Multistrangeness in Heavy-Ion Collisions

AIP Conference Proceedings, 2001

We suppose that overall strangeness production in both high energy elementary and heavy ion collisions can be described within the framework of an equilibrium statistical model in which the effective degrees of freedom are constituent quarks as used in effective lagrangian models. In this picture, the excess of relative strangeness production in heavy ion collisions with respect to elementary particle collisions arises from the unbalance between initial non-strange matter and antimatter and from the exact colour and flavour quantum number conservation over different finite volumes. The comparison with the data and the possible sources of model dependence are discussed.

The European Physical Journal C - Particles and Fields, 2002

The canonical statistical model analysis of strange and multistrange hadron production in central A-A relative to p-p/p-A collisions is presented over the energy range from √ s = 8.73 GeV up to √ s = 130 GeV. It is shown that the relative enhancement of strange particle yields from p-p/p-A to A-A collisions substantially increases with decreasing collision energy. It is largest at √ s = 8.7 GeV, where the enhancement of Ω, Ξ and Λ is of the order of 100, 20 and 3, respectively. In terms of the model these results are due to the canonical suppression of particle thermal phase space at lower energies, which increases with the strangeness content of the particle and with decreasing size of the collision fireball. The comparison of the model with existing data on energy dependence of the kaon/pion ratio is also discussed.

Nature Physics

At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark–gluon plasma (QGP)1. Such an exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed2,3,4,5,6. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions7, is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton–proton (pp) collisions8,9, but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton–proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event ch...

Journal of Physics G: Nuclear and Particle Physics, 2009

Journal of Physics G: Nuclear and Particle Physics, 2010

Prospects for strangeness production in p−p collisions at the Large Hadron Collider (LHC) are discussed within the statistical model. Firstly, the system size and the energy dependence of the model parameters are extracted from existing data and extrapolated to LHC energy. Particular attention is paid to demonstrate that the chemical decoupling temperature is independent of the system size. In the energy regime investigated so far, strangeness production in p − p interactions is strongly influenced by the canonical suppression effects. At LHC energies, this influence might be reduced. Particle ratios with particular sensitivity to canonical effects are indicated. Secondly, the relation between the strangeness production and the charged-particle multiplicity in p − p interactions is investigated. In this context the multiplicity dependence studied at Tevatron is of particular interest. There, the trend in relative strangeness production known from centrality dependent heavy-ion collisions is not seen in multiplicity selected p − p interactions. However, the conclusion from the Tevatron measurements is based on rather limited data samples with low statistics and number of observables. We argue, that there is an absolute need at LHC to measure strangeness production in events with different multiplicities to possibly disentangle relations and differences between particle production in p − p and heavy-ion collisions.

Quark-Gluon Plasma and Heavy Ion Collisions - Proceedings of a Meeting Held in the Framework of the Activities of GISELDA, the Italian Working Group on Strong Interactions, 2002

We develop a model to calculate strangeness production in both elementary and heavy ion collisions, within the framework of a statistical approach to hadronisation. Calculations are based on the canonical partition function of the thermal Nambu-Jona-Lasinio model with exact conservation of flavor and color. It turns out that the growth of strange quarks production in heavy ion collisions is due to the initial excess of non-strange matter over antimatter, whereas a suppression occurs for elementary collisions, owing to the constraint of exact quantum charges conservation over small volumes.

Journal of Physics G-nuclear and Particle Physics, 2010

We present recent results on the production, spectra and elliptic flow of strange particles in dynamic simulations employing hadronic degrees of freedom and from recombination models. The main focus will be on the Ultra-relativistic Molecular Dynamics (UrQMD) Boltzmann approach to relativistic heavy ion collisions and a hybrid approach with intermediate hydrodynamic evolution based on UrQMD (available for download as UrQMD v3.3). Compared to the standard binary collision approach, an enhancement of the strange particle particle yields is found in the hybrid approach due to the assumption of local equilibration. The production origins of the Phi-meson in the hybrid approach are studied in further detail. We also present results on the transverse momentum spectra of baryon to meson ratios of strange particles. Due to the approximate energy independent scaling of this ratio as a function of p_T we argue, that a maximum in these spectra may not be a unique sign for quark coalescence but can be understood in terms of flow and fragmentation.

