Meson Production In Heavy Ion Collisions Research Articles

In-medium effects on hidden strangeness production in heavy-ion collisions

We study ϕ meson production in heavy-ion collisions from subthreshold energies of 1.23 A GeV up to RHIC energies within the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach where novel production channels for ϕ mesons based on a coupled channel T-matrix approach are implemented along with the collisional broadening of the ϕ meson spectral width in medium. Since ϕ meson production is closely related to the production of kaons and antikaons, antikaon properties are described via the self-consistent coupled-channel unitarized scheme within a SU(3) chiral Lagrangian (G-matrix) which incorporates explicitly the s− and p− waves of the kaon-nucleon interaction, while the in-medium modifications of kaons are accounted for via a kaon-nuclear potential, which is assumed to be proportional to the local baryon density.

In-medium effects in ϕ meson production in heavy-ion collisions from subthreshold to relativistic energies

We investigate the hidden strange $\phi$ meson production in heavy-ion collisions from subthreshold ($E_{kin}\approx 1$ A GeV) to relativistic ($E_{kin}\approx 21$ A TeV) energies as well as its coupling to the open strange mesons (kaons, antikaons) and their productions. Our study is based on the off-shell microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which is applicable for the dynamical description of strongly interacting hadronic and partonic degrees-of-freedom created in heavy-ion collisions. Implementing novel meson-baryon and meson-hyperon production channels for $\phi$ mesons, calculated within a T-matrix coupled channel approach based on the extended SU(6) chiral effective Lagrangian model, along with the collisional broadening of the $\phi$-meson in-medium spectral function, we find a substantial enhancement of $\phi$ meson production in heavy-ion collisions, especially at sub- and near-thresholds. This allows to describe the experimentally observed strong enhancement of the $\phi/K^-$ ratio at low energies without including hypothetical decays of heavy baryonic resonances to $\phi$ as in alternative approaches. Moreover, we show that in spite of a stronger contribution from enhanced $\phi$ to $K^-$ production, the majority of the experimental data for different A+A systems at low energies favor the scenario with in-medium modifications of the kaon and antikaon properties in the hot and dense environment. Moreover, we study the influence of the final state interactions of $K, \bar K$ mesons on the reconstruction of $\phi$'s by the by invariant mass method.

Observation of Ds±/D0 Enhancement in Au+Au Collisions at sNN=200 GeV

We report on the first measurement of charm-strange meson D_{s}^{±} production at midrapidity in Au+Au collisions at sqrt[s_{NN}]=200 GeV from the STAR experiment. The yield ratio between strange (D_{s}^{±}) and nonstrange (D^{0}) open-charm mesons is presented and compared to model calculations. A significant enhancement, relative to a pythia simulation of p+p collisions, is observed in the D_{s}^{±}/D^{0} yield ratio in Au+Au collisions over a large range of collision centralities. Model calculations incorporating abundant strange-quark production in the quark-gluon plasma and coalescence hadronization qualitatively reproduce the data. The transverse-momentum integrated yield ratio of D_{s}^{±}/D^{0} at midrapidity is consistent with a prediction from a statistical hadronization model with the parameters constrained by the yields of light and strange hadrons measured at the same collision energy. These results suggest that the coalescence of charm quarks with strange quarks in the quark-gluon plasma plays an important role in D_{s}^{±}-meson production in heavy-ion collisions.

Neutral pion and η meson production at midrapidity in Pb-Pb collisions at sNN=2.76 TeV

Neutral pion and η meson production in the transverse momentum range 1 <pT< 20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at sNN = 2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper pT reach of the previous ALICE π0 measurements from 12 to 20 GeV/c and present the first measurement of η meson production in heavy-ion collisions at the LHC. The η/π0 ratio is similar for the two centralities and reaches at high pT a plateau value of 0.457 ± 0.013stat ± 0.018syst. A suppression of similar magnitude for π0 and η meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the pT range 6 <pT< 10 GeV/c. For pT< 3 GeV/c, hadronization models describe the π0 results while for the η some tension is observed.1 MoreReceived 21 March 2018DOI:https://doi.org/10.1103/PhysRevC.98.044901Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.©2018 CERN, for the ALICE CollaborationPhysics Subject Headings (PhySH)Research AreasNuclear reactionsRelativistic heavy-ion collisionsNuclear Physics

Centrality dependence of subthresholdϕmeson production in Ni + Ni collisions at1.9AGeV

We analyzed the ϕ meson production in central Ni + Ni collisions at a beam kinetic energy of 1.93A GeV with the FOPI spectrometer and found a production probability per event of [8.6±1.6(stat)±1.5(syst)]×10−4. This new data point allows us for the first time to inspect the centrality dependence of subthreshold ϕ meson production in heavy-ion collisions. The rise of ϕ meson multiplicity per event with mean number of participants can be parametrized by a power function with exponent α=1.8±0.6. The ratio of ϕ to K− production yields seems not to depend, within the experimental uncertainties, on the collision centrality, and the average of measured values was found to be 0.36±0.05.Received 13 February 2016DOI:https://doi.org/10.1103/PhysRevC.94.014901©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionRelativistic heavy-ion collisionsNuclear Physics

Understanding the HADES dilepton invariant mass spectra with UrQMD simulations

Photons and lepton pairs emerging from decays of virtual photons are the most promising probes of dense hadronic matter. In the energy domain of 1 − 2 GeV per nucleon, HADES has measured electron pairs in C+C, Ar+KCl, p+p, d+p and p+Nb collisions. An experimentally constrained N+N reference spectrum was established. Moreover, for the first time at this energy the inclusive production cross sections for light vector mesons were extracted. This result allows putting tight constraints on vector meson production in heavy-ion collisions at beam energies of a few GeV per nucleon. In this contribution I will compare the HADES data to predictions from UrQMD microscopic transport model calculations and introduce an approach which will allow to separate in a transparent way the generation of the event background from the emission pattern of a physics observable under consideration.

