Collisions Of Ultrarelativistic Nuclei Research Articles

Off-equilibrium sphaleron transitions in the glasma

We perform first classical-statistical real time lattice simulations of topological transitions in the non-equilibrium Glasma of weakly coupled but highly occupied gauge fields created immediately after the collision of ultra-relativistic nuclei. Simplifying our description by employing SU(2) gauge fields, and neglecting their longitudinal expansion, we find that the rate of topological transitions is initially strongly enhanced relative to the thermal sphaleron transition rate and decays with time during the thermalization process. Qualitative features of the time dependence of this non-equilibrium transition rate can be understood when expressed in terms of the magnetic screening length, which we also extract non-perturbatively. A detailed investigation of auto-correlation functions of the Chern-Simons number ($N_{CS}$) reveals non-Markovian features of the evolution distinct from previous simulations of non-Abelian plasmas in thermal equilibrium.

Momentum imbalance of isolated photon-tagged jet production at RHIC and LHC.

In collisions of ultrarelativistic nuclei, photon-tagged jets provide a unique opportunity to compare jet production and modification due to parton shower formation and propagation in strongly interacting matter at vastly different center-of-mass energies. We present first results for the cross sections of jets tagged by an isolated photon to O(α(em)α(s)(2)) in central Au+Au reactions with sqrt[s(NN)] = 200 GeV at RHIC and central Pb + Pb reactions with sqrt[s(NN)] = 2.76 TeV at LHC. We evaluate the increase in the transverse momentum imbalance of the observed γ + jet state, induced by the dissipation of the parton shower energy due to strong final-state interactions. Theoretical predictions to help interpret recent and upcoming experimental data are presented.

Soft Physics at RHIC

Recent soft physics results from collisions of ultra-relativistic nuclei at Relativistic Heavy Ion Collider (RHIC) operating at Brookhaven National Laboratory (BNL) are reviewed. Topics discussed cover the Beam Energy Scan program with some emphasis on anisotropic particle flow.

Strong-field effect for the process of photon emission in collisions of relativistic nuclei at RHIC and LHC

Virtual Delbrück scattering in heavy-ion collisions contributes to the emission of photons. Specifically, the elastic scattering of an equivalent photon emitted by a nucleus in the Coulomb field of an incoming nucleus can lead to the emission of a photon in the collision of ultra-relativistic nuclei Z 1 Z 2 → Z 1 Z 2 γ . The discussed process has no infrared divergence. The total cross section obtained is 14 barn for the Au–Au collisions at the RHIC collider and 50 barn for the Pb–Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary nuclear bremsstrahlung in the same photon energy range.

Large contribution of virtual Delbrück scattering to the emission of photons by relativistic nuclei in nucleus–nucleus and electron–nucleus collisions

Delbrueck scattering is an elastic scattering of a photon in the Coulomb field of a nucleus via a virtual electron loop. The contribution of this virtual subprocess to the emission of a photon in the collision of ultra-relativistic nuclei Z_1 Z_2 -> Z_1 Z_2 gamma is considered. We identify the incoming virtual photon as being generated by one of the relativistic nuclei involved in the binary collision and the scattered photon as being emitted in the process. The energy and angular distributions of the photons are calculated. The discussed process has no infrared divergence. The total cross section obtained is 14 barn for Au-Au collisions at the RHIC collider and 50 barn for Pb-Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary tree-level nuclear bremsstrahlung in the considered photon energy range m_e << E_\gamma << m_e gamma, where gamma is the Lorentz factor of the nucleus. Finally, photon emission in electron-nucleus collisions e Z -> e Z gamma is discussed in the context of the eRHIC option.

Transverse-energy correlations and jet shape in collisions of ultrarelativistic nuclei

