CERN Accelerating science

Quantum chromodynamics (QCD) is the theory that describes the strong interactions observed in nature. QCD predicts that at a high temperature and/or density, the nuclear matter will make a transition to a strongly interacting matter in which quarks and gluons are free to move, and not confined as in hadrons. Such an extreme state of matter is known as quark-gluon plasma (QGP) which existed in the early stages of the Universe after the Big Bang. QGP can be created in laboratory by colliding heavy ions at relativistic energies. The properties of QGP can be investigated by determining the angular and momentum distribution of vector mesons produced in heavy-ion collisions. In a non-central heavy-ion collision, there is a large orbital angular momentum in the initial state. In the presence of large initial angular momentum, the vector mesons (spin = 1) can be polarized due to spin-orbit interaction or during the hadronization from polarized quarks. The polarization of vector mesons can be determined by measuring the angular distribution of the decay daughters of the vector meson with respect to the quantization axis in the rest frame of the vector meson. The quantization axis is considered along the direction perpendicular to either the production plane (defined by the momentum of vector meson and the beam axis), or perpendicular to the reaction plane (defined by the impact parameter and the beam axis) of the colliding system. The measured angular distribution can be used to estimate the elements of the spin density matrix, in particular the element \rh which is the probability of finding the vector meson in the spin 0 state out of the three possible spin states 1, 0 and -1. Deviation of \rh from the value 1/3 indicates the presence of spin alignment. We report the first evidence of significant spin alignment effect for vector mesons (\kst and \pha) in heavy-ion collisions. The measurements are carried out as a function of transverse momentum (\pta) and collision centrality with the ALICE detector using the particles produced at mid-rapidity ($|y|$ $<$ 0.5) in Pb--Pb collisions at a center-of-mass energy (\snna) of 2.76 TeV. The initial angular momentum due to the extended size of the nuclei and the finite impact parameter in non-central heavy-ion collisions is missing in proton-proton collisions. Determination of \rh for vector mesons produced in pp collisions and for spin zero \kzs produced in heavy-ion collisions provide a null test for spin alignment of vector mesons measured in the present work. Short lifetimes of hadronic resonances compared to other stable hadrons are comparable to the time taken by the dense nuclear matter to evolve to its final state. This can be exploited to investigate the properties of the hadronic phase produced in heavy-ion collisions. \kst yields are expected to be modified due to the interaction of their decay daughters within the hadronic medium. We present \kst production in mid-rapidity for different collision centrality classes in Xe-Xe collisions at \snn = 5.44 TeV. The \pt spectra, \pt integrated yield, mean transverse momentum (\mpta), resonance to stable particle yield ratio (\ksta/$K$) and nuclear modification factor are presented. Recent measurements in high multiplicity pp collisions show striking similarities between small system and heavy-ion collisions. In order to find the possible presence of hadronic phase effect in small collisions system, \kst production as a function of charged particle multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV are also presented. Measured \ksta/$K$ yield ratio decreases with increasing charged particle multiplicity in both heavy-ion (Xe-Xe and Pb-Pb) and pp collisions. Since \kst has a short lifetime, its decay products scatter in their passage through the hadronic medium changing their momenta and hence affecting the reconstruction of the parent particle, thereby decreasing the measured yield. In addition to measuring the \kst production in Xe-Xe collisions at $\sqrt{sNN}$ = 5.44 TeV, we have also compared the measured bulk properties for Xe-Xe collisions to those from A Multiphase Transport (AMPT) model. The shape of the Xe nucleus is non-spherical and is therefore generally referred to as a deformed nucleus. In this work, we have studied the effect of deformation of Xe nucleus on the bulk properties such as charged particle yield, mean transverse momentum, elliptic flow and triangular flow. The results of AMPT model calculations are also obtained for a spherical shape of the Xe nucleus to determine the effect of deformation. The measured charged particle multiplicity and \mpt do not show any effect due to deformation; the elliptic flow is enhanced by $\sim$15\% due to the deformation of the Xe nucleus in central Xe-Xe collisions.