Chatterjee, R.

Eskola, K. J.

Holopainen, H.

Renk, T.

GSI Repository

Centrality dependence of the emission of thermal photons from fluctuatinginitial conditions∗H. Holopainen1, R. Chatterjee2, T. Renk2,3, and K. J. Eskola2,31FIAS, Frankfurt, Germany; 2University of Jyva¨skyla¨, Jyva¨skyla¨, Finland; 3Helsinki Institute of Physics, Helsinki,FinlandEvent-by-event fluctuations in the initial density profilein the hydrodynamical modeling of ultrarelativistic heavyion collisions have been shown to enhance the productionof thermal photons compared to a smooth initial profiles[1]. This enhancement is originating from the fact thathot spots in the fluctuating initial conditions produce muchmore photons with transverse momenta pT > 1 GeV dueto strong temperature dependence of the photon emissionrates.In this work the event-by-event fluctuating initial statesare obtained from Monte Carlo Glauber model and densityis distributed around the positions of the wounded nucleonsusing 2-dimensional Gaussians with width σ. These initialstates are then evolved with (2+1)-dimensional ideal hy-drodynamics [2] and thermal photon emission is obtainedby integrating over the whole medium using thermal emis-sion rates [3, 4]. The switch from plasma emission rateto hadron gas emission rate is assumed to happen instanta-neously at temperature Ts = 170 MeV.In the figure 1 we have plotted the pT -spectra of ther-mal photons in Au+Au collisions at √sNN = 200 GeVfor different centralities. We can see from the figure thatthe enhancement is more prominent in the peripheral col-lisions: the exponential slope of the spectra in the region2 < pT < 4 GeV is 10%, 12% and 16% flatter for fluc-tuating initial conditions at 0–20%, 20–40% and 40–60%centrality bins respectively [5]. This shows that the relativeimportance of the hot spots increases towards peripheralcollisions, where the number of sources, i.e. the number ofparticipants, is smaller.1 2 3 4 5 6pT (GeV/c)10-810-710-610-510-410-310-210-1100dN/d2 pTdY  (GeV-2  c3)  0-20% Fluctuating IC  0-20% Smooth IC20-40% Fluctuating IC20-40% Smooth IC40-60% Fluctuating IC40-60% Smooth ICThermal Photons200 AGeV Au+Au@RHICσ=0.4 fm, τ0=0.17 fm/cFigure 1: Transverse momentum spectra of thermal pho-tons from smooth and fluctuating initial conditions at RHICfor different collision centralities.The initial time τ0 used in these calculations should also∗Work supported by Extreme Matter Institute (EMMI).vary as a function of centrality – it should be larger formore peripheral collisions. This was studied at LHC ener-gies and we found out that the larger initial times in moreperipheral collisions lead to smaller photon yield at largetransverse momenta. However, the net effect from densityfluctuations and initial time centrality dependence is posi-tive, i.e. the photon production is enhanced relative to thesimpler smooth initial state calculation [5].In figure 2 is plotted the ratio of thermal photons fromsmooth and fluctuating initial conditions in 0–20% cen-tral collisions. Here we have compared the calculationsat RHIC and LHC energies and we found out that theenhancement is larger at smaller energies. This suggeststhat photon measurements at even lower energies than√sNN = 200 GeV could be more sensitive to the initialstate density fluctuations.1.0 2.0 3.0 4.0 5.0 6.0pT  (GeV/c)1.01.52.02.5dN(Fluctuating IC)/dN(Smooth IC)  200 AGeV Au+Au@RHIC, τ0=0.17 fm/c 2.76 ATeV Pb+Pb@LHC, τ0=0.14 fm/cThermal Photons, σ=0.4 fmRHIC LHCFigure 2: The ratio of thermal photon spectra with fluctuat-ing and smooth initial conditions for 0–20% centrality binin Au+Au collisions at √sNN = 200 GeV and in Pb+Pbcollisions at √sNN = 2.76 TeV.References[1] R. Chatterjee, H. Holopainen, T. Renk and K. J. Eskola, Phys.Rev. C 83 (2011) 054908.[2] H. Holopainen, H. Niemi and K. J. Eskola, Phys. Rev. C 83(2011) 034901.[3] P. B. Arnold, G. D. Moore and L. G. Yaffe, JHEP 0112 (2001)009.[4] S. Turbide, R. Rapp and C. Gale, Phys. Rev. C 69 (2004)014903.[5] R. Chatterjee, H. Holopainen, T. Renk and K. J. Eskola, Phys.Rev. C 85 (2012) 064910.GSI SCIENTIFIC REPORT 2012 PHN-NQM-THEORY-15121

Centrality dependence of the emission of thermal photons from fluctuating initial conditions

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Centrality dependence of the emission of thermal photons from fluctuating initial conditions

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