Collisions Of Heavy Nuclei Research Articles

Measuring nuclear modification factors for φ-mesons in p + Pb and Pb + Pb collisions in the ALICE experiment at the large hadron collider

Investigations of light vector mesons allow us to understand the mechanisms of hadron production in more detail and to investigate the properties of the dense and hot medium formed in central collisions of heavy ultrarelativistic nuclei. One of the most useful objects for investigations are φ-mesons, since their properties are sensitive to such effects as strangeness enhancement, the restoration of chiral symmetry, and the loss of parton energy that accompanies a phase transition. To investigate these phenomena, meson production must be measured not only in collisions of heavy nuclei but in proton-proton and proton-nuclear collisions as well. Measurements in elementary nucleon-nucleon collisions are used as the basis for comparison to heavier interacting systems. The collision of light and heavy nuclei allows us to evaluate the effects of cold nuclear matter and to interpret the effects of a hot nuclear medium more precisely. Invariant differential production spectra of φ-mesons were measured in the ALICE experiment as a function of transverse momenta in p + p, p + Pb, and Pb + Pb interactions. The distributions of the nuclear modification factors were constructed for φ-mesons on the basis of these data.

Electron-positron pair creation in low-energy collisions of heavy bare nuclei

A new method for calculations of electron-positron pair-creation probabilities in low-energy heavy-ion collisions is developed. The approach is based on the propagation of all one-electron states via the numerical solving of the time-dependent Dirac equation in the monopole approximation. The electron wave functions are represented as finite sums of basis functions constructed from B-splines using the dual-kinetic-balance technique. The calculations of the created particle numbers and the positron energy spectra are performed for the collisions of bare nuclei at the energies near the Coulomb barrier with the Rutherford trajectory and for different values of the nuclear charge and the impact parameter. To examine the role of the spontaneous pair creation the collisions with a modified velocity and with a time delay are also considered. The obtained results are compared with the previous calculations and the possibility of observation of the spontaneous pair creation is discussed.

Layout of new experiments on the observation of spontaneous electron–positron pair creation in supercritical Coulomb fields

Collisions of bare heavy nuclei implemented by means of merging-beam technique are suggested for the study of remarkable phenomena appearing in very strong fields created when distance between the nuclear surfaces approaches zero. Suggested here is to build a collider with two beams of actinide nuclei merging within a common straight section. Principal parameters of this collider are outlined.

Dynamical approach to heavy ion-induced fission

Deep inelastic collisions (DICs) can compete strongly with fusion in collisions of heavy nuclei. However, standard coupled-channels calculations do not take DIC processes into account. As a result, calculations have been shown to overestimate the fusion cross-sections, resulting in a discrepancy between experimental data and theoretical calculations, particularly at energies above the fusion barrier. To investigate this discrepancy, we conducted a series of experiments using the ANU 14UD tandem accelerator and the CUBE 2-body fission spectrometer to examine the competition between transfer/DIC and fusion. In particular, fusion-fission and 3-body fission yields have been extracted for 34 S + 232 Th and 40 Ca + 232 Th systems. This work shows that the transfer-fission probability is enhanced relative to fusion-fission for 40 Ca + 232 Th, when compared to 34 S+ 232 Th. It is suggested that the enhancement of this DIC process in 40 Ca + 232 Th is linked to an increase in the density overlap of the colliding nuclei as a function of the charge product and contributes to fusion hindrance.

Recent highlights from the PHENIX heavy ion program

It is accepted that a QGP can be formed in relativistic collisions of heavy nuclei (A+A). Recently long-range correlations have been observed in p+A collisions at the LHC in high multiplicity events. PHENIX has carried out a series of studies of d+Au collisions at 200 GeV to see if such correlations persist at lower energies compared to those at the LHC. Results of a study of long-range correlations and flow are presented for d+Au collisions. Data from Au+Au collisions collected during the beam energy scan (BES) was used to determine both quark and nucleon number scaling. The HBT method was used to determine radii of the fireball at kinetic freezeout. Implications for the nuclear EOS are discussed. Also results of a search for “dark photons” are presented. Recent PHENIX highlights on heavy flavor, electromagnetic probes, spin and plans for PHENIX upgrades were presented in other talks at this conference.

Recent highlights from the PHENIX heavy ion program

It is accepted that a QGP can be formed in relativistic collisions of heavy nuclei (A+A). Recently long-range correlations have been observed in p+A collisions at the LHC in high multiplicity events. PHENIX has carried out a series of studies of d+Au collisions at 200 GeV to see if such correlations persist at lower energies compared to those at the LHC. Results of a study of long-range correlations and flow are presented for d+Au collisions. Data from Au+Au collisions collected during the beam energy scan (BES) was used to determine both quark and nucleon number scaling. The HBT method was used to determine radii of the fireball at kinetic freezeout. Implications for the nuclear EOS are discussed. Also results of a search for “dark photons” are presented. Recent PHENIX highlights on heavy flavor, electromagnetic probes, spin and plans for PHENIX upgrades were presented in other talks at this conference.

