Strangeness production in nuclear interactions at 200 AGeV and the number of nucleon-nucleon collisions

Data on mean numbers of $\Lambda$, $\bar\Lambda$, K's and on the total number $$of pairs of strange valence quarks in final state hadrons in hadronic and nuclear collisions at CERN- SPS energies are studied as a function of the mean number$$ of nucleon- nucleon collisions. Results give indications of an almost linear dependence over most of the region of $$. This in turn points out to strangeness being produced mostly in the central rapidity region of nucleon- nucleon collisions by a mechanism similar to a hard or semi- hard process. The available data are extrapolated to Pb+Pb interactions by two simple models, leading to $_{Pb+Pb}\approx 300\pm 30$. Observations exceeding these values of $_{Pb+Pb}$ would give evidence of the onset of a new dynamical regime in Pb+Pb interactions.Comment: 16 pages,3 figure

A model of anomalous production of strange baryons in nuclear collisions

We propose a simple model of production of strange baryons and antibaryons in nuclear collisions at the CERN SPS. The model takes into account both the increase of strangeness production in collisions of lighter ions and a possibility of the formation of anomalous, strangeness rich matter in central PbPb interactions. It is shown that ratios like $::$ depend strongly on the presence of anomalous matter and can be used to determine its phenomenological parameters. In the model we assume that particle composition of final state hadrons is essentially given by a rapid recombination of quarks and antiquarks formed in tube-on-tube interactions of incoming nucleons.Comment: 17 pages, 3 picture

Multiplicity and event-scale dependent flow and jet fragmentation in pp collisions at s√ = 13 TeV and in p−Pb collisions at sNN−−−√ = 5.02 TeV

Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s√=13 TeV and p−Pb collisions at sNN−−−√=5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle Δφ and pseudorapidity separation Δη for pairs of primary charged particles within the pseudorapidity interval |η|<0.9 and the transverse-momentum interval 1<pT<4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6<|Δη|<1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events

Multiplicity-dependent production of Σ(1385)^± and Ξ(1530)⁰ in pp collisions at √s=13 TeV

The production yields of the Σ(1385)± and Ξ(1530)0 resonances are measured in pp collisions at √s=13 TeV with ALICE. The measurements are performed as a function of the charged particle multiplicity ⟨dNch/dη⟩, which is related to the energy density produced in the collision. The results include transverse momentum (pT) distributions, pT-integrated yields, mean transverse momenta of Σ(1385)± and Ξ(1530)0, as well as ratios of the pT-integrated resonance yields relative to yields of other hadron species. The Σ(1385)±/π± and Ξ(1530)0/π± yield ratios are consistent with the trend of the enhancement of strangeness production from low to high multiplicity pp collisions, which was previously observed for strange and multi-strange baryons. The yield ratio between the measured resonances and the long-lived baryons with the same strangeness content exhibits a hint of a mild increasing trend at low multiplicity, despite too large uncertainties to exclude the flat behaviour. The results are compared to predictions from models such as EPOS-LHC and PYTHIA 8 with Rope shoving. The latter provides the best description of the multiplicity dependence of the Σ(1385)± and Ξ(1530)0 production in pp collisions at √s=13 TeV

Azimuthal anisotropy of jet particles in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV

The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV down to transverse momentum (pT) of 0.5 GeV/c and 2 GeV/c, respectively, with ALICE. The second-order Fourier coefficient of the jet-particle azimuthal distribution (v2) in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8σ at low pT. Comparisons with the inclusive charged-particle v2 and with AMPT calculations are discussed. The model describes qualitatively the main features of the jet-particle v2 in high-multiplicity p-Pb collisions and indicates that the positive jet-particle v2 is generated by parton interactions