Strangeness Yield Research Articles

Cosmological strangeness abundance

We investigate the strange particle composition of the expanding early Universe in the hadron epoch Th≈150≥T≥10 MeV. We study strangeness yield in thermal and chemical equilibrium constrained by prescribed conserved entropy per baryon in a charge neutral and strangeness neutral 〈s−s¯〉 Universe. Turning to kinetic processes in a Hubble expanding Universe, we determine the conditions at which at decreasing temperature T individual strangeness producing reactions fall out of detailed balance between decay and back-reaction strangeness production rates: This is done by comparing the relevant reaction rates to the Hubble expansion rate. We allow for weak, electromagnetic, and strong interaction processes. The weak interaction μ±+νμ→K± freezeout is at TfK±=33.8 MeV; the electromagnetic process l−+l+→ϕ freezeout is at Tfϕ=23∼25 MeV; and the hadronic reaction π+π→K freezeout is at TfK=19.8 MeV.

Monte-Carlo study of long-range correlations of average transverse momentum and multiplicity for strange particles in pp-collisions at the LHC energies

This study is motivated by the puzzling enhanced production of multi-strange hadrons observed for the first time by the ALICE Collaboration in high-multiplicity pp-collisions at at the LHC. We investigate this effect of strangeness enhancement in pp-collisions in the framework of the MC event generator PYTHIA 8 where the collectivity processes are taken into account by the formation of a so-called flavour rope. The latter is hadronized with a larger effective string tension providing the increase of strangeness yield. This concept of the flavour rope, as a source of multi-strange hadrons, extended in rapidity, is being tested by the study of so called long-range correlations (LRC) between the average transverse momentum 〈pT〉 and multiplicity n for charged particles containing strange quarks produced in high energy pp-collisions.The dependencies of correlation coefficients bn−n, bPT−n and bPT−PT on the gap between forward and backward pseudorapidity windows, and on the width of the forward pseudorapidity windows, are studied and the results are discussed.

Freeze-out conditions from strangeness observables at RHIC

We determine chemical freeze-out conditions from strangeness observables measured at RHIC beam energies. Based on a combined analysis of lowest-order net-kaon fluctuations and strange anti-baryon over baryon yield ratios we obtain visibly enhanced freeze-out conditions at high beam energies compared to previous studies which analyzed net-proton and net-charge fluctuations. Our findings are in qualitative agreement with the recent study (Bellwied et al. in arXiv:1805.00088 [hep-ph], 2018) which utilizes the net-kaon fluctuation data in combination with information from lattice QCD. Our complementary approach shows that also strange hadron yield ratios are described by such enhanced freeze-out conditions.

Complete strangeness measurements in heavy-ion collisions

We discuss strangeness production in heavy-ion collisions within and around the energy range of the planned NICA facility. We describe the minimal statistical model, in which the total strangeness yield is fixed by the observed or calculated $K^+$ multiplicity. We show how the exact strangeness conservation can be taken into account on the event-by-event basis in such a model. We argue that from strange particle yields one can reveal information about the collision dynamics and about possible modifications of particle properties in medium. This can be best achieved if the complete strangeness measurement is performed, i.e. kaons, anti-kaons, hyperons and multistrange hyperons are registered in the same experimental setup. In particular the production of hadrons containing two and more strange quarks, like $\Xi$ and $\Omega$ baryons could be of interest.

Interpretation of strange hadron production at LHC

We extend the SHM analysis of hadron production results showing here consistency with the increased experimental data set, stability of the fit with regard to inclusion of finite resonance widths and 2-star hyperon resonances. We present new results on strangeness yield as a function of centrality and present their interpretation in terms of QGP inspired model of strangeness abundance in the hadronizing fireball.

Strangeness chemical equilibration in a quark-gluon plasma

We study, in the dynamically evolving quark-gluon plasma (QGP) fireball formed in relativistic heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC), the growth of strangeness yield toward and beyond the chemical equilibrium. We account for the contribution of the direct strangeness production and evaluate the thermal-QCD strangeness production mechanisms. The specific yield of strangeness per entropy, $s/S$, is the primary target variable. We explore the effect of collision impact parameter, i.e., fireball size, on kinetic strangeness chemical equilibration in QGP. Insights gained in studying the RHIC data with regard to the dynamics of the fireball are applied to the study of strangeness production at the LHC. We use these results and consider the strange hadron relative particle yields at RHIC and LHC in a systematic fashion. We consider both the dependence on $s/S$ and the direct dependence on the participant number.

