Low-mass Dilepton Enhancement Research Articles

Enhancement of low-mass dileptons in ultraperipheral collisions

It is shown that production of low-mass e^+e^--pairs in ultraperipheral nuclear collisions is enhanced due to the Sommerfeld–Gamow–Sakharov (SGS) factor. This effect is especially strong near the threshold of creation of unbound e^+e^--pairs with low masses in the two-photon fusion. Coulomb attraction of the non-relativistic components of such pairs may lead to the increased intensity of 511 keV photons. It can be recorded at the NICA collider and has some astrophysical implications. The analogous effect can be observed at LHC in dilepton production.

Low mass enhanced probability of pion in hadronic matter due to its Landau cut contributions

In the real-time thermal field theory, the pion self-energy at finite temperature and density is evaluated where the different mesonic and baryonic loops are considered. The interactions of pion with the other mesons and baryons in the medium are governed by the effective hadronic Lagrangian densities whose effective strength of coupling constants have been determined from the experimental decay widths of the mesons and baryons. The detail branch cut structures of these different mesonic and baryonic loops are analyzed. The Landau cut contributions of different baryon and meson loops become only relevant around the pion pole and it is completely appeared in presence of medium. The in-medium spectral function of pion has been plotted for different values of temperature, baryon chemical potential as well as three momentum of the pion. A noticeable low mass probability in pion spectral function promise to contribute in the low mass dilepton enhancement via indirect modification of ρ self-energy for loop.

Effects of chain decays, radial flow and U A(1) restoration on the low-mass dilepton enhancement in $$\sqrt {s_{NN} } = 200$$ GeV Au+Au reactions

Abstract In $$\sqrt {s_{NN} } = 200$$ GeV Au+Au collisions PHENIX reported a significant enhancement in the low-mass region (0.1 < m ee < 0.7 GeV) of the dielectron spectrum, which is still not fully understood. Several theoretical works and an indirect measurement suggest, due to the possible restoration of the U A(1) part of the chiral symmetry in a hot and dense medium, that the mass of the η′ meson may substantially decrease. This work reports on a statistically acceptable description of the PHENIX low-mass dilepton enhancement using a radial flow dominated meson spectra, chain decays of long-lived resonances and an in-medium η′ mass modification.

Can transport peak explain the low-mass enhancement of dileptons at RHIC?

We propose a novel relation between the low-mass enhancement of dielectrons observed at PHENIX and transport coefficients of QGP such as the charge diffusion constant D and the relaxation time τJ. We parameterize the transport peak in the spectral function using the second-order relativistic dissipative hydrodynamics by Israel and Stewart. Combining the spectral function and the full (3+1)-dimensional hydrodynamical evolution with the lattice EoS, theoretical dielectron spectra and the experimental data are compared. Detailed analysis suggests that the low-mass dilepton enhancement originates mainly from the high-temperature QGP phase where there is a large electric charge fluctuation as obtained from lattice QCD simulations.

Medium Effects on Meson Couplings and Dilepton Spectra

The pi omega rho coupling constant is calculated in a relativistic hadronic framework incorporating nucleons and Delta(1232) isobars. Medium modifications are included. The vertex is analyzed in context of the omega to pi e+e- and rho to pi e+e- Dalitz decays in nuclear matter. A sizeable increase of the widths for these decays is found for invariant mass of dileptons in the range 0.2-0.5 GeV. This increase may help, among other medium effects, to explain the problem of the low-mass dilepton enhancement seen in relativistic heavy-ion collisions.

Relativistic heavy ion physics: An experimental review

A review of relativistic heavy ion physics is given, with emphasis on selected interesting results. The charged particle multiplicity is compared from measurements at the AGS, SPS, and RHIC. Collective motion due to a build-up of pressure is discussed, with implications on estimates of the energy density of the produced medium. The suppression of J/ψ and the enhancement of low-mass dileptons observed in experiments at the SPS are highlighted. The high p T phenomena at RHIC are discussed in some detail. The suppression of high p T hadrons in central Au+Au collisions is shown, as well as the absence of such a suppression in d+Au collisions. PACS: 25.75.-q Relativistic heavy-ion collisions

