Dilepton Production In Heavy-ion Collisions Research Articles

Thermal dilepton production in heavy-ion collisions at beam-energy-scan (BES) energies

The lepton pair production rate at finite temperature and at nextto-leading-order (NLO) is calculated for quark-gluon plasma at non-zero net baryon density. Yields are obtained using a (3+1)D multicomponent simulation capable of reproducing hadronic observables measured in the RHIC Beam Energy Scan (BES). Spectra of intermediate invariant mass dileptons are compared with measurements from the STAR collaboration. The result of a study where the temperature information obtained from the dilepton spectrum from heavyion collisions performed at different energies and centralities is presented.

Effects of chiral symmetry restoration on meson and dilepton production in relativistic heavy-ion collisions

We include effects of chiral symmetry and its restoration in the kinetic equations for baryon propagation and explore the consequences for $\eta$, $\pi^0$, $\rho$ and dilepton production in heavy-ion collisions at 1-2A GeV. Numerical calculations are performed using the GiBUU microscopic transport model supplemented by the parity-doublet model for the mean fields of the nucleon and the $N^*(1535)$ resonance. In this chiral model, a strong dropping of the Dirac mass of the $N^*(1535)$ in the high-density stage of a collision leads to a considerable enhancement in the production of this resonance as compared to the standard (non-linear) Walecka model. As the system expands, the Dirac masses of these abundant soft $N^*(1535)$ resonances gradually increase and ultimately cross the $N \eta$ decay threshold. As a result, an enhanced low-energy $\eta$ production is observed in the calculations with chiral mean fields. Comparing with TAPS data on $\eta$ and $\pi^0$ production we find that the chiral model improves the agreement for the $m_t$-spectra of $\eta$'s at small $m_t$ in heavy colliding systems. A similar enhancement is also observed in the soft $\rho$ production due to chiral symmetry and its partial restoration. The resulting dilepton yields at low and intermediate invariant masses are slightly enhanced due to these chiral effects which further improve the agreement between GiBUU transport simulations and HADES data for C+C at 1A GeV.

Dilepton production in microscopic transport theory with an in-medium ρ -meson spectral function

We use the microscopic GiBUU transport model to calculate dilepton ($e^+e^-$) production in heavy-ion collisions at SIS18 energies focusing on the effect of collisional broadening of the $\rho$-meson. The collisional width of the $\rho$-meson at finite temperature and baryon density in nuclear matter is calculated on the basis of the collision integral of the GiBUU model. A systematic comparison with HADES data on dilepton production in heavy-ion collisions is performed. The collisional broadening of the $\rho$ improves the agreement between theory and experiment for the dilepton invariant-mass distributions near the $\rho$ pole mass and for the excess radiation in Au+Au at $1.23 A$ GeV. We furthermore show that some remaining underprediction of the experimental dilepton spectra in C+C at $1 A$ GeV and Au+Au at $1.23 A$ GeV at intermediate invariant masses $0.2-0.4$ GeV can be accounted for by adjusting the $pn$ bremsstrahlung cross section in a way to agree with the inclusive dilepton spectrum from $dp$ collisions at $1.25 A$ GeV.

Electromagnetic emissivity of hot and dense matter

In this study we investigate dilepton production in heavy-ion collisions from GeV to 5 TeV with a focus on the competition between the thermal QGP radiation and the semi-leptonic decays from correlated D—meson pairs. As a ‘tool’ we employ the parton-hadron-string dynamics (PHSD) transport approach incorporating for the first time a fully microscopic treatment of the charm dynamics and their semi-leptonic decays. We find that the dileptons from correlated D—meson decays dominate the ‘thermal’ radiation from the QGP in central Pb+Pb collisions at the intermediate masses (1.2 GeV < M < 3 GeV) for GeV, while for to 20 GeV the contribution from D, decays to the intermediate mass dilepton spectra is subleading such that one should observe a rather clear signal from the QGP radiation. We, furthermore, study the pT-spectra and the RAA(pT) of single electrons at different energies.

