An update on femtoscopic pi-Xi correlations in Au+Au collisions as measured by the STAR experiment at RHIC is presented. Centrality dependence of Gaussian radii and relative emission asymmetry is extracted for the first time.
In these proceedings, we present the measurements of proton, light nuclei, and strange particle with neutral kaons correlation functions in Au+Au collisions at the BES program and top RHIC energy. The experimental results will be compared with model calculations to extract the size of emitting source and the properties of final state interactions. The collision energy and centrality dependence of the source size will be studied. Further, the implications for the production mechanism of light nuclei will be discussed.
In this paper we present the measurement of charged pion two-particle femtoscopic correlation functions in 200 GeV Au+Au collisions, in 31 average transverse mass bins, separately for positive and negative pion pairs. Lévy-shaped source distributions yield a statistically acceptable description of the measured correlation functions, with three physical parameters: correlation strength parameter lambda, Lévy index alpha and Lévy scale parameter R. The transverse mass dependence of these Lévy parameters is then investigated and their physical interpretation is also discussed, and the appearance …
Charged pion two-particle correlation functions were measured in 0-30% centrality 200 GeV Au+Au collisions with the PHENIX experiment at RHIC. The measured correlation functions can be statistically well described based on the assumption of Levy-shaped source distributions. In this proceedings paper we present the Levy parameters of the measured correlation functions: correlation strength parameter lambda, Levy index alpha and Levy scale parameter R as a function of pair transverse mass mT, in 31 bins from 228 to 871 MeV, separately for positive and negative pion pairs. We discuss the physical…
Femtoscopic measurements at a variety of facilities have established a clear dependence of spatial scales with event multiplicity and particle transverse mass ($m_T$) in heavy ion collisions from $\sqrt(s_{NN}) \sim 2-200 GeV$. The $m_T$-dependence is thought to arise from collective, explosive flow of the system, as probed by independent measurements, while the multiplicity dependence reflects the increased spatial extent of the final state with decreasing impact parameter. Qualitatively similar dependences have been reported from high energy hadron and lepton collisions, where the conceptual…
Azimuthally-sensitive femtoscopy for heavy-ion collisions at RHIC and LHC is explored within the approach consisting of the hydrodynamics of perfect fluid followed by statistical hadronization. It is found that for the RHIC initial conditions the very same framework that reproduces the standard soft observables (including the transverse-momentum spectra, the elliptic flow, and the azimuthally-averaged HBT radii) leads to a proper description of the azimuthally-sensitive femtoscopic observables - we find that the azimuthal variation of the side and out HBT radii is very well reproduced for all …
We report on the results of femtoscopic analysis of Pb-Pb collisions at sqrt(s_NN)=2.76 TeV and pp collisions at sqrt(s)=0.9, 2.76 and 7 TeV with identical pions and kaons. Detailed femtoscopy studies in heavy-ion collisions at SPS and RHIC have shown that emission region sizes ("HBT radii") decrease with increasing pair transverse momentum k_T, which is understood as a manifestation of the collective behavior of matter. The trend was predicted to persist at the LHC. The data from Pb-Pb collisions confirm the existence of a flowing medium and provide strict constraints on the dynamical models.…
Femtoscopy of non-central heavy ion collisions provides access to information on the geometry of the effective pion-emitting source. In particular, the source shape can be studied by measuring femtoscopic radii as a function of the pion emission angle relative to the collision symmetry planes. We present the results of azimuthally differential femtoscopy of Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the LHC relative to the second harmonic event plane. We observe a clear oscillation of the extracted radii as a function of the emission angle. We find that $R_{side}$ and $R_{out}$ oscillate …
We report the elliptic flow ($v_{2}$) of $K_{s}^{0}$, $Λ$, $\barΛ$, $ϕ$, $Ξ^{-}$, and $\overlineΞ^{+}$ at mid-rapidity for Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV. The transverse momentum ($p_{T}$) and centrality dependence of $v_{2}$ have been studied. The number of constituent quark (NCQ) scaling for all the (multi-)strange hadrons has also been tested. The dependence in the ratio of average $v_{2}$ ($\langle$$v_{2}$$\rangle$) on centrality shows a deviation from unity indicating a difference in nuclear structure and deformation between the isobars. The results show…