Electromagnetic production of an electron-positron pair in collisions of heavy nuclei

We consider the cross-section of electromagnetic production of $e^+ e^-$ pair in adiabatic scattering of heavy nuclei.

Alternative scenarios of relativistic heavy-ion collisions. III. Transverse momentum spectra

Transverse-mass spectra, their inverse slopes and mean transverse masses in relativistic collisions of heavy nuclei are analyzed in a wide range of incident energies 2.7 GeV $\le \sqrt{s_{NN}}\le$ 39 GeV. The analysis is performed within the three-fluid model employing three different equations of state (EoS's): a purely hadronic EoS, an EoS with the first-order phase transition and that with a smooth crossover transition into deconfined state. Calculations show that inverse slopes and mean transverse masses of all the species (with the exception of antibaryons within the hadronic scenario) exhibit a step-like behavior similar to that observed for mesons and protons in available experimental data. This step-like behavior takes place for all considered EoS's and results from the freeze-out dynamics rather than is a signal of the deconfinement transition. A good reproduction of experimental inverse slopes and mean transverse masses for light species (up to proton) is achieved within all the considered scenarios. The freeze-out parameters are precisely the same as those used for reproduction of particles yields in previous papers of this series. This became possible because the freeze-out stage is not completely equilibrium.

Results on identified particle production in pp, p-Pb and Pb-Pb collisions measured with ALICE at the LHC

Using the unique capabilities of the ALICE detectors for particle identification, different measurements have been performed to study the properties of the hot and dense matter created in the Pb-Pb collisions at sqrt(sNN)= 2.76 TeV. The analysis of the p-Pb data at sqrt(sNN) = 5.02 TeV reveals that the suppression of high pt hadrons observed in heavy nuclei collisions can not be explained as due to initial state effects. The systems created in the p-Pb collisions do not show evidence of jet quenching but, surprisingly, exhibit characteristics of flow. In this paper a review of the main results on identified particle production measured in different colliding systems is presented, data are also compared to models.

Magnetic flux loop in high-energy heavy-ion collisions

We consider the expectation value of a magnetic flux loop in the immediate forward light cone of collisions of heavy nuclei at high energies. Such collisions are characterized by a non-linear scale Q_s where color fields become strong. We find that loops of area greater than ~2/Q_s^2 exhibit area law behavior, which determines the scale of elementary flux excitations ("vortices"). We also estimate the magnetic string tension, sigma_M = 0.12 Q_s^2. By the time t ~ 1/Q_s even small loops satisfy an area law. We describe corrections to the propagator of semi-hard gluons at very early times in the background of fluctuating magnetic fields.

Phase evolution and freeze-out within alternative scenarios of relativistic heavy-ion collisions

Global evolution of the matter in relativistic collisions of heavy nuclei and the resulting global freeze-out parameters are analyzed in a wide range of incident energies 2.7 GeV⩽sNN⩽39 GeV. The analysis is performed within the three-fluid model employing three different equations of state (EoS): a purely hadronic EoS, an EoS with the first-order phase transition and that with a smooth crossover transition. Global freeze-out parameters deduced from experimental data within the statistical model are well reproduced within the crossover scenario. The 1st-order-transition scenario is slightly less successful. The worst reproduction is found within the purely hadronic scenario. These findings make a link between the EoS and results of the statistical model, and indicate that deconfinement onset occurs at sNN≳5 GeV.

Alternative scenarios of relativistic heavy-ion collisions. II. Particle production

Particle production in relativistic collisions of heavy nuclei is analyzed in a wide range of incident energies 2.7 GeV $\ensuremath{\le}\sqrt{{s}_{NN}}\ensuremath{\le}$ 62.4 GeV. The analysis is performed within the three-fluid model employing three different equations of state (EoSs): a purely hadronic EoS, an EoS with the first-order phase transition, and an EoS with a smooth crossover transition. It is found that the hadronic scenario fails to reproduce experimental yields of antibaryons (strange and nonstrange), starting already from lower Super Proton Synchrotron (SPS) energies, i.e., $\sqrt{{s}_{NN}}>$ 5 GeV. Moreover, at energies above the top SPS one, i.e., $\sqrt{{s}_{NN}}>$ 17.4 GeV, the midrapidity densities predicted by the hadronic scenario considerably exceed the available Relativistic Heavy-Ion Collider data on all species. At the same time the deconfinement-transition scenarios reasonably agree (to a various extent) with all the data. The present analysis demonstrates certain advantage of the deconfinement-transition EoSs. However, all scenarios fail to reproduce the strangeness enhancement in the incident energy range near 30$A$ GeV (i.e., a horn anomaly in the ${K}^{+}/{\ensuremath{\pi}}^{+}$ ratio) and yields of $\ensuremath{\phi}$ mesons at 20$A$--40$A$ GeV.

