Thermal Dilepton Research Articles

Thermal dilepton emission rate in terms of spectral functions

We interpret the dilepton emission rate from a hadronic gas expected to be produced in heavy ion collisions in terms of the spectral functions available from the e + e − annihilation and the τ lepton decay experiments. We take into account all possible hadronic states especially the multi-pion contributions to the dilepton emission and the parity mixing phenomenon due to the soft final-state corrections. A new compilation of the experimental data for the spectral functions is presented.

Rapidity dependence of thermal dileptons resulting from hadronizing quark-gluon matter with finite baryon charge

The influence of a non-vanishing baryon charge on the rapidity distribution of dileptons produced in ultrarelativistic heavy-ion collisions is studied. We employ a frozen motion model with scaling invariant expansion of the hadronizing quark-gluon plasma as well as a realistic rapidity distribution of secondary particles (i.e., pions and baryons) expected for RHIC energies. We demonstrate a sizeable suppression of the thermal dilepton yield at large rapidities due to the finite baryon density. This affects the most favorable rapidity window for using dileptons for the diagnostic of the quark-gluon plasma and the early temperature distribution of the hot reaction zone. To discriminate the thermal dileptons from Drell-Yan background we propose to utilize the dilepton yield scaled suitably by the pion multiplicity as function of rapidity.

Rapidity distributions of dileptons from a hadronizing quark-gluon plasma.

It has been predicted that dilepton production may be used as a quark-gluon plasma probe. We calculate the rapidity distributions of thermal dileptons produced by an evolving quark-gluon plasma assuming a longitudinal scaling expansion with initial conditions locally determined from the hadronic rapidity density. These distributions are compared with Drell-Yan production and semileptonic charm decays at invariant mass $M=2,4, \mathrm{and} 6$ GeV.

Chiral symmetry and dileptons in heavy ion collisions

We study the consequences of chiral symmetry restoration for thermal production of dileptons in central nucleus-nucleus collisions. We discuss the temperature dependence of various hadronic parameters entering the dilepton cross-section, putting emphasis on universal properties in the vicinity of a 2nd order chiral symmetry restoring phase transition and the constraints on their temperature dependence known from lattice simulations. It is shown that the modifications of the hadron spectrum due to medium effects can lead to important changes of the thermal dilepton production rate in heavy ion collisions. In particular, it can imply the suppression of a thermalp peak.

Status of Quark-Gluon Plasma

I review the current status of the experimental searches for a Quark-Gluon Plasma in heavy ion collisions. The brief introduction covers important results of finite temperature lattice QCD and the intuitive Bjorken picture of Quark-Gluon Plasma (QGP) formation. Global features of the data, such as transverse energy distributions and the various signatures of QGP such asJ/ϕ-suppression, ψ-enhancement as well as thermal dileptons and photons are then discussed.

Lepton pair production in relativistic heavy ion collisions at CERN SPS energies

The production of lepton pairs as probes for the nuclear collision dynamics at CERN SPS energies is discussed in some detail. We review the present theoretical status of different dilepton sources. Most probably thermal dileptons from the quark gluon plasma, if such a state is formed at all in present CERN heavy ion collision experiments, are covered under a huge background of lepton pairs from meson decays at low invariant masses and Drell-Yan pairs at higher invariant masses. However, dileptons, and in particular muon pairs look promising as probes for the hadron dynamics of the nuclear collision process. We emphasize the implications of non-equilibrium pion dynamics for the production of lepton pairs. We find a strong enhancement in the low invariant mass region, 2 m π ≤ M μ + μ − ≤ 600 MeV, for lepton pairs which are produced via annihilation of pions in the hot and dense collision zone. Various sources of background which could mask this enhancement are discussed in some detail. We present and compare calculations for dilepton mass spectra based on different nuclear collision scenarios proposed so far. Finally we show that the dilepton yields from the low mass vector mesons contain valuable information on the lifetime of the hadronic phase.