Ultra-relativistic gold-gold and proton-proton collisions are investigated in the experiments of the Relativistic Heavy Ion Collider (RHIC). In the last several years large amount of results were revealed about the matter created in these collisions. The latest PHENIX results for femtoscopy and correlations are reviewed in this paper. Bose-Einstein correlations of charged kaons in 200 GeV Au+Au collisions and of charged pions in 200 GeV p+p collisions are shown. They are both compatible with previous measurements of charged pions in gold-gold collisions, with respect to transverse mass or numb…
We report STAR measurements of mid-rapidity yields for the $Λ$, $\barΛ$, $K^{0}_{S}$, $Ξ^{-}$, $\barΞ^{+}$, $Ω^{-}$, and $\barΩ^{+}$ particles in Cu+Cu and Au+Au $\sqrt{s_{NN}} = 200$ GeV collisions. We show that at a given number of participating nucleons, bulk strangeness production is higher in Cu+Cu collisions compared to Au+Au collisions at the same center of mass energy, counter to predictions from the Canonical formalism. We compare both the Cu+Cu and Au+Au yields to AMPT and EPOS predictions, and find they reproduce key qualitative aspects of the data. Finally, we investigate other sca…
We report on the first femtoscopic measurement of baryon pairs, such as p-p, p-$Λ$ and $Λ$-$Λ$, measured by ALICE at the Large Hadron Collider (LHC) in proton-proton collisions at $\sqrt{s}$ = 7 TeV. This study demonstrates the feasibility of such measurements in pp collisions at ultrarelativistic energies. The femtoscopy method is employed to constrain the hyperon-nucleon and hyperon-hyperon interactions, which are still rather poorly understood. A new method to evaluate the influence of residual correlations induced by the decays of resonances and experimental impurities is hereby presented.…
We present the first azimuthally differential measurements of the pion source size relative to the second harmonic event plane in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of $\sqrt{s_{\rm NN}}=2.76$ TeV. The measurements have been performed in the centrality range 0-50% and for pion pair transverse momenta $0.2 < k_{\rm T} < 0.7$ GeV/$c$. We find that the $R_{\rm side}$ and $R_{\rm out}$ radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate out of phase, similar to what was observed at …
Strangeness measurement at RHIC energies constitutes one of the favorite theme of the STAR Collaboration. Besides the fact that strangeness enhancement has been proposed as a quark gluon plasma signature, its production provides various and relevant information about the collision evolution and especially on the hadronization process. The investigation of short-lived strange particles as well as their anti-particles and resonances allows an attempt of characterization of the matter created in RHIC heavy ion collisions.
The study of the strength and behaviour of the antikaon-nucleon ($\mathrm{\overline{K}N}$) interaction constitutes one of the key focuses of the strangeness sector in low-energy Quantum Chromodynamics (QCD). In this letter a unique high-precision measurement of the strong interaction between kaons and protons, close and above the kinematic threshold, is presented. The femtoscopic measurements of the correlation function at low pair-frame relative momentum of (K$^+$ p $\oplus$ K$^-$ $\overline{\mathrm{p}}$) and (K$^-$ p $\oplus$ K$^+$ $\overline{\mathrm{p}}$) pairs measured in pp collisions at …
We will present a review of selected results on strange baryon production from nucleus-nucleus collisions at the AGS, SPS and RHIC. From the AGS and SPS heavy ion physics program several intriguing aspects: centrality dependence of strangeness production, anti-hyperon yield, hyperon production enhancement and baryon number transport will be highlighted. Our discussion on RHIC results will focus on experimental probes of partonic degree of freedom. Measurements of the elliptic flow v2 and the nuclear modification factor and their distinctive particle dependence are presented as a function of tr…
Analyses of two-particle correlations have provided the chief means for determining spatio-temporal characteristics of relativistic heavy ion collisions. We discuss the theoretical formalism behind these studies and the experimental methods used in carrying them out. Recent results from RHIC are put into context in a systematic review of correlation measurements performed over the past two decades. The current understanding of these results is discussed in terms of model comparisons and overall trends.