Perturbative dynamics of strangeness production at RHIC

Strange quark production in Au–Au collisions at RHIC is studied in the framework of the parton cascade model (PCM). The yields of (anti-) strange quarks for three production scenarios—primary–primary scattering full scattering and full production—are compared to a proton–proton baseline. Enhancement of strange quark yields in central Au–Au collisions compared to scaled p–p collisions increases with the number of secondary interactions. The centrality dependence of strangeness production for the three production scenarios is studied as well. For all production mechanisms, the strangeness yield increases with (Npart)4/3. The perturbative QCD regime described by the PCM is able to account for up to 50% of the observed strangeness at RHIC.

Continuum limit of quark number susceptibilities

We report the continuum limit of quark number susceptibilities in quenched QCD. Deviations from ideal gas behaviour at temperature T increase as the lattice spacing is decreased from T/4 to T/6, but a further decrease seems to have very little effect. The measured susceptibilities are 20% lower than the ideal gas values, and also 10% below the hard thermal loop (HTL) results. The off-diagonal susceptibility is several orders of magnitude smaller than the HTL results. We verify a strong correlation between the lowest screening mass and the susceptibility. We also show that the quark number susceptibilities give a reasonable account of the Wroblewski parameter, which measures the strangeness yield in a heavy-ion collision.

Excitation function of strangeness in A+A reactions from SIS to RHIC energies

The properties of K + and K − mesons are studied in nuclear reactions from SIS to RHIC energies within the covariant transport approach HSD in comparison to the experimental data whenever available. Whereas kaon abundancies and spectra indicate little repulsive or vanishing selfenergies in the medium, antikaons are found to experience strong attractive potentials in nucleus-nucleus collisions at SIS energies. However, even when including these potentials the K + and K − spectra at AGS energies are noticeably underestimated showing an experimental excess of strangeness that points towards a nonhadronic phase in these reactions. On the other hand the K +, K − production at SPS energies is again well described by the HSD approach based on quark, diquark, string and hadronic degress of freedom. At RHIC energies hadronic rescatterings are found to enhance the strangeness yield from the partonic phase quite substantially.

A numerical study of an expanding plasma of quarks in a chiral model

We numerically solve the transport equations for a quark gas described by the the Nambu-Jona-Lasinio model. The mean field equations of motion, which consist of the Vlasov equation for the density and the gap equation for the mean field, are discussed, and energy and momentum conservation are proven. Numerical solutions of the partial differential equations are obtained by applying finite difference methods. For an expanding fireball the light quark mass evolves from small values initially to the value of 350 MeV. This leads to a depletion of the high energy part of the quark spectrum and an enhancement at low momenta. When collisions are included one obtains an equation of the Boltzmann type, where the transition amplitudes depend on the properties of the medium. These equations are given for flavor SU(3), i.e. including strangeness. They are solved numerically in the relaxation time approximation and the time evolution of various observables is given. Medium effects in the relaxation times do not significantly influence the shape of the spectra. The mass of the strange quark changes little during the expansion. The strangeness yield and the slope temperatures of the final spectra are studied as a function of the size of the initial fireball.

Thermal strangeness and charm in QGP

Using QCD methods we re-evaluate strangeness and charm production in a thermal QGP fireball. QCD renormalization group is employed to evaluate running of the parameters αs(μ) andm s(μ). We resum even-αs Feynman diagrams involving two particles in initial and final states. We obtain thermal relaxation times in a simple dynamical description of the fireball and we use these results to study two generic strangeness observables as function of collision energy: specific (per baryon) strangeness yield\({{\left\langle {\bar s} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\bar s} \right\rangle } B}} \right. \kern-\nulldelimiterspace} B}\), and phase space occupancy γs. From these two quantities, knowing the hadronization process in rough detail, we obtain the final state strange particle abundances.

