Antibaryon Production Research Articles

Multistrange hyperon production on nuclear targets

We consider the experimental data on yields of protons, strange $\Lambda^{\prime}$s, and multistrange baryons ($\Xi$, $\Omega$), and antibaryons production on nuclear targets, and the experimental ratios of multistrange to strange antibaryon production, at the energy region from SPS up to LHC, and compare them to the results of the Quark-Gluon String Model calculations. In the case of heavy nucleus collisions, the experimental dependence of the $\overline{\Xi}^+/\overline{\Lambda}$, and, in particular, of the $\overline{\Omega}^+/\overline{\Lambda}$ ratios, on the centrality of the collision, shows a manifest violation of quark combinatorial rules.

Centrality dependence of multistrangeness production in high-energy heavy-ion collisions

We compare the experimental data on yields of protons, strange Λ′s, and multistrange baryons (Ξ, Ω), and antibaryons production on nuclear targets, and the experimental ratios of multistrange to strange antibaryon production, at the energy region from SPS up to LHC, with the corresponding results of the Quark-Gluon String Model calculations. In the case of heavy nucleus collisions, the experimental dependence of the Ξ̅+/Λ̅, and Ω̅+/Λ̅ ratios, on the centrality of the collision, shows a manifest violation of quark combinatorial rules.

Role of baryon number conservation in measurements of fluctuations

I discuss the role and impact of net-baryon number conservation in measurements of net proton fluctuations in heavy-ion collisions. I show that the magnitude of the fluctuations is entirely determined by the strength of two particle correlations. At LHC and top RHIC energy, this implies the fluctuations are proportional to the integral of the balance function (BF), $B^{p\bar p}$ of protons and anti-protons, while in the context of the RHIC beam energy scan (BES), one must also account for correlations of "stopped" protons. The integral of $B^{p\bar p}$ measured in a $4\pi$ detector depends on the relative cross-sections of processes yielding $p\bar p$ and those balancing the proton baryon number via the production of other anti-baryons. The accepted integral of $B^{p\bar p}$ further depends on the shape and width of the BF relative to the width of the acceptance. The magnitude of the measured second order cumulant of net proton fluctuations thus has much less to do with QCD susceptibilities than with the creation/transport of baryons and anti-baryons in heavy-ion collisions, and most particularly the impact of radial flow on the width of the BF. I thus advocate that net-proton fluctuations should be studied by means of differential BF measurements rather integral correlators.

Prospects of Ξ(1530)0Ξ̄(1530)0pair production at BESIII

The study of the baryon–antibaryon pair production from the charmonium states provides a favorable test and an input for the perturbative quantum chromodynamics calculation. Based on a Monte Carlo study via a single tag baryon strategy, a clear observation of [Formula: see text](3686)[Formula: see text](1530)[Formula: see text](1530)0is predicted at BESIII with the current 3.08[Formula: see text]106[Formula: see text](3686) dataset. The averaged precision of the [Formula: see text] parameters in the decay angular distribution formula is estimated as 0.25, and is not effective to separate the theoretical predictions. It will be an opportunity to distinguish them with 3.0[Formula: see text]109[Formula: see text](3686) events taken by BESIII detector in the next few years.

Multiplicity fluctuation and correlation of identified baryons in a quark combination model

The dynamical fluctuation and correlation of multiplicity distributions of identified baryons and antibaryons produced by the hadronization of the bulk quark system are systematically studied in quark combination model. Starting from the most basic dynamics of quark combination which are necessary for multiplicity study, we analyze moments (variance, skewness and kurtosis) of inclusive multiplicity distribution of identified baryons, two-baryon multiplicity correlations, and baryon-antibaryon multiplicity correlations after the hadronization of quark system with given quark number and antiquark number. We obtain a series of interesting findings, e.g., binomial behavior of multiplicity moments, coincide flavor dependent two-baryon correlation and universal baryon-antibaryon correlation, which can be regarded as general features of quark combination. We further take into account correlations and fluctuations of quark numbers before hadronization to study their influence on multiple production of baryons and antibaryons. We find that quark number fluctuation and flavor conservation lead to a series of important results such as the negative $p\bar{\Omega}^{+}$ multiplicity correlation and universal two-baryon correlations. We also study the influence of resonance decay in order to compare our results with future experimental data in ultra-relativistic heavy ion collisions at LHC.

