Ultrarelativistic Ion Collisions Research Articles

Model calculations of φ-meson production in small collision systems

Ultrarelativistic ion collisions provide the unique possibility to study the quark-gluon plasma, a state of matter formed in the universe at the very first moments after the Big Bang. The minimal temperature and baryon density for the quark-gluon plasma formation requires scrutiny, since the signatures of the quark-gluon plasma formation are observed in large systems (such as Au+Au) at , whereas collective effects in p+p collisions are not revealed. The φ-meson production measurements are considered to be a convenient tool to investigate the collision dynamics, as it is sensitive to the quark-gluon plasma effects. To interpret the nuclear modification effects and to study the process of the possible QGP formation the comparison with different theoretical models predictions is needed. This paper presents the comparison of the obtained experimental results on φ-meson production in small collision systems (p+Al, p+Au) at to default and string melting versions of the AMPT model and PYTHIA model predictions. The results indicate that the minimal conditions (temperature and baryon density) for a QGP formation may lie in between in p+Al and p+Au collisions.

Measurement of flow harmonics correlations with mean transverse momentum in lead\u2013lead and proton\u2013lead collisions at\xa0sqrt{s_{mathrm{NN}}} = 5.02~hbox {TeV} with the ATLAS detector

To assess the properties of the quark–gluon plasma formed in ultrarelativistic ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the flow harmonics. The analysis uses data samples of lead–lead and proton–lead collisions obtained at the centre-of-mass energy per nucleon pair of 5.02 TeV, corresponding to total integrated luminosities of 22~upmu text {b}^{-1} and 28~text {nb}^{-1}, respectively. The measurement is performed using a modified Pearson correlation coefficient with the charged-particle tracks on an event-by-event basis. The modified Pearson correlation coefficients for the 2nd-, 3rd-, and 4th-order flow harmonics are measured in the lead–lead collisions as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision. The measurements are performed for several intervals of the charged-particle transverse momentum. The correlation coefficients for all studied harmonics exhibit a strong centrality evolution, which only weakly depends on the charged-particle momentum range. In the proton–lead collisions, the modified Pearson correlation coefficient measured for the 2nd-order flow harmonics shows only weak centrality dependence. The lead-lead data is qualitatively described by the predictions based on the hydrodynamical model.

Formation of exotic baryon clusters in ultrarelativistic heavy-ion collisions

Recent experiments at RHIC and LHC have demonstrated that there are excellent opportunities to produce light baryonic clusters of exotic matter (strange and anti-matter) in ultra-relativistic ion collisions. Within the hybrid-transport model UrQMD we show that the coalescence mechanism can naturally explain the production of these clusters in the ALICE experiment at LHC. As a consequence of this mechanism we predict the rapidity domains where the yields of such clusters are much larger than the observed one at midrapidity. This new phenomenon can lead to unique methods for producing exotic nuclei.

Bound–free pair production in ultra–relativistic ion collisions at the LHC collider: analytic approach to the total and differential cross sections

A theoretical investigation of the bound-free electron-positron pair production in relativistic heavy ion collisions is presented. Special attention is paid to the positrons emitted under large angles with respect to the beam direction. The measurement of these positrons in coincidence with the down--charged ions is in principle feasible by LHC experiments. In order to provide reliable estimates for such measurements, we employ the equivalent photon approximation together with the Sauter approach and derive simple analytic expressions for the differential pair--production cross section, which compare favorably to the results of available numerical calculations. Based on the analytic expressions, detailed calculations are performed for collisions of bare Pb$^{82+}$ ions, taking typical experimental conditions of the LHC experiments into account. We find that the expected count rate strongly depends on the experimental parameters and may be significantly enhanced by increasing the positron-detector acceptance cone.

On multiplicity and transverse-momentum correlations in collisions of ultrarelativistic ions

A theoretical description of long-range correlations between observables measured in separated rapidity intervals for nucleus-nucleus collisions at high energies is given. With allowance for an actual distribution of nucleons in nuclei, the ptn and nn correlation functions and respective correlation coefficients are calculated at various values of the impact parameter. It is shown that fluctuations of the impact parameter at a level of 1 fm, which are irremovable in experiments, affect the correlation coefficients substantially. It is shown that ptn correlations become much more pronounced upon going over from the SPS to the RHIC and LHC energies. The ptn and nn correlation functions calculated for the minimumbias case (that is, without selection in centrality) are compared with experimental data on PbPb collisions at the SPS energies.

Anomalous J/ψ suppression: A fingerprint for quark matter?

We summarize the experimental results available today on J/ψ suppression in ultra-relativistic ion collisions. Systematic measurements show a monotonic behaviour of the J/ψ production yield from p-p up to semi-peripheral Pb Pb interactions. A clear departure from this normal trend is found for more central Pb Pb collisions whiich could be the first observed fingerprint of Quark-Gluon Plasma production

Search for quark-gluon plasma and jets production in ultra-relativistic ion collisions

We analyze the sensitivity of the collisional and radiative energy losses of hard partonic jets to the main parameters and space-time evolution of the dense matter, which is expected to be created in ultra-relativistic heavy ion collisions. The problem of “true” QCD-jets recognition against the “false” jets background (statistical fluctuations of the transverse energy flux arising from a huge multiplicity of “thermal” secondary particles in an event) is also considered.

Interactions of 200xA GeV 16O and 32S studied by emulsion and CR39 in hybrid experiments

Ultrarelativistic ion collisions were studied by solid state particle track detectors. Physics results obtained by nuclear emulsions, stand-alone or with electronic counters, are reviewed, concerning both peripheral and central interactions. A new hybrid experiment, making use of nuclear emulsions and of CR39, is described. Data were taken last August. Preliminary technical results are presented, obtained by automatic track measurements on projectile fragments collected in previously exposed CR39 sheets.