Measurement of angular correlations between energy fluxes is one of the promising methods for analyzing the structure of events in ultrarelativistic heavy-ion collisions. The possibility of diagnosing the rescattering and the energy loss of hard partons in dense QCD matter is studied here on the basis of an analysis of the transverse-energy correlation function. It is shown that, if events are chosen for an analysis in a special way (that is, if at least one jet is required to have a transverse energy above some threshold) and if the procedure of background subtraction is applied in each event, the energy correlation function is sensitive to the parton energy loss and the angular spectrum of gluons emitted in a medium. The transverse-energy correlation function calculated for all events reflects the global structure of the transverse energy flux: it is independent of the azimuthal anisotropy of the energy flux for central collisions and is sensitive to the azimuthal anisotropy of the energy flux, reproducing all of its Fourier harmonics for noncentral collisions, but the coefficients of these harmonics are squared. A special correlation function in the vicinity of the maximum energy deposition in each event makes it possible to study changes in the jet shape. Within the conventional scenario of the scattering of hard jet partons on accompanying medium constituents, the correlation function is independent of the rapidity position of the jet axis and becomes much broader (symmetrically in the rapidity and azimuthal angle) than in proton-proton collisions. In the case of scattering on slow medium constituents, the broadening of the correlation function depends on the rapidity position of the jet axis and, in relation to the preceding scenario, increases sizably for jets of rather high rapidity.

Modification of the jet fragmentation function for ultrarelativistic collisions of nuclei and its determination in the channel involving the production of a leading neutral pion

The modification of the jet fragmentation function due to the parton-energy loss in dense quark-gluon matter is studied in channels involving the production of leading particles. The “softening” effect for the jet fragmentation function is shown to be determined by the angular distribution of gluons emitted in matter. This effect anticorrelates with the “suppression” of the total yield of jets owing to the energy loss of jet partons outside the jet cone. The possibility of measuring the jet fragmentation function by using leading electromagnetic clusters in heavy-ion collisions at the LHC energies is analyzed.

Multiple Exchanges in Lepton Pair Production in High-Energy Heavy Ion Collisions

As an archetype reaction for pQCD multigluon hard processes in collisions of ultrarelativistic nuclei, we analyze generic features of lepton pair production via multiphoton processes in peripheral heavy ion scattering. We report explicit results for collisions of two photons from one nucleus with two photons from the other nucleus, 2γ + 2γ → l + l −. The results suggest that the familiar eikonalization of Coulomb distortions breaks down for oppositely moving Coulomb centers. The breaking of eikonalization in QED suggests that multigluon pQCD processes cannot be described in terms of collective nuclear gluon distributions. We discuss a logarithmic enhancement of the contribution from the 2γ + 2γ → l + l − process to production of lepton pairs with large transverse momentum; similar enhancement is absent for the nγ + mγ → l + l − processes with m, n > 2. We comment on the general structure of multiphoton collisions and properties of higher-order terms that cannot be eikonalized.

Structure of the Coulomb and unitarity corrections to the cross section ofe+e−pair production in ultrarelativistic nuclear collisions

We analyze the structure of the Coulomb and unitarity corrections to the single e + e - pair production, as well as the cross section σ n for the multiple pair production in collision of ultra-relativistic nuclei. In the external field approximation, we consider the probability of one pair production at a fixed impact parameter p between colliding nuclei. We obtain the analytical result for this probability at large p as compared to the electron Compton wavelength. The energy dependence of this probability, as well as that of σ n , differ essentially from widely cited but incorrect results. We estimate also the unitary corrections to the total cross section of the process.

Kinetic description of vacuum particle production in collisions of ultrarelativistic nuclei

The dynamics of partons that emerge as the result of quantum tunneling in a spatially uniform time-dependent field is studied under conditions prevalent in ultrarelativistic heavy-ion collisions. A self-consistent set of coupled equations that consists of the renormalized Maxwell equation and the Vlasov kinetic equation that involves a source and which is derived on a dynamical basis is solved numerically. The time dependence of the distributions of internal fields and currents for bosons and fermions is investigated within this back-reaction mechanism, and their momentum spectra are constructed. Clear evidence that oscillations in the time dependence of parton distributions in phase-space cells are of a stochastic character is obtained, and a significant irregularity in the momentum distribution on large time scales is found. If the influence of the back reaction is disregarded, these effects disappear completely, the oscillations becoming regular. A possible thermalization scenario for such a quasiparticle plasma is considered in the relaxation-time approximation. A locally equilibrium state is described within the two-component thermodynamics of particles and antiparticles. The possibility of introducing temperature under conditions of a strong vacuum polarization is discussed.

Coherent photon radiation from nuclei as a probe of impact-parameter and nucleon-velocity distribution in ultrarelativistic nuclear collisions.

Photons which are coherently produced when nucleons are accelerated during the collisions of ultrarelativistic nuclei are studied as probes of the impact parameter of the collision and of the ra- pidity distribution of the recoil nucleons.