(Anti-)strangeness production in heavy-ion collisions

The production and dynamics of strange and antistrange hadrons in heavy-ion reactions from √sNN ≈ 3 GeV to 200 GeV is analyzed within the Parton-Hadron-String- Dynamics (PHSD) transport model. The PHSD results for strange baryon and antibaryon production are roughly consistent with the experimental data starting from upper SPS energies. Nevertheless, hadronic final state flavor-exchange reactions are important for the actual abundances, in particular at large rapidities where hadronic dynamics, parton fragmentation and string decay dominate. A striking disagreement between the PHSD results and the available data persists, however, for bombarding energies below √sNN ≈ 8 GeV where the strangeness production is significantly underestimated as in earlier HSD studies. This finding implies that the strangeness enhancement seen experimentally at FAIR/NICA energies cannot be attributed to a deconfinement phase transition or crossover but probably involves the approximate restoration of chiral symmetry in the hadronic phase.References

Net-Baryon Production in Nucleus-Nucleus Collisions and Rare QGP Events

The experimental data on net proton and net Λ-hyperon spectra obtained by the NA35 Collaboration, as well as the inclusive densities of Λ and obtained by NA49, NA57, and STAR collaborations, are compared with the predictions of the Quark-Gluon String Model. The contributions of String Junction diffusion, interactions with nuclear clusters, and percolation corrections are accounted for. The level of numerical agreement of the calculations with the experimental data is of about 20-30%. We also consider the experimental ratios of multistrange to strange antibaryon production. The significant differences between the experimental and, especially, values and the QGSM predictions can be interpreted as the signal of the existence of an additional source of multistrange antibaryon production.

The P̅ANDA apparatus

The PANDA experiment will make use of cooled antiproton beams of unprecedented quality that will be available at the Facility for Antiproton and Ion Research (FAIR) in Darmstadt. The envisaged physics program includes: meson spectroscopy, baryon antibaryon production, baryon spectroscopy, hypernuclar physics, hadron properties in the nuclear medium and electromagnetic processes. This rich physics program asks for a general purpose apparatus. The design of the experiment is an advanced stage and the R&D phase is approaching its final phase, as resulted by most of the Technical Design Reports (TDRs) being already completed or under writing. In addition the production phase has already started for the electromagnetic calorimeter.

Evolution of antibaryon abundances in the early universe and in heavy-ion collisions

We study the kinetics of antibaryon production and annihilation in an expanding system, assuming that it is spatially homogeneous and chemically equilibrated at the initial stage. By solving simplified rate equations for (anti)baryon abundances we study the deviations from chemical equilibrium at late stages. The calculations are done for different expansion rates and net-baryon-to-entropy ratios, covering the conditions from early Universe to heavy-ion collisions. Our analysis includes both stable (anti)baryons and resonances. We conclude that residual antibaryon abundances are very sensitive to the time scales of expansion. Our calculations naturally explain noticeable deviations of antiproton-to-pion and proton-to-pion ratios observed in nuclear collisions at the LHC energy from the thermal model predictions. We conclude that at high bombarding energies the chemical freeze-out of (anti)baryons should occur at lower temperatures as compared to mesons.