Catalytic ϕ meson production in heavy-ion collisions

The phi meson production on hyperons πY → ϕY and anti-kaons is argued to be a new efficient source of phi mesons in a nucleus–nucleus collision. These reactions are not suppressed according to the Okubo–Zweig–Izuka rule in contrast to the processes with non-strange particles in the entrance channels, πB and BB with B = N, Δ. A rough estimate of the cross sections within a simple hadronic model shows that the cross sections of πY → ϕY and reactions can exceed that of the πN → ϕN reaction by factors 50 and 60, respectively. In the hadrochemical model for nucleus–nucleus collisions at SIS and lower AGS energies, we calculate the evolution of strange particle populations and the ϕ meson production rate due to the new processes. It is found that the catalytic reactions can be operative if the maximal temperature in nucleus–nucleus collisions is larger than 130 MeV and the collision time is larger than 10 fm. A possible influence of the catalytic reactions on the centrality dependence of the ϕ yield at AGS energies and the ϕ rapidity distributions at SPS energies is discussed.

Subthreshold ϕ meson production in heavy-ion collisions

Within a BUU type transport model we study ϕ meson production in subthreshold Ni + Ni and Ru + Ru reactions. For the first time we included in our model the elementary reaction channels ρ + N, Δ → ϕ + N, π + N(1520) → ϕ + N and πρ → ϕ. In spite of a substantial increase of the ϕ multiplicities by these channels, our results stay significantly below the preliminary experimental data.

Subthreshold φ meson production in heavy-ion collisions

Within a transport code of BUU type the production of φ mesons in the reactions Ni+Ni at 1.93 A GeV and Ru+Ru at 1.69 A GeV is studied. New elementary reaction channels ρN( Δ)→ φN and πN(1520)→ φN are included. In spite of a substantial increase of the φ multiplicities by these channels the results stay below the tentative numbers extracted from experimental data.

Interference in exclusive vector meson production in heavy-Ion collisions

Vector mesons are produced copiously in peripheral relativistic heavy-ion collisions. Virtual photons from one ion can fluctuate into quark-antiquark pairs and scatter from the second ion, emerging as vector mesons. The emitter and target are indistinguishable, so emission from the two ions will interfere. Vector mesons have negative parity so the interference is destructive, reducing the production of mesons with small transverse momentum. The mesons are short lived, and decay before emission from the two ions can overlap. However, the decay-product wave functions overlap and interfere since they are produced in an entangled state, providing an example of the Einstein-Podolsky-Rosen paradox.

Phi meson production in heavy-ion collisions at SIS energies

Phi meson production in heavy-ion collisions at SIS/GSI energies (∼ 2 GeV/nucleon) is studied in the relativistic transport model. We include contributions from baryon-baryon, pion-baryon, and kaon-antikaon collisions. The cross sections for the first two processes are obtained in a one-boson-exchange model, while that for the last process is taken to be of Breit-Wigner form through the phi meson resonance. The dominant contribution to phi meson production in heavy-ion collisions at these energies is found to come from secondary pion-nucleon collisions. Effects due to medium modifications of kaon masses, which are needed to account for the observed enhancement of antikaon yield, are also studied and are found to reduce the phi meson yield by about a factor of two, mainly because of an increased phi decay width as a result of dropping kaon-antikaon masses. In this case, the ø K − ratio is about 4%, which is a factor of 2–3 below preliminary experimental data from the FOPI collaboration at GSI. Including also the reduction of the phi meson mass in medium increases this ratio to about 8%, which is then in reasonable agreement with the data.

Dilepton and vector meson production in heavy-ion reactions

We present a nonperturbative dynamical study of dilepton and vector meson production in heavy-ion collisions from 1 to 2 GeV/A bombarding energies incorporating all known sources relevant in this energy range. The dynamical evolution of the nucleus-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type evolving phase-space distribution functions for nucleons, baryon resonances, pions,η’s,ρ’s andσ’s with their isospin degrees of freedom. We investigate in particular the sensitivity of the calculated yields to predicted changes of theρ andω mesons in dense matter.

Phi meson production cross sections in pion-baryon and baryon-baryon interactions

Phi meson production in pion-baryon and baryon-baryon interactions near the production threshold is studied in a boson-exchange model. Parameters in this model are either taken from the Bonn model for the nucleon-nucleon potential or determined from the measured phi meson decay width and the experimental data for the reactions π − p→ φn and pp→ ppφ. The isospin-averaged cross sections for the reactions πN→ φN, πΔ→ φN, NN→ NNφ, which are needed in transport models to study phi meson production in heavy-ion collisions at subthreshold energies, have been evaluated. For the reactions NΔ→ NNφ and ΔΔ→ NNφ, which become singular when the exchanged pion is on-shell, the Peierls method is used to calculate their cross sections. We find that the cross section for the reaction πN→ φN is much larger than the other cross sections.