.7 • 0.4 4.8 • 0.6 5.4 • 0.8 5.7 • 1.0 5.9

11. t0 = 0.8 fm/c t0 = 0.6 fm/c t0 = 0.4 fm/c

12. Spectra & elliptic flow for pions at different t0. Spectra and elliptic flow parameter v2 for pions from Pb+Pb collisions having b = 7 fm @ CERN SPS for different initial times keeping entropy fixed.  p spectra are flatter for smaller t0. For pT 1 GeV, spectra are indifferent to change in t0.  Elliptic flow of p is insensitive to the initial thermalization time. similar results are observed for other hadrons.

13. pT dependent elliptic flow at different t0 • v2 for single photons. Results for pT< 2.15 GeV • are obtained by arbitrarily using the normalizing factor • at pT = 2.15 GeV. • Thermal photon v2 for different t0. Results for photons from hadronic matter are also given.

14. Summary & conclusions • Single photon data from relativistic collisions of Pb nuclei @ CERN SPS (measured by WA98) have been re-analyzed. • Iso-spin and shadowing corrected NLO pQCD treatment is used for prompt photon production along with an optimized scale for factorization, renormalization, and fragmentation. • For thermal part centrality dependent hydrodynamic model with a well tested EOS has been used considering a set of initial thermalization times. • The data can be explained well at different t0 using varying multiplicative factors for prompt photons. • Photon v2 is sensitive to initial thermalization time and can be used to estimate the value of t0.

15. Thank you