Excitation functions of hadronic observablesfrom SIS to RHIC energies

We calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ( q , q ̄ , qq , q ̄ q ̄ ) and hadronic degrees of freedom without including any explicit phase transition to a quark–gluon plasma (QGP). It is argued that the failure of this more `conventional' approach in comparison to experimental data should indicate the presence of a different phase which might be either attributed to space–time regions of vanishing scalar quark condensate ( 〈q q ̄ 〉 = 0) or to the presence of a QGP phase with strongly interacting partons. We study the K/π ratio, the low mass dilepton enhancement in the invariant mass regime from 0.2–1.2 GeV as well as charmonium suppression for central Au + Au collisions as a function of the bombarding energy and present predictions for these observables as well as hadron rapidity distributions at RHIC energies. Whereas all observables studied within HSD show smooth increasing/decreasing excitation functions, the experimental K +/π + ratio indicates a maximum at 11 A GeV (or above) which is interpreted as a signature for a chirally restored phase in the course of the reaction.

Constraints on vector mesons with finite momentum in nuclear matter

Using the QCD operator product expansion, we derive the real part of the transverse and longitudinal vector-vector correlation function with the quantum numbers of the ϱ and ω mesons to leading order in density and three-momentum ( q 2 ) for ω 2 → −∞. The operator product expansion provides, through the Borel transformed energy dispersion relation, a model independent constraint for the momentum dependence of the vector-meson spectral density in nuclear matter. Existing model calculations for the dispersion effect of the ϱ, where the vector-meson nucleon scattering amplitude is obtained by resonance saturation in the s-channel, in general violate this constraint. We trace this to an inconsistent choice for the form factor of the ΔNϱ vertex. With a consistent choice, where both the form factor and the coupling constant are obtained from the Bonn potential, the contribution of the Δ is substantially reduced and we find good agreement with the constraint equation. We briefly comment on the implications of our result for attempts to interpret the enhancement of low-mass dileptons in heavy-ion collisions.

Bremsstrahlung dileptons in ultrarelativistic heavy ion collisions

We consider production of dilepton pairs through coherent electromagnetic radiation during nuclear collisions. We show that the number of pairs produced through bremsstrahlung is about two orders of magnitude smaller than the yield measured by the CERES Collaboration. Therefore, coherent bremsstrahlung can be ruled out as an explanation for the observed enhancement of low mass dileptons in CERES and HELIOS data. {copyright} {ital 1998} {ital The American Physical Society}

Enhancement of low-mass dileptons in SPS heavy-ion collisions: possible evidence for dropping rho meson mass in medium

Dilepton production in proton- and nucleus-induced reactions at SPS energies is studied in the relativistic transport model using initial conditions determined by the string dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanism of Dalitz decay observed dilepton spectra in central S+Au and S+W collisions requires contributions other than these direct decays. Introducing a decrease of vector meson masses in hot dense medium, we find that these heavy-ion data can also be satisfactorily explained. We also give predictions for Pb+Au collisions at 160 GeV/nucleon using current CERES mass resolution and acceptance.

Effects of in-medium vector meson masses on low-mass dileptons from SPS heavy-ion collisions

Using a relativistic transport model to describe the expansion of the fire-cylinder formed in the initial stage of heavy-ion collisions at SPS/CERN energies, we study the production of dileptons with a mass below about 1 GeV from these collisions. The initial hadron abundance and their momentum distributions in the fire-cylinder are determined by following the general features of the results from microscopic models based on the string dynamics and further requiring that the final proton and pion spectra and rapidity distributions are in agreement with available experimental data. For dilepton production, we include the Dalitz decay of π0, η, η′, ω and a1 mesons, the direct decay of primary ϱ0, ω and φ mesons, and the pion-pion annihilation that proceeds through the ϱ0 meson, the pion-rho annihilation that proceeds through the a1 meson, and the kaon-antikaon annihilation that proceeds through the φ meson. We find that the modification of vector meson properties, especially the decrease of their mass due to the partial restoration of chiral symmetry, in hot and dense hadronic matter, provides a quantitative explanation of the recently observed enhancement of low-mass dileptons by the CERES collaboration in central S+Au collisions and by the HELIOS-3 collaboration in central S+W collisions.

Enhancement of low-mass dileptons in heavy ion collisions.

Using a relativistic transport model for the expansion stage of S + Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of low-mass dileptons by the CERES Collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter.