1  /e + e - arXiv: [nucl.th]

2

3 Sometime ago it was noted that: “The ratio of the production rates (  /  +  - ) and (  o,  /  +  - ) from quark gluon plasma is independent of the space time evolution of the fireball”. Universal Signal : Only a function of universal constants. (1) (2) (3) B.S. PLB 1983

4 B.S. PLB 1983 R   /  +  - = const( ,  s  q q Light from QGP   qq   +  - ~ T 4

5 Invariant yield of thermal photons can be written as i  Q  QGP M  Mixed (coexisting phase of QGP and hadrons) H  Hadronic Phase is the static rate of photon production  convoluted over the space time expansion. Thermal Photons

6 Thermal photons from QGP : using hard thermal loop approximation. Again, Resumming ladder diagrams in the effective theory Thermal photons from hadrons : (i)    (ii)    (with , , ,  and a 1, in the intermediate state) (iii)    (iv)   ,    and    & Similarly from strange meson sector

7 Rather similar to photons, dileptons can be efficient probe for QGP – again not suffering from final state interactions. One has to subtract out contributions from: (a) Drell–Yan process, (b) Decays of vector mesons within the life time of the fireball (c) Hadronic decays occurring after the freeze out. Invariant transverse momentum distribution of thermal dileptons (e + e - or virtual photons,  *): integrated over the invariant mass region: Dileptons

8 Dileptons from light vector mesons ( ,  ) &  (Hadronic Sector) : Consistent with e + e - V(  ) data f V (V) : coupling between electromagnetic current and vector meson fields m V and  V are the mass and width of the vector V and w 0 are the continuum threshold above which the asymptotic freedom is restored.

9 Isentropic expansion : Hydrodynamics takes care of the evolution of the transverse motion.

10 The number density as a function of temperature. Effect of mass modification and width modification is shown.

11 Photons at SPS

12 Thermal Photon reproduce WA98 data

13 Di-electrons at SPS

14 Photons at RHIC (J. Phys. G 2007, J. Alam, J. Nayak, P.Roy, A. Dutt-Mazumder, B.S.)

15 Thermal Photon reproduce PHENIX data

16 Di-electrons at RHIC

17 Photons at LHC

18 Di-electrons at LHC

19 RESULTS from the ratio: The variation of R em (the ratio of the transverse momentum spectra of photons and dileptons) has been studied for SPS, RHIC and LHC. We argue that simultaneous measurements of this quantity will be very useful to determine the value of the initial temperature of the system formed after heavy ion collisions. We observe that Rem reaches a plateau beyond PT=0.5 GeV. The value of R em in the plateau region depends on T i but largely independent of T c, v o, T f and the EOS.

20 Ratio (R em ) at SPS

21 Ratio (R em ) at RHIC

22 Ratio (R em ) at LHC

23 Ratio (R em ) for pQCD processes FILTERING OUT pQCD PHOTONS

24 arXiv: [nucl.th] Ratio (R em ) vs. Initial Temperature

25 OBSERVATIONS: 1.The medium effect on R em is negligibly small 2.Hydrodynamic effects such as viscosity, flow get sort of erased out by observing the ratio, R em 3.Equivalently, model dependent uncertainties also get cancelled out through R em 4.Contributions from Quark Matter increase with the increase of the initial temperature – a)thermal photons mostly for hadronic phase at SPS b)thermal photons from RHIC and more so from LHC originate from QGP 5.R em flattens out beyond p T ~ 0.5GeV 6.In the plateau region: R em LHC > R em RHIC >R em LHC

26 OBSERVATIONS, contd. WHY & HOW R em (in Born approx.) => At the end R em still remains by far and large model independent: SPS => RHIC => LHC Thus R em is a universal signal of QGP, model independent and unique.

27 We see that is a function of the universal constants and the temperature. Because of the slow (logarithmic) variation as with temperature, one can assume In an expanding system, however, R em involves the superposition of results for all temperatures from T i to T f, so the effective (average) temperature, T eff will lie between T i and T f and This explains: It is also interesting to note that for  s = 0.3, T=0.4GeV, (  M) 2 ~ 1 (M max =1.05, M min =0.28), we get: R s ~ 260. This is comparable to R em obtained in the present calculation.

28 WHAT DO WE EXPECT at LHC

29 Photons and di-electrons in the ALICE experiment PHOS: Photons TRD: Electron-pairs

30 Muon chambers PMD Modules PMD photons PMD photons MUON arm  -pairs MUON arm  -pairs

31  /e + e - as well as     at the Large Hadron Collider LOOKING FORWARD TO THE VERIFICATION OF THE UNIVERSAL SIGNATURE: