Hadronic Cocktail Research Articles

Measurements of dielectron production in Au+Au collisions at sNN=27 , 39, and 62.4 GeV from the STAR experiment

We report systematic measurements of dielectron (e+e−) invariant-mass Mee spectra at midrapidity in Au+Au collisions at sNN = 27, 39, and 62.4 GeV taken with the STAR detector at the Relativistic Heavy Ion Collider. For all energies studied, a significant excess yield of dielectrons is observed in the low-mass region (0.40<Mee<0.75 MeV/c2) compared to hadronic cocktail simulations at freeze-out. Models that include an in-medium broadening of the ρ-meson spectral function consistently describe the observed excess. In addition, we report acceptance-corrected dielectron-excess spectra for Au+Au collisions at midrapidity (|yee|< 1) in the 0–80% centrality bin for each collision energy. The integrated excess yields for 0.4<Mee<0.75GeV/c2, normalized by the charged particle multiplicity at midrapidity, are compared with previously published measurements for Au+Au at sNN1 = 19.6 and 200 GeV. Models that include an in-medium broadening of the ρ-meson spectral function consistently describe the observed excess. The normalized excess yields in the low-mass region show no significant collision energy dependence. The data, however, are consistent with model calculations that demonstrate a modest energy dependence.Received 27 February 2023Accepted 4 May 2023DOI:https://doi.org/10.1103/PhysRevC.107.L061901©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionRelativistic heavy-ion collisionsNuclear Physics

Low-pT μ+μ− pair production in Au+Au collisions at √SNN = 200 GeV at STAR

We report the measurements of μ+μ− pair production for pT &lt; 0.10 GeV/c in 40-80% Au+Au collisions at √SNN = 200 GeV at STAR. A significant enhancement with respect to the hadronic cocktail is observed. The pT2 and ∆ϕ distributions of the excess yields are also reported and compared with model calculations. The EPA-QED calculations can describe the data very well within uncertainties.

Dielectron measurements with the HADES at GSI

In this work we present the dielectron analysis of 4.5 billion Ag+Ag collisions ((0−40)% centrality) at a center-of-mass energy of √sNN = 2.55 GeV measured with HADES. The obtained dielectron signal spectrum is compared to simulated hadronic cocktail and nucleon-nucleon reference spectra revealing a strong contribution from the hot and dense phases. The average temperature of the collision system can be extracted from the slope of the in-medium contribution. In a momentum differential analysis we observe modifications of the dielectron signal shape in the ω−ρ invariant mass region. Furthermore we compare the results to previous measurements in Au+Au collisions at slightly lower energy. The suggested modifications of the ρ spectral function is further studied within the HADES pion-beam program. Here, we present recently published results on the electromagnetic and hadronic coupling of baryonic resonances to the ρ − N final state.

Soft-Dielectron Excess in Proton-Proton Collisions at s=13 TeV

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\rm ee}<0.6$ GeV/$c^2$ and for $p_{\rm T,ee} < 0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13\,(\rm{stat.})\pm 0.17\,(\rm{syst., data}) \pm 0.34\,(\rm{syst., cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\rm ee}$ and $p_{\rm T,ee}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

Dielectrons and Direct Virutal Photons at RHIC-STAR

Electromagnetic probes are unique probes of the hot and dense medium cre- ated in relativistic heavy-ion collisions due to their minimal interactions with the partonic and hadronic medium. They can be produced at all stages of a collision, thus they provide unique ways to study the medium properties of the whole collision evolution. We present the dielectron invariant mass spectrum at √sNN = 27, 39, 39, and 62.4 GeV. Comparing to hadronic cocktail simulation, significant excesses of dielectron invariant mass spectrum at low mass are observed. The direct virtual photon invariant yields derived from the low-mass e+e− continuum in Au+Au collisions at √sNN = 200 GeV is presented and the excesses from thermal photons in low transverse momentum (pT) have been observed. Model calculations can simultaneously describe both dielectron low mass excesses and direct virtual photon low pT excesses.

Low-p_{T} e^{+}e^{-} Pair Production in Au+Au Collisions at sqrt[s_{NN}]=200 GeV and U+U Collisions at sqrt[s_{NN}]=193 GeV at STAR.

We report first measurements of e^{+}e^{-} pair production in the mass region 0.4<M_{ee}<2.6 GeV/c^{2} at low transverse momentum (p_{T}<0.15 GeV/c) in noncentral Au+Au collisions at sqrt[s_{NN}]=200 GeV and U+U collisions at sqrt[s_{NN}]=193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40%-80% centrality of these collisions. The excess yields peak distinctly at low p_{T} with a width (sqrt[⟨p_{T}^{2}⟩]) between 40 and 60 MeV/c. The absolute cross section of the excess depends weakly on centrality, while those from a theoretical model calculation incorporating an in-medium broadened ρ spectral function and radiation from a quark gluon plasma or hadronic cocktail contributions increase dramatically with an increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the p_{T}^{2} distributions.