Strangeness study is one of the major goal of the STAR experiment at RHIC. Results presented here have been obtained from analyses restricted to the mid-rapidity region of Au-Au collisions at $\sqrt{s_{NN}}$ = 130 GeV. An onset of the understanding of the quark matter behavior produced at this relativistic energy can be stressed from the investigation of the relative and absolute yields of strange particle production, their transverse mass distributions and from comparisons with previous results obtained by AGS and SPS experiments.
We present data on strange particle production in elementary proton-proton collisions at RHIC energies. Comparison to leading order and next-to-leading order (NLO) calculations shows that the fragmentation process is flavor dependent and that higherorder corrections are needed to describe all spectra, in particular at these collision energies, which are modest compared to the Tevatron energies. A model (EPOS) which takes into account multiple scattering between projectile constituents seems to describe the data best.
Strange particles have been a very important observable in the search for a deconfined state of strongly interacting matter, the quark-gluon plasma (QGP), which is expected to be formed in ultra-relativistic heavy ion collisions. We review the main experimental observations made at the Super Proton Synchrotron (SPS) at CERN, Geneva, and at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). The large amount of recently collected data allows for a comprehensive study of strangeness production as a function of energy and system size. We review results on yields, t…
Two-particle high-$p_T$ triggered correlations in $Cu+Cu$ and $Au+Au$ collisions at $\sqrt{s_{NN}}$ = 62 $GeV$ and $\sqrt{s_{NN}}$ = 200 $GeV$ in STAR at RHIC are presented. The $N_{part}$, $p_T^{associated}$, and $p_T^{trigger}$ dependence of the yield per trigger in the \textit{Jet} and \textit{Ridge} components of the near-side is investigated for h, $K^0_S$, $Λ$, and $Ξ$ trigger and associated particles. The system and energy dependence of these components is a potentially powerful tool to distinguish between models used to describe their production.
We present measurements of strange and multi-strange hadrons in p+p collisions at $\sqrt{s}$= 200 GeV measured by STAR. We will compare these preliminary results to leading-order (LO) and next-to-leading order (NLO) perturbative QCD models widely believed to describe the production mechanisms. In particular we will point out recent changes of the model calculations which improve the agreement with our data significantly and will discuss the physics consequences. In larger collision systems, produced with heavy ions at RHIC, we observe the centrality dependence of strange and multi-strange part…
Multi-strange baryon azimuthal correlations were observed in d+Au and Au+Au data taken by the STAR detector at RHIC. We extract the same-side per-trigger yields for a variety of strange particle species and trigger pt, but do not observe any species dependence. We also report the observation of an elongation in the Delta-eta; direction of the Xi correlation peak, the ridge. Comparing the same-side yields in d+Au and Au+Au data, we conclude that the same-side yield of the azimuthal-only Xi correlations is greatly enhanced by the ridge yield. We also report the first observation of a strong same…
Two particle azimuthal correlations with strange trigger particles ($K^0_S,Λ,Ξ$) are presented in Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV in STAR at RHIC. The near-side associated yield is investigated as a function of centrality, $p_T$, and strangeness content of the trigger to investigate possible flavour or meson/baryon differences. The system size dependence of both the jet-like correlations and the long range pseudorapidity correlations is studied to give insight into the origin of these two components.
We present STAR measurements of K^{0}_{S}, ϕ, Λ, Ξ, and Ωat mid-rapidity from Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). Nuclear modification factors and baryon-to-meson ratios are measured to understand recombination and parton energy loss mechanisms. Implications on partonic versus hadronic dynamics at low beam energies are discussed.