At suuciently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark–gluon plasma (QGP) 1. Such an exotic state of strongly interacting quantum chro-modynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed 2–6. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions 7 , is more pronounced for multi-strange baryons. Several eeects typical of heavy-ion phenomenology have been observed in high-multiplicity proton–proton (pp) collisions 8,9 , but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton–proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with the p–Pb collision results 10,11 , indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb–Pb collisions, where a QGP is formed. The production of strange hadrons in high-energy hadronic interactions provides a way to investigate the properties of quantum chromodynamics (QCD), the theory of strongly interacting matter. Unlike up (u) and down (d) quarks, which form ordinary matter, strange (s) quarks are not present as valence quarks in the initial state, yet they are sufficiently light to be abundantly created during the course of the collisions. In the early stages of high-energy collisions, strangeness is produced in hard (perturbative) 2 → 2 partonic scattering processes by flavour creation (gg → s¯ s, q¯ q → s¯ s) and flavour excitation (gs → gs, qs → qs). Strangeness is also created during the subsequent partonic evolution via gluon splittings (g → s¯ s). These processes tend to dominate the production of high transverse momentum (p T) strange hadrons. At low p T , non-perturbative processes dominate the production of strange hadrons. In string fragmentation models the production of strange hadrons is generally suppressed relative to hadrons containing only light quarks, as the strange quark is heavier than up and down quarks. The amount of strangeness suppression in elementary (e + e − and pp) collisions is an important parameter in Monte Carlo (MC) models. For this reason, measurements of strange hadron production place constraints on these models. The abundances of strange particles relative to pions in heavy-ion collisions from top RHIC (Relativistic Heavy-Ion Collider) to LHC (Large Hadron Collider) energies do not show a significant dependence on either the initial volume (collision centrality) or the initial energy density (collision energy). With the exception of the most peripheral collisions, particle ratios are found to be compatible with those of a hadron gas in thermal and chemical equilibrium and can be described using a grand-canonical statistical model 12,13. In peripheral collisions, where the overlap of the colliding nuclei becomes very small, the relative yields of strange particles to pions decrease and tend toward those observed in pp collisions, for which a statistical-mechanics approach can also be applied 14,15. Extensions of a pure grand-canonical description of particle production , such as statistical models implementing strangeness canon-ical suppression 16 and core–corona superposition 17,18 models, can effectively produce a suppression of strangeness production in small systems. However, the microscopic origin of enhanced strangeness production is not known, and the measurements presented in this Letter may contribute to its understanding. Several effects, such as azimuthal correlations and mass-dependent hardening of p T distributions , which in nuclear collisions are typically attributed to the formation of a strongly interacting quark–gluon medium, have been observed in high-multiplicity pp and proton–nucleus collisions at the LHC 8–11,19–25. Yet, enhanced production of strange particles as a function of the charged-particle multiplicity density (dN ch /dη) has so far not been observed in pp collisions. The study of pp collisions at high multiplicity is thus of considerable interest as it opens the exciting possibility of a microscopic understanding of phenomena known from nuclear reactions. In this Letter, we present the multiplicity dependence of the production of primary strange (K 0 S , Λ, Λ) and multi-strange (Ξ − , Ξ + , Ω − , Ω +) hadrons in pp collisions at the centre-of-mass energy of √ s = 7 TeV. Primary particles are defined as all particles created in the collisions, except those coming from weak decays of light-flavour hadrons and of muons. The measurements have been performed at midrapidity (the particle rapidity is defined as y = (1/2) ln((E + p z c)/(E − p z c)), where E is the energy and p z is the component of momentum along the beam axis), y < 0.5, with the ALICE detector 26 at the LHC. Similar measurements of the multiplicity and centrality dependence of strange and multi-strange hadron production have been performed by ALICE in proton– lead (p–Pb) collisions at a centre-of-mass energy per nucleon pair √ s NN = 5.02 TeV (refs 10,11) and in lead–lead (Pb–Pb) collisions at √ s NN = 2.76 TeV (refs 6,27). The measurements reported here have been obtained in pp collisions at √ s = 7 TeV for events having at least one charged particle produced in the pseudorapidity (the particle pseudorapidity is defined as η = − ln(tan(θ/2)), where θ is the angle with respect to the beam axis) interval |η| < 1 (INEL > 0), corresponding to about 75% of the total inelastic cross-section. To study the multiplicity dependence of strange and multi-strange hadron production, the sample is divided into event classes based on the total ionization energy deposited in the forward detectors, covering the pseudorapidity regions 2.8 < η < 5.1 and −3.7 < η < −1.7. Particle/antiparticle production yields are identical within uncertainties. The p T distributions of K 0 S , Λ + Λ, Ξ − + Ξ + and Ω − + Ω + (in the following denoted as K 0 S , Λ, Ξ and Ω) are shown in Fig. 1 for a selection of event classes with progressively decreasing † A full list of authors and aaliations appears at the end of the paper. NATURE PHYSICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephysics

Physical Review C, 2001

A systematic study is performed of fully integrated particle multiplicities in central Au-Au and Pb-Pb collisions at beam momenta of 1.7A GeV/c, 11.6A GeV/c (Au-Au) and 158A GeV/c (Pb-Pb) by using a statistical-thermal model. The close similarity of the colliding systems makes it possible to study heavy ion collisions under definite initial conditions over a range of centre-of-mass energies covering more than one order of magnitude. In order to further study the behaviour of strangeness production, an updated study of Si-Au collisions at 14.6A GeV is also presented. The data analysis has been performed with two completely independent numerical algorithms giving closely consistent results. We conclude that a thermal model description of particle multiplicities, with additional strangeness suppression, is possible for each energy. The degree of chemical equilibrium of strange particles and the relative production of strange quarks with respect to u and d quarks are higher than in e + e − , pp and pp collisions at comparable and even at lower energies. The behaviour of strangeness production as a function of centre-of-mass energy and colliding system is presented and discussed. The average energy per hadron in the comoving frame is close to 1 GeV per hadron despite the fact that the energy increases more than 10-fold. PACS:24.10.Pa,25.75.Dw,25.75.-q

Nuclear Physics A, 2003

An effective model with constituent quarks as fundamental degrees of freedom is used to predict the relative strangeness production pattern in both high energy elementary and heavy ion collisions. The basic picture is that of the statistical hadronization model, with hadronizing color-singlet clusters assumed to be at full chemical equilibrium at constituent quark level. Thus, by assuming that at least the ratio between strange and non-strange constituent quarks survives in the final hadrons, the apparent undersaturation of strange particle phase space observed in the data can be accounted for. In this framework, the enhancement of relative strangeness production in heavy ion collisions in comparison with elementary collisions is mainly owing to the excess of initial non-strange matter over antimatter and the so-called canonical suppression, namely the constraint of exact color and flavor conservation over small volumes.