Emission of thermal dileptons in ultrarelativistic heavy-ion collisions

The emission of thermal dileptons from a quark-gluon plasma and hot pion gas formed in a collision of heavy nuclei, is studied in the framework of hydrodynamics. Thermal rates are compared with the estimates of rates for Drell-Yan production. Dependence on the mass number and impact parameter of colliding nuclei, and the collision energy is considered.

Is the thermal dilepton spectrum a signature for diagnosing first-order detonation QCD phase transition?

Detonation has two different scenarios: the Gyulassy's scenario and the Kapusta's one. The dilepton production rate in the Kapusta's scenario is calculated. Associated with the Gyulassy's scenario, the peculiar property of the spectrum and its dynamical origin in the detonation phase transition are discussed and analyzed. Its spectrum is not a signature for diagnosing the detonation phase transition.

Production of thermal dileptons in high energy nucleus-nucleus collisions

A method for including realistic nuclear geometry and impact parameter effects in computations of production rates of thermal dileptons of mass in the 1.5 to 3 GeV range andy≈0 in nucleus nucleus collisions at\(\sqrt s = 20 - 200\) GeV is given. A comparison with the Drell-Yan rate indicates that for large nuclei and energies thermal production gives a sizable contribution even atM=MJψ and thus may contribute significantly toJ/ψ suppression as background enchancement.

Evaporation of hadrons with largeP T and dilepton emission from the mixed phase of QCD matter in ultrarelativistic nuclear collisions

To probe the mixed phase of QCD matter we study thermal dilepton emission and evaporation of hadrons with largeP T in the framework of hydrodynamical model for ultrarelativistic nuclear collisions.

Photon and dilepton emission from the quark-gluon plasma: Some general considerations.

The emission rates for photons and dileptons from a quark-gluon plasma are related to the thermal expectation value of an electromagnetic current-current correlation function. This correlation function possesses an invariant-tensor decomposition with structure functions entirely analogous to ${W}_{1}$ and ${W}_{2}$ of deep-inelastic scattering of leptons from hadronic targets. The thermal scaling properties of the appropriate structure functions for thermal emission are derived. The thermal structure functions may be computed in a weak-coupling expansion at high plasma temperature. The rates for thermal emission are estimated, and for dileptons, using conservative estimates of the plasma temperature, the thermal-emission process is argued to dominate over the Drell-Yan process for dilepton masses 600 MeV<M<1--2 GeV. We argue that higher temperatures are entirely possible within the context of the inside-outside cascade model of matter formation, perhaps temperatures as high as 500--800 MeV. If these high temperatures are achieved, the maximum dilepton masses arising from thermal emission are argued to be 5 GeV. Pre-equilibrium emission might dominate over Drell-Yan emission at somewhat higher masses. Signals for thermal emission are presented as the relative magnitude of invariant thermal structure functions, thermal scaling relations, and transverse momenta of thermal dilepton pairs which increase with and are proportional to the dilepton-pair mass. The transverse-mass spectrum is shown to be dN/${\mathrm{dM}}^{2}$dy ${d}^{2}$${q}_{\ensuremath{\perp}}$\ensuremath{\propto}${M}_{\ensuremath{\perp}}$${\mathrm{}}^{\mathrm{\ensuremath{-}}6}$ and upon integrating over transverse momentum \ensuremath{\propto}${M}^{\mathrm{\ensuremath{-}}4}$, for a high-temperature plasma. The spectrum is power law, not exponential. The dependence of the spectrum of thermal emission upon the existence of a first-phase transition is studied, and the possibility that the spectrum might change its slope as a function of ${M}_{\ensuremath{\perp}}$ or have a sharp break is pointed out. We argue that if there is a first-order phase transition, as beam energy or nuclear baryon number is raised through the threshold for production of a plasma, the rate for photon or dilepton emission might dramatically increase. In the case of a first-order phase transition, in addition to the power-law spectrum of transverse mass, there is an additional contribution of ${e}^{\mathrm{\ensuremath{-}}{M}_{\ensuremath{\perp}}/T}$, where T is the phase-transition temperature.