Global flow of glasma in high energy nuclear collisions

We discuss the energy flow of the classical gluon fields created in collisions of heavy nuclei at collider energies. We show how the Yang–Mills analog of Faradayʼs Law and Gaussʼ Law predicts the initial gluon flux tubes to expand or bend. The resulting transverse and longitudinal structure of the Poynting vector field has a rich phenomenology. Besides the well-known radial and elliptic flow in transverse direction, classical quantum chromodynamics predicts a rapidity-odd transverse flow that tilts the fireball for non-central collisions, and it implies a characteristic flow pattern for collisions of non-symmetric systems A+B. The rapidity-odd transverse flow translates into a directed particle flow v1 which has been observed at RHIC and LHC. The global flow fields in heavy ion collisions could be a powerful check for the validity of classical Yang–Mills dynamics in high energy collisions.

The nonlinear glasma

We study the evolution of quantum fluctuations in the glasma, created immediately after the collision of heavy nuclei. We show how the presence of instabilities leads to an enhancement of nonlinear interactions among initially small fluctuations. The nonlinear dynamics leads to an enhanced growth of fluctuations in a large momentum region, exceeding by far the originally unstable band. We investigate the dependence on the coupling constant at weak coupling using classical statistical lattice simulations for $SU(2)$ gauge theory and show how these nonlinearities can be analytically understood within the framework of two-particle irreducible effective action techniques. The dependence on the coupling constant is only logarithmic, in accordance with analytic expectations. Concerning the isotropization of bulk quantities, our results indicate that the system exhibits an order-one anisotropy on parametrically large time scales. Despite this fact, we find that gauge invariant pressure correlation functions seem to exhibit a power law behavior characteristic for wave turbulence.

Heavy-flavour and quarkonia in heavy-ion collisions

The comparison of heavy-flavour hadron production in proton-proton, proton-Pb and Pb-Pb collisions at the LHC offers the opportunity to investigate the properties of the high-density colour-deconfined state of strongly-interacting matter (Quark Gluon Plasma, QGP) that is expected to be formed in high-energy collisions of heavy nuclei. A review of the main quarkonium and open heavy-flavour results obtained by the ALICE, ATLAS and CMS experiments is presented.

Nuclear-nuclear collision centrality determination by the spectators calorimeter for the MPD setup at the NICA facility

The work conditions of the hadron calorimeter for spectators registration (Zero Degree Calorimeter, ZDC) were studied for the heavy nuclei collisions with the several GeV invariant energy. The ZDC simulations were performed for the MPD (Multi-Purpose Detector) at the NICA (Nuclotron-based Ion Collider fAcility) collider, which are under developement at the Joint Institute for Nuclear Research (JINR, Dubna). Taking into account the spectator nuclear fragments leads to a nonmonotonic dependence of the ZDC response on the impact parameter. The reason for this dependence studied with several event generators is the primary beam hole in the ZDC center. It is shown, that the ZDC signal should be combined with a data from other MPD@NICA detector subsystems to determine centrality.

Primordial Matter Comes Into Focus in Many Tiny Big Bangs

A standard model is emerging of the quark-gluon plasma produced in collisions of heavy nuclei. But will a key atom smasher run long enough to flesh it out?

Turbulent thermalization of weakly coupled non-Abelian plasmas

We study the dynamics of weakly coupled non-abelian plasmas within the frameworks of classical-statistical lattice gauge-theory and kinetic theory. We focus on a class of systems which are highly occupied, isotropic at all times and initially characterized by a single momentum scale. These represent an idealized version of the situation in relativistic heavy ion-collisions in the color-glass condensate picture, where on a time scale $1/Q_s$ after the collision of heavy nuclei a longitudinally expanding plasma characterized by the saturation scale $Q_s$ is formed. Our results indicate that the system evolves according to a turbulent Kolmogorov cascade in the classical regime. Taking this into account, the kinetic description is able to reproduce characteristic features of the evolution correctly.

Empirical determination of charm quark energy loss and its consequences for azimuthal anisotropy

We propose an empirical model to determine the form of energy loss of charm quarks due to multiple scatterings in quark gluon plasma by demanding a good description of production of D mesons and non-photonic electrons in relativistic collision of heavy nuclei at RHIC and LHC energies. The best results are obtained when we approximate the momentum loss per collision ΔpT = α pT, where α is a constant depending on the centrality and the centre of mass energy. Comparing our results with those obtained earlier for drag coefficients estimated using the Langevin equation for heavy quarks, we find that up to half of the energy loss of charm quarks at the highest RHIC energy could be due to collisions while that at LHC energy at 2760 GeV/A the collisional energy loss could be about one-third of the total. Estimates are obtained for azimuthal anisotropy in momentum spectra of heavy mesons, due to this energy loss. We further suggest that the energy loss of charm quarks may lead to an enhanced production of D-mesons and single electrons at low pT in AA collisions.Communicated by SA Bass

Large transverse momentum dilepton and photon production by photoproduction processes

We calculate the production of large transverse momentum dileptons and photons by using direct and resolved photoproduction processes in relativistic heavy ion collisions. Considering the central collisions of heavy nuclei at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies, we find that the photoproduction processes modify the dilepton and photon production in the large transverse momentum region.