Energy, centrality, and momentum dependence of dielectron production at collider energies in a coarse-grained transport approach

Dilepton production in heavy-ion collisions at collider energies - i.e., for the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) - is studied within an approach that uses coarse-grained transport simulations to calculate thermal dilepton emission applying in-medium spectral functions from hadronic many-body theory and partonic production rates based on lattice calculations. The microscopic output from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model is hereby put on a grid of space-time cells which allows to extract the local temperature and chemical potential in each cell via an equation of state. The resulting dilepton spectra are in good agreement with the experimental results for the range of RHIC energies, $\sqrt{s_{NN}}=19.6 - 200$ GeV. The comparison of data and model outcome shows that the newest measurements by the PHENIX and STAR collaborations are consistent and that the low-mass spectra can be described by a cocktail of hadronic decay contributions together with thermal emission from broadened vector-meson spectral functions and from the Quark-Gluon Plasma phase. Predictions for dilepton results at LHC energies show no significant change of the spectra as compared to RHIC, but a higher fraction of thermal contribution and harder slopes of the transverse momentum distributions due to the higher temperatures and flow obtained.

Vector Meson Spectral Functions in a Coarse-Graining Approach

Dilepton production in heavy-ion collisions at top SPS energy is investigated within a coarse-graining approach that combines an underlying microscopic evolution of the nuclear reaction with the application of medium-modified spectral functions. Extracting local energy and baryon density for a grid of small space-time cells and going to each cell's rest frame enables to determine local temperature and chemical potential by application of an equation of state. This allows for the calculation of thermal dilepton emission. We apply and compare two different spectral functions for the $\rho$: A hadronic many-body calculation and an approach that uses empirical scattering amplitudes. Quantitatively good agreement of the model calculations with the data from the NA60 collaboration is achieved for both spectral functions, but in detail the hadronic many-body approach leads to a better description, especially of the broadening around the pole mass of the $\rho$ and for the low-mass excess. We further show that the presence of a pion chemical potential significantly influences the dilepton yield.

Magnetic contribution to dilepton production in heavy-ion collisions

We calculate a novel "magnetic contribution" to the dilepton spectrum in heavy-ion collisions arising from interaction of relativistic quarks with intense magnetic field. Synchrotron radiation by quarks, which can be approximated by the equivalent photon flux, is followed by dilepton decay of photons in intense magnetic field. We argue that "magnetic contribution" dominates the dilepton spectrum at low lepton energies, whereas a conventional photon dilepton decay dominates at higher lepton energies.

System size and energy dependence of dilepton production in heavy-ion collisions at 1-2 GeV/nucleon energies

We study the dilepton production in heavy-ion collisions at energies of 1-2 AGeV as well as in proton induced pp, pn, pd and p+A reactions from 1 GeV up to 3.5 GeV. For the analysis we employ three different transport models - the microscopic off-shell Hadron-String-Dynamics (HSD) transport approach, the Isospin Quantum Molecular Dynamics (IQMD) approach as well as the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) approach. We confirm the experimentally observed enhancement of the dilepton yield (normalized to the multiplicity of neutral pions $N_{\pi^0}$) in heavy-ion collisions with respect to that measured in $NN = (pp+pn)/2$ collisions. We identify two contributions to this enhancement: a) the $pN$ bremsstrahlung which scales with the number of collisions and not with the number of participants, i.e. pions; b) the dilepton emission from intermediate $\Delta$'s which are part of the reaction cycles $\Delta \to \pi N ; \pi N \to \Delta$ and $NN\to N\Delta; N\Delta \to NN$. With increasing system size more generations of intermediate $\Delta$'s are created. If such $\Delta$ decays into a pion, the pion can be reabsorbed, however, if it decays into a dilepton, the dilepton escapes from the system. Thus, experimentally one observes only one pion (from the last produced $\Delta$) whereas the dilepton yield accumulates the contributions from all $\Delta$'s of the cycle. We show as well that the Fermi motion enhances the production of pions and dileptons in the same way. Furthermore, employing the off-shell HSD approach, we explore the influence of in-medium effects like the modification of self-energies and spectral functions of the vector mesons due to their interactions with the hadronic environment.

