Hadronic Production of Thermal Photons

We study the thermal emission of photons from hot and dense strongly interacting hadronic matter at temperatures close to the expected phase transition to the Quark-Gluon Plasma (QGP). Earlier calculations of photon radiation from ensembles of interacting mesons are re-examined with additional constraints, including new production channels as well as an assessment of hadronic form factor effects. Whereas strangeness-induced photon yields turn out to be moderate, the hitherto not considered t-channel exchange of omega mesons is found to contribute appreciably for photon energies above ~1.5 GeV. The role of baryonic effects is assessed using existing many-body calculations of lepton pair production. We argue that our combined results constitute a rather realistic emission rate, appropriate for applications in relativistic heavy-ion collisions. Supplemented with recent evaluations of QGP emission, and an estimate for primordial (hard) production, we compute photon spectra at SPS, RHIC and LHC energies.Comment: 19 pages, 13 figures, 1 appendix. Discussion added, improved parameterisation

Electromagnetic radiation from nuclear collisions at RHIC energies

The hot and dense strongly interacting matter created in collisions of heavy nuclei at RHIC energies is modeled with relativistic hydrodynamics, and the spectra of real and virtual photons produced at mid-rapidity in these events are calculated. Several different sources are considered, and their relative importance is compared. Specifically, we include jet fragmentation, jet-plasma interactions, the emission of radiation from the thermal medium and from primordial hard collisions. Our calculations consistently take into account jet energy loss, as evaluated in the AMY formalism. We obtain results for the spectra, the nuclear modification factor (R_AA), and the azimuthal anisotropy (v_2) that agree with the photon measurements performed by the PHENIX collaboration at RHIC

High Momentum Dilepton Production from Jets in a Quark Gluon Plasma

We discuss the emission of high momentum lepton pairs in central Au+Au collisions at RHIC ($\sqrt{s_{NN}}=$200 GeV) and Pb+Pb collisions at the LHC ($\sqrt{s_{NN}}=$5500 GeV). Yields of dileptons produced through interactions of jets with thermal partons have been calculated, with next-to-leading order corrections through hard thermal loops (HTL) resummation. They are compared to thermal dilepton emission and the Drell-Yan process. A complete leading order treatment of jet energy loss has been included. Jet-plasma interactions are found to dominate over thermal dilepton emission for all values of the invariant mass $M$. Drell-Yan is the dominant source of high momentum lepton pairs for $M >$ 3 GeV at RHIC, after the background from heavy quark decays is subtracted. At LHC, the range $M<7$ GeV is dominated by jet-plasma interactions. Effects from jet energy loss on jet-plasma interactions turn out to be weak, but non-negligible, reducing the yield of low-mass dileptons by about 30%.Comment: 20 pages, 12 figures, discussions adde

High momentum lepton pairs from jet-plasma interactions

We discuss the emission of high momentum lepton pairs (p_T>4 GeV) with low invariant masses (M << p_T) in central Au+Au collisions at RHIC (\sqrt{s_{NN}}=200 GeV). The spectra of dileptons produced through interactions of quark and antiquark jets with the quark-gluon plasma (QGP) have been calculated. Annihilation and Compton scattering processes, as well as processes benefitting from collinear enhancement, including Landau-Pomeranchuk-Migdal (LPM) effects, are calculated and convolved with a one dimensional hydrodynamic expansion. The jet-induced contributions are compared to thermal dilepton emission and Drell-Yan processes, and are found to dominate around p_T=4 GeV.Comment: Parallel talk given at QM2006, Shanghai November 2006. 4 pages, 3 figure

Energy Loss of Leading Hadrons and Direct Photon production in Evolving Quark-Gluon Plasma

We calculate the nuclear modification factor of neutral pions and the photon yield at high p_T in central Au-Au collisions at RHIC (\sqrt{s}=200 GeV) and Pb-Pb collisions at the LHC (\sqrt{s}=5500 GeV). A leading-order accurate treatment of jet energy loss in the medium has been convolved with a physical description of the initial spatial distribution of jets and a (1+1) dimensional expansion. We reproduce the nuclear modification factor of pion R_{AA} at RHIC, assuming an initial temperature T_i=370 MeV and a formation time \tau_i=0.26 fm/c, corresponding to dN/dy=1260. The resulting suppression depends on the particle rapidity density dN/dy but weakly on the initial temperature. The jet energy loss treatment is also included in the calculation of high p_T photons. Photons coming from primordial hard N-N scattering are the dominant contribution at RHIC for p_T > 5 GeV, while at the LHC, the range 8<p_T<14 GeV is dominated by jet-photon conversion in the plasma.Comment: 21 pages, 16 figures. Discussions and references added. New figure includind photon dat

Photon production in relativistic nuclear collisions at SPS and RHIC energies

Chiral Lagrangians are used to compute the production rate of photons from the hadronic phase of relativistic nuclear collisions. Special attention is paid to the role of the pseudovector a_1 meson. Calculations that include reactions with strange mesons, hadronic form factors and vector spectral densities consistent with dilepton production, as well as the emission from a quark-gluon plasma and primordial nucleon-nucleon collisions, reproduce the photon spectra measured at the Super Proton Synchrotron (SPS). Predictions for the Relativistic Heavy Ion Collider (RHIC) are made.Comment: Work presented at the 26th annual Montreal-Rochester-Syracuse-Toronto conference (MRST 2004) on high energy physics, Montreal, QC, Canada, 12-14 May 2004. 8 pages, 3 figure

Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration

Extensive experimental data from high-energy nucleus-nucleus collisions were recorded using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The comprehensive set of measurements from the first three years of RHIC operation includes charged particle multiplicities, transverse energy, yield ratios and spectra of identified hadrons in a wide range of transverse momenta (p_T), elliptic flow, two-particle correlations, non-statistical fluctuations, and suppression of particle production at high p_T. The results are examined with an emphasis on implications for the formation of a new state of dense matter. We find that the state of matter created at RHIC cannot be described in terms of ordinary color neutral hadrons.Comment: 510 authors, 127 pages text, 56 figures, 1 tables, LaTeX. Submitted to Nuclear Physics A as a regular article; v3 has minor changes in response to referee comments. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.htm

Electromagnetic radiation from matter under extreme conditions

The subject of this thesis is the production of electromagnetic radiations during relativistic heavy ions collisions. Since they constitute one of the major ways to probe the presence of a quark-gluon plasma (QGP), their evaluation through theoretical models is very important. The photon production at low-to intermediate transverse momentum (pT) is first studied. The photon production rate in a mesonic gas is evaluated within a massive Yang-Mills (MYM) approach. Earlier calculations are reexamined with additional constraints, including new production channels and with the inclusion of form-factors. Adding primordial N-N contribution and existing baryonic and QGP production rates, we can reproduce the photon spectra observed at the Super Proton Synchrotron (SPS). The intermediate to high-p T region is dominated by the physics of jets. A treatment, complete to leading-order in the strong coupling, is used to calculate energy loss in the strongly interacting medium. This approach is convolved with a physical description of the initial spatial distribution of jets and with an expansion of the emission zone. The role played by jet-plasma interactions is highlighted, showing that they dominate in the range 2 &lt; p T &lt; 4 GeV, at the Relativistic Heavy Ion Collider (RHIC). This mechanism has an important impact on both the total photon yield and the photon azimuthal asymmetry, turning the coefficient v 2 negative. Finally, the dilepton production at high p T is calculated with hard-thermal loops (HTL) effects, showing, that in perfect analogy with real photons, jet-plasma interactions also dominate the dilepton yield around pT = 4 GeV