Resonances from PHSD

The multi-strange baryon and vector meson resonance production in relativistic nucleus-nucleus collisions is studied within the parton-hadron- string dynamics (PHSD) approach which incorporates explicit partonic degrees- of-freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. We find a significant effect of the partonic phase on the production of multi-strange antibaryons at SPS energies due to a slightly enhanced s¯ s pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process. We, futhermore, obtain a visible in-medium effects in the low mass dilepton sector from dynamical vector-meson spectral functions from SIS to SPS energies whereas at RHIC and LHC energies such medium ef- fects become more moderate. In the intermediate mass regime from 1.1 to 3 GeV pronounced traces of the partonic degrees of freedom are found at SPS energies which superseed the hadronic (multi-meson) channels as well as the correlated and uncorrelated semi-leptonic D-meson decays. The dilepton production from the strongly interacting quark-gluon-plasma (sQGP) becomes already visible at top SPS energies and more pronounced at RHIC and LHC energies.

Nonequilibrium parton dynamics in the strongly interacting QGP

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results—including the partonic equation of state—in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energymomentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant ‘pre-hadronic’ color-dipole states (q\(\bar q\) or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. When applying the PHSD approach to Pb + Pb colllisions at 158 A GeV we find a significant effect of the partonic phase on the production of multi-strange antibaryons due to a slightly enhanced s\(\bar q\) pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.

Strangeness production within parton–hadron–string dynamics (PHSD)

Parton-hadron–string dynamics (PHSD) transport approach consistently simulates the full evolution of a relativistic heavy ion collision from the initial hard scatterings and string formation through the dynamical deconfinement phase transition to quark gluon plasma (QGP) to the hadronization and to the subsequent interactions in the hadronic phase. The transport theoretical description of quarks and gluons is based on a dynamical quasiparticle model for partons matched to reproduce lattice QCD results in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark–antiquark pairs or three quarks (antiquarks). Studying Pb+Pb reactions from 40 to 158 A GeV, we find that at most 40% of the collision energy is stored in the dynamics of the partons. This implies a large fraction of non-partonic, i.e. hadronic or string-like matter, which can be viewed as a hadronic corona; thus, neither hadronic nor purely partonic models can be employed to extract physical conclusions in comparing model results with data. On the other hand, comparing the PHSD results to those of the hadron-string dynamics (HSD) approach without the phase transition to QGP, we observe that the existence of the partonic phase has a sizeable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect of the QGP on the production of multi-strange antibaryons due to a slightly enhanced pair production in the partonic phase from massive time-like gluon decay and to a more abundant formation of strange antibaryons in the hadronization process.

Covariant transport approach for strongly interacting partonic systems

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results – including the partonic equation of state – in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy-momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant 'pre-hadronic' color-dipole states (qq̄ or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. When applying the PHSD approach to Pb+Pb colllisions at 158 A·GeV we find a significant effect of the partonic phase on the production of multi-strange antibaryons due to a slightly enhanced ss̄ pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.

Parton–hadron–string dynamics: An off-shell transport approach for relativistic energies

The dynamics of partons, hadrons and strings in relativistic nucleus–nucleus collisions is analyzed within the novel Parton–Hadron–String Dynamics (PHSD) transport approach, which is based on a Dynamical QuasiParticle Model (DQPM) for partons matched to reproduce recent lattice-QCD results – including the partonic equation of state – in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark–antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy–momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant ‘pre-hadronic’ color-dipole states ( q q ¯ or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. The PHSD approach is applied to nucleus–nucleus collisions from 20 to 160 A GeV in order to explore the space–time regions of ‘partonic matter’. We find that even central collisions at the top-SPS energy of 158 A GeV show a large fraction of non-partonic, i.e. hadronic or string-like matter, which can be viewed as a hadronic corona. This finding implies that neither hadronic nor only partonic ‘models’ can be employed to extract physical conclusions in comparing model results with data. On the other hand – studying in detail Pb + Pb reactions from 40 to 158 A GeV – we observe that the partonic phase has a very low impact on rapidity distributions of hadrons but a sizeable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect on the production of multi-strange antibaryons due to a slightly enhanced s s ¯ pair production in the partonic phase from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.

Systematic measurements of identified particle spectra inpp,d+Au, andAu+Aucollisions at the STAR detector

Identified charged-particle spectra of pi(+/-), K(+/-), p, and (p) over bar at midrapidity (vertical bar y vertical bar < 0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d + Au collisions at root s(NN) = 200 GeV and for Au + Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm(3) for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au + Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au + Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d + Au, and peripheral Au + Au collisions, drops significantly with centrality in Au + Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au + Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

JET CORRELATIONS WITH IDENTIFIED PARTICLES FROM PHENIX: METHODS AND RESULTS

Azimuthal angle two particle correlations have been shown to be a powerful probe for extracting novel features of the interaction between hard scattered partons and the medium produced in Au + Au collisions at RHIC. At intermediate pT, 2–5 GeV/c, the jets have been shown to be significantly modified in both their particle composition and their angular distribution compared to p + p collisions. Additionally, angular two particle correlations with identified hadrons provide information on the possible role of modified hadronization scenarios such as partonic recombination, which might allow medium modified jet fragmentation by connecting hard scattered partons to low pT thermal partons. PHENIX has excellent particle identification capabilities and has developed robust techniques for extracting jet correlations from the large underlying event. We present recent PHENIX results from Au + Au collisions for a variety of pT and particle type combinations. We also present p + p measurements as a baseline. We show evidence that protons and anti-protons in the pT region of enhanced baryon and anti-baryon single particle production are produced in close angle pairs of opposite charge and that the strong modifications to the away side shape observed for charged hadron correlations are also present when baryons are correlated.

Results from NA57

The NA57 experiment has measured strange baryon and antibaryon production in Pb–Pb collisions at 40 A GeV/c and 158 A GeV/c beam momenta. Expansion dynamics has been investigated; transverse dynamics using blast wave fits, longitudinal dynamics obtained from rapidity distributions. HBT analysis gives results compatible with those obtained by the other methods. Similar values are observed for the transverse and longitudinal flow parameters. Particles carrying all levels of strangeness have been studied. Enhancement factors increase with the strangeness content of the particle, reaching above 20 for Ωs in central collisions. Enhancement values are strikingly similar for SPS and RHIC energies. Central-to-peripheral nuclear modification factors have been measured. The baryon/meson pattern displays similarities with that observed at RHIC.

Can $\phi$ mesons give an answer to the baryon puzzle at RHIC?

The PHENIX experiment at RHIC has observed a large enhancement of baryon and anti-baryon production at p(T) approximate to 2-5 GeV/c, compared to expectations from jet fragmentation. While a number of theoretical interpretations of the data are available, there is not yet a definitive answer to the "baryon puzzle". We investigate the centrality dependence of phi-meson production at mid-rapidity in Au+Au collisions with root s(NN) = 200 GeV. Comparison with the proton and anti-proton spectra reveal similar shapes, as expected for soft production described by hydrodynamics. However, the absolute yields show a different centrality dependence. The nuclear modi. cation factors for phi are similar to those of pions, rather than (anti) protons that have similar mass. At intermediate p(T), baryon/meson effects seem to be more important than the mass effects, in support of recombination models. (Less)

Comments on particle production in p+p, p+A and A+A reactions

Pion production, strangeness production and baryon stopping in elementary and nuclear collisions are discussed. It is argued that particle production in A+A reactions is closely related to that observed in p+p and p+A collisions. New features of baryon and anti-baryon production are pointed out. This leads to further conclusions about the similarities between p+p, p+A and A+A interactions.

Energy dependence of hyperon production in nucleus–nucleus collisions at SPS

A measurement of strange baryon and antibaryon production in Pb–Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 AGeV/c beam momentum. Results on Λ, Ξ and Ω hyperon yields at mid-rapidity in the most central 53% of Pb–Pb collisions at 40 AGeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range.The Λ and Ξ− yields per unit rapidity stay roughly constant while those of Ω−, Λ̄, Ξ̄+ and Ω̄+ increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in sNN=130 GeV Au–Au collisions at RHIC.