Low-mass Dilepton Research Articles

Dilepton yield in heavy-ion collisions with Bose enhancement of decay widths

The excess of low invariant mass dilepton yield in heavy ion collisions arising from reduction in the rho meson mass at finite temperatures is partially suppressed because of the effect on the width of the rho meson induced by Bose enhancement, essentially due to emission of pions in a medium of the pion gas in the central rapidity region. The sensitivity of the effect on the initial temperature of the hadronic phase is also examined.

Signatures of quark-gluon plasma formation in nuclear collisions: News from QM96

Three generic signatures of quark-gluon plasma production in nuclear collisions are reviewed. First, we discuss whether the recent J Ψ suppression data in Pb + Pb are the “smoking gun” for the deconfinement transition. Second, claims that the excess production of low mass dileptons provides evidence for chiral restoration are reviewed. Finally, the pion interferometry time delay signature of a rapid cross-over region in the QCD equation of state is analyzed.

Low mass dilepton and photon production

First continuum dilepton data from S-beam have brought an experimental breakthrough in heavy ion physics. Three experiments — CERES, HELIOS-3, and NA38 — have reported an enhanced production of dileptons in S-induced reactions, with a yield clearly above a simple extrapolation from p-p and p-nucleus collisions. These interesting results have now been corroborated by Pb-beam data from CERES and NA50 presented at this conference. The search for direct photons remains more tedious and so far no clear evidence was found.

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.

First results from CERES/NA45 on low-mass electron pair production in PbAu collisions

We report preliminary results of the first measurement of low-mass electron pairs in 160 GeV/nucleon PbAu collisions at central rapidities in a wide multiplicity range. This work is the continuation of our systematic studies on pair production in p-Be, p-Au and SAu interactions. The motivation for this effort derives from a new source of lowmass dileptons recently observed in SAu and S-W collisions by CERES and HELIOS/3, respectively. The results obtained in the analysis of the PbAu data confirm our previous finding that pair production in the mass range 0.2 < m ee < 1.5 GeV/ c 2 is enhanced over the contributions from hadron decays while no significant excess could be observed for m ee ≲ 2m π . A comparison of the results for different multiplicity bins favours a non-linear dependence of the e +e −-yield on the accompanying charged particle densities.

Upper limits on η and η′ production in central SAu collisions at 200 GeV/nucleon

Inclusive photon measurements from heavy ion experiments at the CERN SPS limit the ratio ν π o in central S-induced collisions at mid rapidity to less than 0.15 which excludes an η yield larger than 1.5 times the value measured in p-p collisions. In addition, the photon data show that the η production cross section cannot increase by more than 25% going from peripheral to central SAu collision. The same data also limits the η′ η ratio to less than 0.75. These results exclude an enhanced η or η′ production as a major contribution to the recently discovered strong low mass dilepton excess in heavy ion collisions at the CERN SPS.

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.

Collision broadening of the rho meson in a dropping mass scenario.

Vector mesons containing light quarks are thought to have their masses reduced in dense nuclear matter, sacrificing some of their energy to the scalar field which becomes appreciable at finite baryon density. Model calculations find masses which fall by a couple tens of percents in normal nuclear matter, and by several hundred MeV in dense matter. We estimate the collision rate for \ensuremath{\rho} mesons in such a scenario and at finite temperature. Compared to its free-mass value, the collision rate changes by nearly a factor of 2 both above and below, depending on the density. This collision broadening effect could be important for estimates of low-mass dilepton production in heavy-ion collisions. \textcopyright{} 1996 The American Physical Society.

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.

Summary of low-mass dilepton and direct photon results

An enhanced production of dileptons −e +e − or μ +μ − pairs -, over those expected from known hadronic sources, has been observed by CERES, HELIOS-3 and NA38 in S-induced interactions on heavy targets at 200 GeV/nucleon. The excess covers the mass range from 200 MeV / c 2 up to the J gy . There is no clear evidence of a signal in the search for direct photons. All the experimental results seem to be fairly consistent.

Kinetics of an expanding pion gas and low-mass dilepton emission.

We study the space-time dependence of muon pair production from dense, nonequilibrium pion matter in the framework of a Boltzmann type transport model which properly includes effects of Bose statistics. Starting from an initially supersaturated pion gas with a large positive effective chemical potential we find that this pion chemical potential stays approximately constant during the evolution. As a consequence, such a scenario leads to an increased dilepton yield near the kinematical threshold for muon pair production via pion annihilation. Depending on the lifetime of the dense hadron matter formed in relativistic heavy ion collisions, this component can be observed above the background of muon pairs from resonance decays and the \ensuremath{\rho},\ensuremath{\omega} bump. In such a way the measurement of the dimuon invariant mass spectrum could help to understand the initial state and the dynamical evolution of ultrarelativistic heavy ion collisions.