Di-lepton production in heavy-ion collisions and the QCD phase diagram

Abstract We reproduce di-electron spectra in the region of 0 &lt; m e+e &lt; 4 GeV in both minimum bias and central Au+Au collisions at $\sqrt {s_{NN} } $ = 200 GeV measured by the STAR experiment. A cocktail simulation, incorporating STAR acceptance and detector responses, is able to describe the “enhancement” of the low mass region by including an in-medium modification of vector mesons and a thermal di-lepton calculation. We also predict the di-lepton mass spectra in RHIC lower energies via an extrapolation method. The evolution of Di-lepton mass spectra, effective temperature, and possible medium modifications versus colliding energies are studied to explore the QCD phase diagram.

Electromagnetic radiation from QGP with chromo-magnetic monopoles

We consider the radiation of photons from quarks scattering on color-magnetic monopoles in the Quark-Gluon Plasma. The calculation is performed in the classical, non-relativistic approximation and results are compared to photon emission from Coulomb scattering of quarks, known to provide a significant contribution to the photon emission rates from the QGP. The present study is a first step towards understanding whether this scattering process can give a sizable contribution to dilepton production in heavy-ion collisions.

Radiation of an electric charge in the field of a magnetic monopole

We consider the radiation of photons from quarks scattering on color-magnetic monopoles in the Quark-Gluon Plasma. We consider a temperature regime $T\gsim2T_c$, where monopoles can be considered as static, rare objects embedded into matter consisting mostly of the usual "electric" quasiparticles, quarks and gluons. The calculation is performed in the classical, non-relativistic approximation and results are compared to photon emission from Coulomb scattering of quarks, known to provide a significant contribution to the photon emission rates from QGP. The present study is a first step towards understanding whether this scattering process can give a sizeable contribution to dilepton production in heavy-ion collisions. Our results are encouraging: by comparing the magnitudes of the photon emission rate for the two processes, we find a dominance in the case of quark-monopole scattering. Our results display strong sensitivity to finite densities of quarks and monopoles.

Dilepton production in heavy-ion collisions with in-medium spectral functions of vector mesons

The in-medium spectral functions of \ensuremath{\rho} and \ensuremath{\omega} mesons and the broadening of nucleon resonances at finite baryon density are calculated self-consistently by combining a resonance dominance model for the vector meson production with an extended vector meson dominance model. The influence of the in-medium modifications of the vector meson properties on the dilepton spectrum in heavy-ion collisions is investigated. The dilepton spectrum is generated for the $\mathrm{C}+\mathrm{C}$ reaction at 2.0$A$ GeV and compared with recent HADES Collaboration data. The collision dynamics is then described by the T\ubingen relativistic quantum molecular dynamics transport model. We find that an iterative calculation of the vector meson spectral functions that takes into account the broadening of the nucleon resonances due to their increased in-medium decay branchings is convergent and provides a reasonable description of the experimental data in the mass region $0.45\ensuremath{\leqslant}M\ensuremath{\leqslant}0.75$ GeV. On the other side, the theoretical calculations slightly underestimate the region ${m}_{\ensuremath{\pi}}\ensuremath{\leqslant}M\ensuremath{\leqslant}0.4$ GeV. Popular in-medium scenarios such as a schematic collisional broadening and dropping vector mesons masses are discussed as well.

J/ $\psi$ production in Indium-Indium collisions

The study of dilepton production in heavy-ion collisions at the CERN SPS provided some of the most interesting observations done so far in the search for the quark gluon plasma. However, several aspects of the understanding of these measurements need to be clarified. For example, the study of the J/\(\psi\) production in different colliding systems should help to disentangle which is the variable driving the J/\(\psi\) suppression. The NA60 experiment, with a new radiation tolerant Silicon pixel detector, studied Indium-Indium interactions in the year 2003. In this paper, we present the J/\(\psi\) / DY ratio, integrated over all the centralities of the collisions, together with a first study of the J/\(\psi\) transverse momentum and polarization.