Low-mass electron pairs from Au+Au collisions at 1.23A GeV with HADES

Lepton pairs are considered as an ideal probe to access the microscopic properties of strongly-interacting matter under extreme conditions of temperature and density. The spectral distribution of lepton pairs in the region below the low mass vector mesons is characterized by an excess radiation above contributions from late meson decays, the so-called hadronic cocktail. It is theoretically understood as a thermal radiation based on Vector Meson Dominance assuming a strong broadening of the in-medium spectral function due to coupling to baryons and anti-baryons.In continuation of a systematic investigation of the emissivity of strongly interacting matter, HADES has recently measured the dielectron emission in Au+Au collisions at 1.23A GeV beam energy. The first measurement for Au+Au completes the systematics of dilepton production in NN/pA/AA collisions in the SIS18 energy range.This contribution will present new results for the Au+Au system based on a data sample of 5.0 billion events of the 40% most central collisions.

Low-Mass Dielectron Production in p-Pb and Pb–Pb Collisions with ALICE

The production of low-mass dielectrons is measured with ALICE at the LHC in pp, p-Pb and Pb–Pb collisions. The dielectron yield in p-Pb collisions in comparison to the expected hadronic cocktail shows an overall reasonable agreement. The production of direct virtual photons, visible as an excess over the hadronic cocktail, in Pb–Pb collisions is measured in two transverse momentum ranges. A feasibility study for the future LHC Run 3 indicates that it should be possible to extract an effective temperature with a combined statistical and systematic uncertainty of around 30%.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p–Pb and Pb–Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located at mid-rapidity. The dielectron yield in p–Pb collisions shows an overall agreement with the hadronic cocktail. The pair transverse momentum distributions are sensitive to the contributions from open heavy-flavours. In Pb–Pb collisions, uncorrected background-subtracted yields have been extracted in two centrality classes. In pp collisions the production of virtual photons relative to the inclusive yield is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. A feasibility study for LHC Run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Dielectron measurements in pp, p-Pb, and Pb-Pb collisions with the ALICE detector

Dielectrons are excellent tools to investigate the properties of the medium created in heavy-ion collisions. Since dielectrons generated in the medium are not affected by final state interactions, they carry information about the space-time evolution of the collision. The dielectron invariant mass shape is expected to be sensitive to the in-medium modification of vector mesons and thermal radiation from the hot medium created in heavy-ion collisions. In order to understand the nuclear effects, dielectron measurements in pp collisions serve as reference, while p-A collisions are important for the evaluation of the cold nuclear matter effects on the modification of the invariant mass shape. In this proceedings, the dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass spectra in pp at s=7 TeV and p-Pb collisions at sNN=5.02 TeV are extracted and in agreement with the estimated hadronic cocktail calculation. The cross section of the direct photons measured via virtual photons is consistent with NLO pQCD calculations within uncertainties. The uncorrected dielectron yields are also extracted in central and semi-central Pb-Pb collisions at sNN=2.76 TeV. The performance study of the dielectron excess measurement in Pb-Pb collisions by the ALICE detector upgrade is also discussed.

Dilepton production and reaction dynamics in heavy-ion collisions at SIS energies from coarse-grained transport simulations

Dilepton invariant-mass spectra for heavy-ion collisions at SIS 18 and BEVALAC energies are calculated using a coarse-grained time evolution from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. The coarse-graining of the microscopic simulations enables to calculate thermal dilepton emission rates by application of in-medium spectral functions from equilibrium quantum-field theoretical calculations. The results show that extremely high baryon chemical potentials dominate the evolution of the created hot and dense fireball. Consequently, a significant modification of the $\rho$ spectral shape becomes visible in the dilepton invariant-mass spectrum, resulting in an enhancement in the low-mass region $M_{ee} = 200$ to 600 MeV/$c^{2}$. This enhancement, mainly caused by baryonic effects on the $\rho$ spectral shape, can fully describe the experimentally observed excess above the hadronic cocktail contributions in Ar+KCl ($E_{\mathrm{lab}}=1.76$ $A$GeV) reactions as measured by the HADES collaboration and also gives a good explanation of the older DLS Ca+Ca ($E_{\mathrm{lab}}=1.04$ $A$GeV) data. For the larger Au+Au ($E_{\mathrm{lab}}=1.23$ $A$GeV) system, we predict an even stronger excess from our calculations. A systematic comparison of the results for different system sizes from C+C to Au+Au shows that the thermal dilepton yield increases stronger ($\propto A^{4/3}$) than the hadronic background contributions, which scale with $A$, due to its sensitivity on the time evolution of the reaction. We stress that the findings of the present work are consistent with our previous coarse-graining results for the NA60 measurements at top SPS energy. We argue that it is possible to describe the dilepton results from SIS 18 up to SPS energies by considering the modifications of the $\rho$ spectral function inside a hot and dense medium within the same model.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p-Pb and Pb-Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located around mid-rapidity. The production of virtual photons relative to the inclusive yield in pp collisions is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. Results from the invariant mass analysis in p-Pb collisions show an overall agreement between data and hadronic cocktail. In Pb-Pb collisions, uncorrected background- subtracted yields have been extracted in two centrality classes. A feasibility study for LHC run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Reconstruction of π0 and η mesons via conversion method in Au+Au at 1.23 GeV/u with HADES

