Low mass dilepton production in relativistic heavy ion collisions

Mechanisms of formation for low mass dilepton excess observed in the relativistic heavy ion collisions are considered. The experimental data are reviewed. In addition to discussing the standard mechanisms of dilepton production specific to the collisions of relativistic nuclei (the pion annihilation in the hadron-gas stage and the quark-antiquark annihilation in the quark-gluon phase), the mechanism of dilepton production in the mixed phase of nuclear matter is proposed, and its contribution to the low mass dilepton spectrum is estimated. In addition, the first-order corrections in the strong-interaction coupling constant to the dilepton production in the parton medium and the nonperturbative approaches are considered.     

Drell-Yan dilepton production in relativistic heavy-ion collisions at RHIC

We investigate the lepton pair production with the Drell-Yan process in relativistic heavy ion collisions by computing the double differential cross section dσ/dM²dy and dσ/dM²dx_F at the next-to-leading order in p+Au and Au+Au collisions with √s_NN =200 GeV at RHIC. The resulting nuclear modification factors R^pAu and R^AuAu show strong sensitivity to the cold nuclear matter (CNM) effects and could probe the CNM effects at a very wide region of the longitudinal momentum fraction x. The variation of R with the invariant mass M, the rapidity y and the Feynman variable x_F is shown and we find that the nuclear modification factor for the double differential cross section could be smaller than 0.4 in some kinematic regions of high-energy nucleus-nucleus reactions at RHIC.     

A clear-cut test of low mass dilepton production mechanism in hadronic collisions

It is pointed out that the dependence of the mean number of low mass dileptons per event on the multiplicity of charged hadrons can disentangle various mechanisms contributing to the low mass dilepton production. Specific predictions are presented for the contribution due to annihilations of quarks and antiquarks created during the collision.     

Prompt contributions to the dilepton yield in heavy ion collisions

The mass spectrum is calculated for those dileptons which are produced in the early phase of a heavy ion collisions via the direct production NN → l⁺l⁻X and via the Compton process GN → l⁺l⁻X with prompt gluons radiated in preceding NN interactions. Both mechanisms produce a mass spectrum which decreases steeply with invariant mass of the l⁺l⁻ pair and which is below the CERES data for Pb-Au collisions by about one order of magnitude. Received: 1 December 1999 / Revised version: 18 February 2000     

Large transverse momentum dilepton production in heavy ion collisions with two-photon processes

The cold component of large transverse momentum dilepton production via semi-coherent two-photon interaction is calculated. The cold contribution is essential to the dilepton spectra in the soft region for different mass bins. The results are compared with the PHENIX experimental data at RHIC, and we find that the modification of semi-coherent two-photon processes is more evident with the rising dilepton mass bins.     

A thermodynamical approach to hadron production ine + e − collisions

The hadron production ine ⁺ e ^? collisions is studied assuming that particles originate in a hadron gas at thermal and chemical equilibrium. The parameters of the hadron gas are determined with a fit to the average multiplicities of various hadron species measured at LEP and PEP-PETRA. A strikingly good agreement is found between the predictions of this model and data for almost all particles over a range of production rate of four orders of magnitude. The methods and assumptions of the fit are described and discussed in detail. The temperature of the hadron gas estimated from the fit is around 166 MeV at √s=91.2 GeV and 181 MeV at √s=29÷35 GeV.     

A simple geometrical approach to particle production in collisions with nuclei

It is argued that hadron collisions with nuclei are similar to hadron-hadron collisions, having similar properties for the impact parameter distributions and the leading particle spectra. The relevant existing high-energy data, including the universality of multiplicity distributions and the possibility of geometrical scaling in reactions with nuclei, are easily understood in the framework of geometrical models by extending to p-nucleus collisions what was learnt about impact parameter and leading particles in pp collisions. The questions of forward-backward correlations and photo- and electroproduction are briefly discussed.     

High-twist contribution to large-p T dilepton production

The high-twist contribution to large-transversemomentum lepton pair production in pion-hadron collisions is investigated. In calculations, various pion model wave functions are used. The high-twist contribution is shown to depend on the choice of wave functions and to exceed the leading one by an order of magnitude in the phase space boundary.     

A pQCD-based approach to parton production and equilibration in high-energy nuclear collisions

A pQCD-based model for parton production and equilibration in ultrarelativistic heavy-ion collisions is reviewed. The model combines pQCD processes including initial and final state radiations together with string phenomenology for nonperturbative soft processes. Nuclear effects on the initial parton production, such as multiple parton scattering and nuclear shadowing of parton distribution functions are considered. Comparisons with existing data are made and further tests of the model to constrain model parameters are proposed. With the obtained space-time history of the parton production, evolution of the minijet gas toward a fully equilibrated parton plasma is studied. Direct probes of the early parton dynamics, such as pre-equilibrium photon and dilepton production, open charm production, J/ψ suppression and jet quenching are also reviewed.     

Baryonic channels of dilepton production

A calculation of the Dalitz-decay width of baryon resonances is presented and applied to dilepton production in proton-proton collisions in the BEVALAC/SIS energy range. The results indicate, that for these energies the contribution of Dalitz decay of higher baryon resonances (i.e. other that the δ(1232)) are negligible.     

Influence of In-medium properties of vector mesons on dilepton production in heavy ion collisions at SIS energies

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy ion collisions. They allow the study of meson properties, like mass and decay width, at various density and temperature regimes. Up to now, in the Tübingen model for dilepton production, modification of meson properties in nuclear medium has been accounted for by allowing a density dependence of the mass (Brown–Rho scaling) together with an ad hoc dependence of the meson decay widths on the same variable. We use the extended vector meson dominance (eVMD) model to extract meson properties in nuclear matter by computing the in-medium meson spectral functions. Dilepton spectra for C+C at 1.0 and 2.0 AGeV are calculated and compared with previous results.     

Open charm contribution to dilepton spectra produced in nuclear collisions at SPS energies

Measurements of open charm hadro-production from CERN and Fermilab experiments are reviewed, with particular emphasis on the absolute cross sections and on their A and √s dependences. Differentialp _T andx _F cross sections calculated with thePythia event generator are found to be in reasonable agreement with recent data. The calculations are scaled to nucleus-nucleus collisions and the expected lepton pair yield is deduced. The charm contribution to the low mass dilepton continuum observed by the CERES experiment is found to be negligible. In particular, it is shown that the observed low mass dilepton excess in S-Au collisions cannot be explained by charm enhancement.     

On the migdal-watson approach to FSI effects in meson production in NN collisions

The influence of the nucleon-nucleon final-state interaction (FSI) on properties of the meson-production amplitude near threshold is discussed. For nucleon-nucleon interaction, a simple Yamaguchi potential and realistic potential models are considered. It is shown that FSI effects cannot be factorized from the production amplitude. The absolute magnitude of FSI effects depends on the momentum transfer (or on the mass of the produced meson) and hence is not universal. Only in the case of the production of rather heavy mesons like η′ or φ FSI do effects become universal. The Jost function approach to FSI effects is critically examined.