Via the study of strange particle production within the STAR experiment, we try to address the surprising amount of baryon transport at the Relativistic Heavy Ion Collider (RHIC). We report here preliminary results showing that, at mid-rapidity and for the top energy of RHIC, the number of created baryons exceeds the number transported from the colliding nuclei. However, thanks to the large acceptance of the experimental setup, one could expect to observe the transition between the ``soft'' regime (low transverse momentum -$\pt$- region corresponding to a bulk of hot and dense matter hadronizi…
Strange particle ratios and pT-spectra are calculated in a thermal model with single freeze-out, previously used successfully to describe non-strange particle production at RHIC. The model and the recently released data for phi, Lambda, anti-Lambda, and K*(892) are in very satisfactory agreement, showing that the thermal approach can be used to describe the strangeness production at RHIC.
We present results on two-particle correlations using singly-strange particles ($Λ$, $\barΛ$, $K^0_S$) and charged hadrons at intermediate p_T in d+Au and Au+Au collisions at RHIC. We discuss properties of the near-side correlation peak in both azimuthal and pseudo-rapidity space and separate jet-like contributions from the long-range pseudo-rapidity correlations (the ridge). In particular, we study the centrality and p_T dependence of the jet and ridge yields for various trigger and associated particle species and compare the results to model predictions.
We report STAR preliminary results on multi-strange baryon production in Au+Au collisions at sqrt(s_NN)=200 GeV at RHIC. Its implication for the formation of a new state of matter is discussed. The system size dependence on the production of strange baryons is investigated to study the onset of strange quark equilibration in the medium. The nuclear modification factor of Lambda, Xi and Omega is also presented. Its suppression at p_T>3 GeV/c supports the formation of a dense interacting medium at RHIC. The spectra of multi-strange baryons reveal that within a hydro-inspired model, they may deco…
Identified charged kaon, pion, and proton spectra and ratios from sqrt(s_NN) = 200 and 62.4 GeV Cu+Cu collisions are studied with a hydro-motivated blast-wave and a statistical model framework in order to explore the strangeness production at RHIC and characterize the bulk freeze-out properties of the created system. The spectra are measured at mid-rapidity |y|<0.1 over the transverse momentum range of 0.25 < p_T < 1.2 GeV/c with particle identification derived from the ionization energy loss in the STAR Time Projection Chamber. The multi-dimensional systematic study of pi(+/-), K(+/-), p and …
Strange particle production is an important experimental observable that allows the study of the strongly interacting matter created in relativistic heavy-ion collisions. The STAR experiment at RHIC has a unique capability of measuring identified strange particles over a wide range of acceptance providing a rich set of data to perform a systematic study. In addition to the data from Au+Au collisions, strange particles from p+p and d+Au collisions are also available for comparison and normalization. A new set of data from Cu+Cu reactions at 62 GeV and 200 GeV provides the chance to compare the …
With the advent of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL), Heavy Ion Physics will enter a new energy regime. The question is whether the signatures proposed for the discovery of a phase transition from hadronic matter to a Quark Gluon Plasma (QGP), that were established on the basis of collisions at the BEVALAC, the AGS, and the SPS, respectively, are still useful and detectable at these high incident energies. In the past two decades, measurements related to strangeness formation in the collision were advocated as potential signatures and were teste…
Resonance yields and spectra from elementary p+p and Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV from the STAR experiment at RHIC are presented and discussed in terms of chemical and thermal freeze-out conditions. Thermal models do not adequately describe the yields of the resonance production in central Au+Au collisions. The approach to include elastic hadronic interactions between chemical freeze-out and thermal freeze-out suggests a time of $Δτ>$5 fm/c.
The study of the multi-strange production in the final state of ultra-relativistic Au+Au collisions at top RHIC energy gives information on the freeze-out conditions and probably on earlier stages of the collision. The chemical freeze-out temperature as well as the strangeness phase space occupancy factor gamma_s extracted from statistical models are studied as a function of the collision centrality. Within the framework of a hydrodynamically-inspired model the transverse collectivity of the collision is investigated. In this view, multi-strange particles seem to develop significant radial tra…
An experimental overview of the energy dependence of strangeness production is presented. The strange hadrons are considered a good probe to study the QCD matter created in relativistic nucleus-nucleus collisions. The heavy-ion experiments at SPS, RHIC, and LHC have recorded a wealth of data in proton-proton, proton-nucleus and nucleus-nucleus collisions at several beam energies. In this proceeding, I discuss the invariant yield and azimuthal anisotropy measurement of strange hadrons in nucleus-nucleus collisions at SPS, RHIC, and LHC.