The Landau-Pomeranchuk effect and soft lepton-pair production in heavy ion collisions

A detailed discussion of the properties of low mass dilepton production in heavy-ion collisions is presented taking into account the Landau-Pomeranchuk suppression effect in an expanding medium. Our results are then compared with non-thermal Dalitz η-decay into dileptons.

Influence of the Landau-Pomeranchuk effect on lepton-pair production in a hadronic gas.

An estimate is made of the Landau-Pomeranchuk effect on the production of dileptons in a hadronic gas. For low-mass dilepton pairs this effect reduces the production rate for bremsstrahlung in a dense pion gas by an order of magnitude. For high invariant masses its influence is negligible. This behavior is of importance for the theoretical analysis of low-mass dilepton pairs produced in relativistic heavy ion collisions.

Low-mass dileptons at high momenta in ultra-relativistic heavy-ion collisions

Abstract We calculate the emission rate of low-mass dileptons at high momenta from a quark-gluon plasma. We find a large enhancement compared with the naive Born rate. Numerical estimates for relativistic heavy ions collisions at RHIC and LHC suggest, however, that the thermal signal is dominated by the background coming from meson decays and from hadronic bremsstrahlung.

Hadronic sources of dileptons from heavy ion collisions at intermediate and relativistic energies

The kinetic model developed previously for soft hadron-nucleus and nucleus-nucleus collisions has been extended to describe low mass dilepton production. A fairly good description of available experimental data on dileptons created in both p+Be collisions from 12 to 1 GeV and Ca+Ca collisions at energies of 2 and 1 GeV/nucleon, is attained taking into account a variety of e +e − sources. Confirming a previous estimate of the ππ annihilation process, it has been found that the ω/ϱ→e +e − decay contributes as much as ππ annihilation does and to the same region of invariant masses, even for Ca+Ca (1 GeV/nucleon) collisions.

Results of the HELIOS Collaboration on low mass dilepton and soft photon production in pBe, pW and SW collisions

We report results of the HELIOS Collaboration on low-mass dilepton and soft photon production in 450 GeV/ c proton-beryllium interactions and preliminary results on sulphur tungsten collisions. Both electron and muon pairs were measured in a double spectrometer with similar kinematic acceptance for both parts. The observed dilepton yield is found to be in good agreement with the expected cumulative lepton pair production by meson decays. Based on the simultaneous direct measurements of the η, via the Dalitz decay channel, which represents the main component of the decay background, the remaining systematic errors of this experiment are as small as 20–30%. With a specially modified experimental set-up the HELIOS-3 collaboration measured low-mass muon pairs in 200 GeV/ c proton-tungsten and sulfur-tungsten collisions. We show first preliminary results. For this analysis the mesonic Dalitz decay contributions are evaluated using cross-section parametrization leading to larger systematic errors. No significant additional dilepton source is detected within the framework of this preliminary analysis. Soft ( p T ≤ 50 MeV/ c) photon production was measured with two special independent set-ups in 450 GeV/ c proton beryllium collisions. The observed production rates and spectra are well reproduced by hadronic bremsstrahlung calculations.

Low-mass dileptons in pA and AA collisions

The low-mass ( M < 1 GeV) e +e − continuum has not yet been understood even though the large yield has been documented in a number of high-energy hadron-nucleon and hadron-nucleus experiments. In heavy-ion collisions, effects resulting from the presence of the excited nuclear medium are expected. An understanding of dilepton production at excitation energies in the few-GeV range will aid in the interpretation of dilepton production at ultra-relativistic energies, where it may be a probe of the quark-gluon plasma. The objective of the Dilepton Spectrometer (DLS) collaboration at the BEVALAC is to study the production of e +e − pairs in nucleon-nucleon, proton-nucleus and nucleus-nucleus collisions at bombarding kinetic energies between 1 and 5 A · GeV. The transverse momentum distributions, the mass dependence, and the excitation function of dilepton production give information about the production mechanisms.

Low mass dilepton production in pp and pA collisions as a signature of the thermalization of hadronic matter

We point out that an increase of the ratio ( dN ee /dy) ( dN π/ dy) 2 (rapidity density of low mass dileptons over the square of the rapidity density of final hadrons) as a function of dN π dy is a signature of the thermalized quark-hadron mixed phase in pp and pA collisions.

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.

Will the production of low-mass dileptons ine+e−collisions be a further test of quark-parton-model ideas?

The production rates and spectra of low-mass dileptons coming from the annihilations of quarks and antiquarks created during the space-time evolution of parton jets in e/sup +/e/sup -/ collisions at ..sqrt..s-bar = 3, 8, and 30 GeV are predicted using a model that describes well the low-mass dilepton production in hadronic collisions throughout a wide incident-energy range. If these predictions are confirmed experimentally, further support will be given to the idea of parton origin of low-mass dileptons, which have already been observed in hadron-hadron and lepton-hadron interactions.