Dilepton production in heavy-ion collisions at intermediate energies

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons ($\rho,\omega$) is described via the excitation of nuclear resonances ($R$). The theoretical framework is an extended vector meson dominance model (eVMD). The treatment of the resonance decays $R\longmapsto NV$ with arbitrary spin is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. This ensures correct asymptotics and provides a unified description of photonic and mesonic decays. The resonance model is successfully applied to the $\omega$ production in $p+p$ reactions. The same model is applied to the dilepton production in elementary reactions ($p+p, p+d$). Corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. As a possible solution of this problem the destruction of quantum interference in a dense medium is discussed. A decoherent emission through vector mesons decays enhances the corresponding dilepton yield in heavy ion reactions. In the vicinity of the $\rho/\omega$-peak the reproduction of the data requires further a substantial collisional broadening of the $\rho$ and in particular of the $\omega$ meson.

“Free” constituent quarks and dilepton production in heavy-ion collisions

An approach is suggested that invokes vitally the notion of constituent massive quarks (valons) that can survive and propagate rather than hadrons (except of pions) within hot and dense matter formed below the chiral-transition temperature in the course of heavy-ion collisions at high energies. This approach is shown to be quite good for describing the experimentally observed excess in the dilepton yield at masses 250≤M ee≤700 MeV over the prompt-resonance-decay mechanism (CERES cocktail) predictions. In certain aspects, it appears to be even more successful than conventional approaches: it seems to match the data somewhat better at dilepton masses below the two-pion threshold and below the ρ-meson peak, as well as at higher dilepton masses (beyond the φ-meson one). The approach implies no specific assumptions on the special features of phase transitions in expanding nuclear matter, and the ideal gas approximation is motivated to be still workable for describing the pion-valonic system under consideration.

Intermediate-Mass Dilepton Production in Heavy-Ion Collisions at200AGeV

Through the analysis of HELIOS-3 data obtained at the CERN Super Proton Synchrotron (SPS), we demonstrate the importance of secondary processes for dilepton production in heavy-ion collisions in the intermediate invariant mass region. We find that while the dilepton spectra between 1 and 2.5 GeV from proton-induced reactions can be attributed to the decay of primary vector mesons, charmed hadrons, and initial Drell-Yan processes, the strong enhancement seen in the heavy-ion data comes mainly from secondary processes. Furthermore, we find $\ensuremath{\pi}{a}_{1}\ensuremath{\rightarrow}l\overline{l}$ to be the most important process in this mass region, as was found by thermal rate calculations.

Intermediate mass dilepton production in heavy-ion collisions at SPS energies

Through the analysis of HELIOS-3 data on dilepton spectra, we demonstrate the importance of secondary processes for the dilepton production in heavy-ion collisions in the intermediate mass region. We find that, while the dilepton spectra in proton-induced reactions can be nicely explained by the decay of primary vector mesons, charmed hadrons, and initial Drell-Yan processes, the strong enhancement seen in the heavy-ion data comes mainly from the secondary meson-meson interactions which are unique to heavy 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.

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.

Dilepton production in heavy-ion collisions

The dynamical evolution of nucleus-nucleus collisions is described by a transport equation of the Uehling-Uhlenbeck type. Our model evolves phase-space distribution functions for nucleons, Δ's, N ∗(1440)-resonances and pions with their isospin degrees of freedom. The equations are solved by the test-particle simulation method. We apply this model to proton-nucleus and nucleus-nucleus collisions from 400 MeV/ A to 2.1 GeV/ A. Our approach is found to correctly describe the spectra of baryons and pions. Furthermore, we calculate the production of dileptons considering proton-neutron and pion bremsstrahlung, Δ and N ∗ decay and π +π − annihilation. The calculations reproduce the data for p+ 9Be and for 40Ca + 40Ca at the bombarding energies of 1 GeV/ A and 2 GeV/ A except for small invariant masses ( M⩽ 200 MeV) where the situation is not fully understood. Above 300 MeV dilepton invariant mass the relevant contributions arise from proton-neutron bremsstrahlung, the Δ Dalitz-decay and π + π − annihilation. At 2 GeV/ A for 40Ca + 40Ca, however, we find the bremsstrahlung contribution to be higher than that from pion annihilation.