We present results on two-particle azimuthal correlations of high-$p_T$ neutral strange baryons ($Λ$,$\barΛ$) and mesons ($K^0_S$) associated with non-identified charged particles in d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. In particular, we discuss properties of the near-side yield of associated charged particles as a function of centrality, transverse momentum and $z_T$, as well as possible baryon/meson and particle/antiparticle differences. The results are compared to the proton and pion triggered correlations and to fragmentation and recombination models.
We present results on two-particle azimuthal correlations with strange trigger particles ($K^0_S$, $Λ$, $Ξ$, $Ω$) associated with unidentified charged particles %at intermediate $p_T$ in d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We investigate, in detail, the near-side associated yield as a function of centrality, $p_T$ and strangeness content in the trigger particle to look for possible flavor and baryon/meson differences. We compare our results to a fragmentation and recombination model, where the study of $Ω$-triggered correlations is used as a critical test of the validity of…
New experimental data on transverse momentum spectra of strange particles (KS0, K-, K*, phi,...) produced in pp collisions at sqrt s = 200 GeV obtained by the STAR and PHENIX collaborations at RHIC are analysed in the framework of z-scaling approach. Scaling properties of the data z-presentation are illustrated. Self-similarity of strange particle production is discussed. A microscopic scenario of constituent interactions developed within the z-scaling approach is used to study constituent energy loss, proton momentum fraction and recoil mass in dependence on the transverse momentum, strangene…
We present the recent results of strangeness production at the mid-rapidity in Au + Au collisions at RHIC, from $\sqrt{s_{\rm NN}}$ = 7.7 to 200 GeV. The $v_2$ of multi-strange baryon $Ω$ and $ϕ$ mesons are similar to that of pions and protons in the intermediate $p_T$ range (2 - 5 GeV/$c$) in $\sqrt{s_{\rm NN}}$ = 200 GeV Au + Au collisions, indicating that the major part of collective flow has been built up at partonic stage. The breaking of mass ordering between $ϕ$ mesons and protons in the low $p_T$ range ($<$ 1 GeV/$c$) is consistent with a picture that $ϕ$ mesons are less sensitive to l…
A nontrivial space-time structure of the hot system created at RHIC is the defining aspect of the physics of relativistic heavy ion collisions. Femtoscopy through pion intensity interferometry provides direct access to the dynamic geometric substructure of the freeze-out stage, and appears to confirm source evolution via collective flow. Since flow is usually considered to be a bulk phenomenon, it is surpring to find a simple scaling in preliminary HBT radii from p+p, d+Au and Au+Au collisions. Investigating the light system data in detail, we discuss a new way to visualize the ``fine structur…
The production and freeze-out conditions of strange particles, produced in Au+Au collisions at RHIC energies, are studied within microscopic transport model. The system of final particles can be represented as a core, containing the particles which are still interacting with each other, and a halo, in which the particles have already decoupled from the system. In microscopic calculations hadrons are continuously emitted from the whole reaction volume. At RHIC, however, significant fractions of both mesons and baryons are emitted from the surface region within the first two fm/c. Different spec…
Due to its large acceptance, the STAR experiment has acquired a wealth of data on strangeness production for a variety of collisions systems and energies, from p+p to Au+Au. By using the yields and spectra, we address the evolution of the bulk system, including strangeness enhancement and the flavour dependence of radial and elliptic flow. Utilising the fact that we can identify strange baryons and mesons, we investigate different hadronization mechanisms in the intermediate and high p$_{T}$ regions. The ratios of the particle yields, measured to high p$_{T}$, are used to further investigate t…
Exploration of the rich structure of the QCD phase diagram is an important topic in the RHIC heavy ion program. One of the ultimate goals of this program is to search for the critical endpoint. Investigation of the space-time structure of hadron emissions at various phase transition points using Bose-Einstein correlations of identical bosons may provide insight on the location of the critical endpoint. PHENIX has performed comprehensive measurements of the Bose-Einstein correlation in Au+Au collisions at sqrt(sNN) = 15, 19, 27, 39, 62.4, and 200 GeV, where we incorporated Levy-type source func…
Experiments using heavy ion collisions at ultrarelativistic energies aim to explore the QCD phase transition and map out the QCD phase diagram. A wealth of remarkable results in this field have been reported recently, for example the Upsilon suppression discovered recently. We discuss recent results from the STAR experiment focusing on strangeness, charm and beauty production.
