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Hard Photon Production in a Chemically Equilibrating QGP at Finite Baryon Density Zejun He Zejun He Shanghai Institute of Applied Physics Research Chinese Academy of Sciences

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I. INTRODUCTION Lattice QCD results show: hadronic matter undergoes a phase transition into QGP. RHIC and LHC provide the opportunity to study the formation and evolution of the QGP. QGP exists: for several fm, in about 100 fm3. Indirect signatures have to be used for its detection, such as dileptons strangeness, and photon. The QGP is regarded as a thermodynamic equilibrium system, dileptons are suppressed with increasing the initial quark chemical potential. In recent years, some authors indicate:

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The partons suffer many collisions in ( :0.3~0.7fm), the system may attain kinetic equilibrium, but away from the chemical equilibrium. From this, K.Geiger, T.S.Biro et al have studied the effect of the chemical equilibration on the dilepton production in baryon-free QGP. N.Hammon, K.Geiger indicated: the initial system has finite baryon density. C.Gale et al. have discussed the dileptons from QGP with finite baryon density. Thus one may study the effect of the chemical equilibration on the dilepton production in a QGP with finite baryon density.

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Distributions of partons in a chemically non-equilibrated system: Jüttner distributions: for quarks (anti-quarks) for gluons Boltzmann form error of the order of 40% Factorized distribution for quarks (anti-quarks) for gluons T.S.Biro et al. pointed out: the calculated thermal screening mass in the intermediate region of the the deviation from that calculated via the Jüttner distributions quite large.

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Nucl. Phys. A724 (2003) 477; Phys. Rev. C68 (2003) 042902; Phys. Rev. C69 (2004) 034906; Chin. Phys. Lett. 20 (2003) 836; Chin. Phys. Lett. 21 (2004) 795; 《物理学报》. 52 (2003) 145. In this work we mainly study the PHOTON PRODUCTION. From Jüttner distribution, studied the evolution of the chemically equilibrating QGP (CEQGP) system with finite baryon density, and calculate the dilepton yields from processes:,, and Compton-like, and discuss strangeness for and.

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II. EVOLUTION OF THE SYSTEM A.THERMODYNAMIC RELATIONS OF THE SYSTEM Expanding densities of quarks (antiquarks) (1) over quark chemical potential, to get the baryon density (2)

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and corresponding energy density including the contribution of s quarks (3) and : degeneracy factors of quarks (antiquarks) and gluons, the integral factors appearing in the above: (4) the integration related to the mass of the s quark.

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B. EVOLUTION EQUATION OF THE SYSTEM Considering chemical equilibration processes:,, and, taking, combining the master equations together with equation of energy- momentum conservation and of baryon number conservation, one can get a set of coupled relaxation equations describing evolutions of the T, and for quarks and for gluons on the basis of the above thermodynamic relations of the system with finite baryon density (5)

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(6) (7) (8)

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(9) is at,,, and The gluon, quark and g-s production rates, and are: (10) (11) (12) (13) : the Debye screening mass,, and the function of and are from thermal masses of quarks and s quarks, respectively.

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III. PHOTON PRODUCTION Considering Compton scattering, and annihilation, we have rate: taking (14) thus (15)

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For Compton scattering (16) should be a quantity, or IV. CALCULATED RESULTS AND DISCUSSIONS

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Fig 1: The calculated evolution paths of the system in the phase diagram.

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Fig 2: The calculated equilibration rates.

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Fig 4: Photon spectra

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V. CONCLUSION (1)The photon production is heightened with increasing the quark chemical potential; (2) The Photon production sensitively depends on the initial conditions of the QGP system; (3) The photon production of a thermodynamic equilibrium QGP system is much faster than that of a chemically equilibrating system. Thus from the production we can understand the thermodynamic properties of the QGP. The increase of the initial quark chemical potential will change the hydrodynamic behavior of the system to cause both the quark phase life-time to increase and the evolution path of the system in the phase diagram to become even longer. These effects are to heighten the photon yield of these three processes to compensate the photon suppression of the process.

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Thank you!!

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