Lepton pairs emerging from decays of virtual photons represent promising probes of matter under extreme conditions. In the energy domain of 1 - 2 GeV per nucleon, the HADES experiment at GSI Helmholtzzentrum fuer Schwerionenforschung in Darmstadt studies di-electrons and strangeness production in various reactions, i.e. collisions of pions, protons, deuterons, heavy-ions, and nuceous with nuclei. An accurate determination of the medium radiation depends on a precise knowledge of the underlying hadronic cocktail composed of various sources contributing to the net spectra. Therefore, a measurement of the neutral meson yields together with the dileptons is crucial. In this contribution, the capability of HADES to detect e+e− pairs from conversion of real photons will be demonstrated. We will present results from a two-photon analysis of Au+Au collisions at 1.23 GeV/u providing information on neutral π0 and η mesons.

Dielectron mass spectra from Au+Au collisions at √[s(NN)]=200 GeV.

We report the STAR measurements of dielectron (e(+)e(-)) production at midrapidity (|y(ee)|<1) in Au+Au collisions at √[s(NN)]=200 GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 (ρ-like), 0.76-0.80 (ω-like), and 0.98-1.05 (ϕ-like) GeV/c(2). The spectrum in the ω-like and ϕ-like regions can be well described by the hadronic cocktail simulation. In the ρ-like region, however, the vacuum ρ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77±0.11(stat)±0.24(syst)±0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the ρ meson. The excess yield in the ρ-like region increases with the number of collision participants faster than the ω and ϕ yields. Theoretical models with broadened ρ contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.

Low-mass dielectron measurement in pp and Pb–Pb collisions in ALICE

We report on the first dielectron measurement in pp collisons at = 7 TeV with the ALICE detector system. The results are compared to the expected hadronic sources. The hadronic cocktail agrees to the measured dielectron continuum within statistical and systematic uncertainties. The status of the dielectron measurement in Pb–Pb collisions at = 2.76 TeV is addressed.

Dielectron measurements by PHENIX using the HBD

Abstract PHENIX presents first dielectron spectra for p + p and Au + Au collisions measured using the HBD detector. The p + p results are compatible with cocktail calculations that take into account the modified aperture of PHENIX due to a change in the magnetic field configuration. The Au + Au results are compared to the expected yield of dielectrons from meson decays (hadronic cocktail) in centrality selections 20–40% and 60–92%. Our results do not show any significant deviations from these predictions and are compatible with previous measurements published by PHENIX.

Di-lepton production in p+p collisions at √s = 200 GeV from STAR

The di-electron mass continuum in the mass range 0 < Mee < 3.3 GeV/c has been measured at midrapidity (|y| < 1) by the STAR experiment in p+p collisions at √s = 200 GeV. We will compare the measured di-lepton continuum to a hadronic cocktail simulation. In addition, the omega yields via di-leptonic decays have been measured for 0 < pT < 1.6 GeV/c.

A unique parametrization of the shapes of secondary dilepton spectra observed in central heavy-ion collisions at CERN-SPS energies

A unique parametrization of secondary (thermal) dilepton yields in heavy-ion experiments at CERN-SPS is proposed. This parametrization resembles a thermal q q ̄ annihilation rate. This is inspired by the observation that lepton pair production rates are quantitatively similar, whether expressed in a hadronic or partonic basis. Adding the thermal yield and the background contributions (hadronic cocktail, Drell–Yan, correlated semileptonic decays of open charm) the spectral shapes of the CERES/NA45, NA38, NA50 and HELIOS/3 data from experiments with lead and sulfur beams can be well described.

Are We Close to an Equilibrated Quark-Gluon Plasma? Nonequilibrium Analysis of Particle Production in Ultrarelativistic Heavy Ion Collisions

Ratios of hadronic abundances are analyzed for $\mathrm{pp}$ and nucleus-nucleus collisions at $\sqrt{s}\ensuremath{\approx}20\mathrm{GeV}$ using the microscopic ultrarelativistic quantum molecular dynamics transport model. Secondary interactions significantly change the primordial hadronic composition of the system. A strong dependence on rapidity is predicted. Without assuming thermal and chemical equilibrium, predicted hadron yields and ratios agree with many of the data ( $\ensuremath{\pi}/p$, $d/p$, $\overline{p}/p$, $\overline{\ensuremath{\Lambda}}/\ensuremath{\Lambda}$, $\overline{\ensuremath{\Xi}}/\overline{\ensuremath{\Lambda}}$, etc.).