Anisotropic flow of K's, anti-K's, and lambdas is studied in heavy ion collisions at SPS and RHIC energies within the microscopic quark-gluon string model. At SPS energy the directed flow of kaons differs considerably at midrapidity from that of antikaons, while at RHIC energy kaon and antikaon flows coincide. The change is attributed to formation of dense meson-dominated matter at RHIC, where the differences in interaction cross-sections of hadrons become unimportant. The directed flows of strange particles, $v_1^{K,\bar{K}, Λ}(y)$, have universal negative slope at $|y| \leq 2$ at RHIC. The e…
The baryon-baryon continuum invariant mass spectrum generated from relativistic nucleus + nucleus collision data may reveal the existence of doubly-strange dibaryons not stable against strong decay if they lie within a few MeV of threshold. Furthermore, since the dominant component of these states is a superposition of two color-octet clusters which can be produced intermediately in a color-deconfined quark-gluon plasma (QGP), an enhanced production of dibaryon resonances could be a signal of QGP formation. A total of eight, doubly-strange dibaryon states are considered for experimental search…
We present results on systematic measurements of strange and multi-strange particles with the STAR detector for center of mass energies per nucleon pair of 62.4 and 200 GeV in ultra-relativistic Au+Au collisions at RHIC. We use these results to characterize the chemical and kinetic freeze-out properties of the fireball produced by the collision. This is done by comparison to statistical and hydrodynamical models as well as parameterization. We emphasize particularly the energy dependence of these features comparing measurements obtained at RHIC with different energies but also at the lower ene…
The first measurements of the scattering parameters of $Λ$K pairs in all three charge combinations ($Λ$K$^{+}$, $Λ$K$^{-}$, and $Λ\mathrm{K^{0}_{S}}$) are presented. The results are achieved through a femtoscopic analysis of $Λ$K correlations in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV recorded by ALICE at the LHC. The femtoscopic correlations result from strong final-state interactions, and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies w…
We study effects of the hadronic rescattering on final observables especially for multi-strange hadrons such as $ϕ$, $Ξ$ and $Ω$ in high-energy heavy-ion collisions within an integrated dynamical approach. In this approach, (3+1)-dimensional ideal hydrodynamics is combined with a microscopic transport model, JAM. We simulate the collisions with or without hadronic rescatterings and compare observables between these two options so that we quantify the effects of the hadronic rescattering. We find that the mean transverse momentum and the elliptic flow parameter of multi-strange hadrons are less…
The experimental data on hadron yields and ratios in central Pb+Pb and Au+Au collisions at SPS and RHIC energies, respectively, are analysed within a two-source statistical model of an ideal hadron gas. These two sources represent the expanding system of colliding heavy ions, where the hot central fireball is embedded in a larger but cooler fireball. The volume of the central source increases with rising bombarding energy. Results of the two-source model fit to RHIC experimental data at midrapidity coincide with the results of the one-source thermal model fit, indicating the formation of an ex…
The STAR Experiment at RHIC is capable of a wide variety of measurements of the production of strange hadrons in nuclear collisions. Measurements of the relative production of strange baryons, antibaryons, and kaons can shed light on the baryon densities achieved in these collisions and on the validity of models for production yields. We will present here preliminary results on these measurements at RHIC energies of sqrt(s_NN) = 130 GeV and 200 GeV and discuss comparisons to models.