Double strangeness production in low-energy antiproton-nucleus annihilation

Double strangeness production, i.e. the net production of two s quarks and two s quarks (carried inside hadrons) in antiproton-nucleus annihilations is investigated theoretically. The various mechanisms are identified, both for conventional B = 0 and for unusual B = 1,2 annihilations. The double strangeness yield is estimated. The lack of precise input data for some reactions is underlined. The results are compared with the recent data of the DIANA collaboration. They seem to be consistent with the conventional processes.

Strange particle abundance in QGP formed in 200 GeV A nuclear collisions

We investigate the relative abundance of strange particles produced in nuclear collisions at the SPS energies (∼ 9 GeV A in CM frame) assuming that the central reaction fireball consists of quark-gluon plasma. We show that the total strangeness yield observed in Sulphur induced reactions is compatible with this picture.

Unified approach to theJ/Ψ suppression and strangeness enhancement in high-energy nucleus-nucleus collisions

A picture of nuclear interactions at high energies (pLab≧100 GeV/c) is proposed, where the only essential difference between nucleon-neucleon and central nucleus-nucleus collisions is the size of the interaction zone. In both cases intermediate multi-parton systems are created, which achieve thermodynamical equilibrium with respect to characteristics of light quarks and gluons. The characteristics of heavier quarks are far from equilibrium ones in nucleon-nucleon collisions, but the degree of equilibration is substantially increased in central heavy-ion collisions. Therefore, the multi-parton systems produced in central nucleus-nucleus interactions can manifest themseleves as the quark-gluon plasma. A simple model based on this picture is formulated and confronted with the experimental data concerningJ/Ψ suppression and strangeness yield enhancement, both revealed recently in central nucleus-nucleus collisions.

Strangeness production in ultra-relativistic heavy-ion collisions: Flavour kinetics II

A recently presented model for the non-equilibrium expansion and hadronization of and the strangeness production in a quark-gluon plasma is developed further. The model includes all members of the lowest flavour SU(3) multiplets, the effect of particle evaporation from the surface of the system and a hydrodynamic treatment of the volume expansion. The evaporation and the decay of resonances are found to be equally important for the production of pions and kaons, and to give about 60% of the total yield. If a quark-gluon plasma is formed, the relative strangeness yield is strongly enhanced compared to pp collisions. The predictions of the model are also compared to recent data for nucleus-nucleus collisions.

Strangeness production in antiproton annihilation on nuclei.

The strangeness production in antiproton annihilation on nuclei is investigated by means of a cascade-type model, within the frame of the conventional picture of the annihilation on a single nucleon followed by subsequent rescattering proceeding in the hadronic phase. The following hadrons are introduced: N, \ensuremath{\Lambda}, \ensuremath{\Sigma}, \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, \ensuremath{\pi}, \ensuremath{\eta}, \ensuremath{\omega}, K, and K\ifmmode\bar\else\textasciimacron\fi{} and, as far as possible, the experimental reaction cross sections are used in our simulation. The numerical results are compared with experimental data up to 4 GeV/c. The \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{} yield is correctly reproduced, while the \ensuremath{\Lambda} and ${\mathit{K}}_{\mathit{s}}$ yields are overestimated in the p\ifmmode\bar\else\textasciimacron\fi{}Ta and p\ifmmode\bar\else\textasciimacron\fi{}Ne cases. On the other hand, the rapidity and perpendicular momentum distributions are well reproduced. It is shown that total strange yield is not very much affected by the associated production taking place during the rescattering process. It is also shown that the \ensuremath{\Lambda}/${\mathit{K}}_{\mathit{s}}$ ratio is largely due to the strangeness exchange reactions induced by antikaons. In particular, values of the order of 1 to 3 are expected in the energy range investigated here, independently of the detail of the hadronic phase dynamics. Finally, it is stressed that rapidity distributions are consistent with the rescattering process. Comparison with other works and implications of our results are examined.

Jets in heavy-ion collisions as a probe of QCD flux-tube dynamics at finite temperature.

The properties of hadronic jets in high-energy heavy-ion collisions are discussed and it is found that thermal effects in the QCD fragmentation process can lead to a broader ${p}_{T}$ distribution and an increased strangeness yield. It is speculated that a phase transition in the central region would cause a significant increase of the effective jet temperature. A theoretical estimate of the thermal correction to the quark-pair-production rate in a strong color-electric field is given. It can be expressed as an effective perturbation of the background color-electric field. This result is applicable to QCD-inspired fragmentation models.