Fields In Heavy Ion Collisions Research Articles

Paths to equilibrium in non-conformal collisions

We extend our previous analysis of holographic heavy ion collisions in non-conformal theories. We provide a detailed description of our numerical code. We study collisions at different energies in gauge theories with different degrees of non-conformality. We compare four relaxation times: the hydrodynamization time (when hydrodynamics becomes applicable), the EoSization time (when the average pressure approaches its equilibrium value), the isotropization time (when the longitudinal and transverse pressures approach each other) and the condensate relaxation time (when the expectation value of a scalar operator approaches its equilibrium value). We find that these processes can occur in several different orderings. In particular, the condensate can remain far from equilibrium even long after the plasma has hydrodynamized and EoSized. We also explore the rapidity distribution of the energy density at hydrodynamization. This is far from boost-invariant and its width decreases as the non-conformality increases. Nevertheless, the velocity field at hydrodynamization is almost exactly boost-invariant regardless of the non-conformality. This result may be used to constrain the initialization of hydrodynamic fields in heavy ion collisions.

Quantum analysis of fluctuations of electromagnetic fields in heavy-ion collisions

We perform quantum calculations of fluctuations of the electromagnetic fields in $AA$ collisions at RHIC and LHC energies. The analysis is based on the fluctuation-dissipation theorem. We find that in the quantum picture the field fluctuations are very small. They turn out to be much smaller than the predictions of the classical Monte-Carlo simulation with the Woods-Saxon nuclear density.

Novel quantum phenomena induced by strong magnetic fields in heavy-ion collisions

The relativistic heavy-ion collisions create both hot quark-gluon matter and strong magnetic fields, and provide an arena to study the interplay between quantum chromodynamics and quantum electrodynamics. In recent years, it has been shown that such an interplay can generate a number of interesting quantum phenomena in hadronic and quark-gluon matter. In this short review, we first discuss some properties of the magnetic fields in heavy-ion collisions and then give an overview of the magnetic field-induced novel quantum effects. In particular, we focus on the magnetic effect on the heavy flavor mesons, the heavy-quark transports, and the phenomena closely related to chiral anomaly.

Initial value problem for magnetic fields in heavy ion collisions

When quark-gluon plasma emerges in the wake of a heavy-ion collision, magnetic field created by the valence charges has already permitted the entire interaction region. Evolution of this "initial" field in plasma is governed by Maxwell equations in electrically conducting medium. As the plasma expands external valence charges induce magnetic field that also contributes to the total magnetic field in plasma. We solve the initial value problem describing these processes and argue that the initial magnetic field often dominates over the one induced by the valence charges. In particular, it grows approximately proportional to the collision energy, unlike the induced component, which is energy-independent. As a result, magnetic field has a significant phenomenological influence on quark-gluon plasma at the LHC energies over its entire lifetime.

The Initial Flow of Classical Gluon Fields in Heavy Ion Collisions

Using analytic solutions of the Yang-Mills equations we calculate the initial flow of energy of the classical gluon field created in collisions of large nuclei at high energies. We find radial and elliptic flow which follows gradients in the initial energy density, similar to a simple hydrodynamic behavior. In addition we find a rapidity-odd transverse flow field which implies the presence of angular momentum and should lead to directed flow in final particle spectra. We trace those energy flow terms to transverse fields from the non-abelian generalization of Gauss' Law and Ampere's and Faraday's Laws.

Comments about the electromagnetic field in heavy-ion collisions

In this short article we discuss the properties of electromagnetic fields in heavy-ion collisions and consequences for observables. We address quantitatively the issue of the magnetic field lifetime in a collision including the electric and chiral magnetic conductivities. We show that for reasonable parameters, the magnetic field created by spectators in a collision is not modified by the presence of matter. Based on this, we draw definite conclusions on observed effects in heavy-ion collisions.

Physics of Quark Gluon Plasma: An Update and the Status Report

The microsecond old universe was too hot for the nucleons to survive as bound states of quarks. When the temperature of the universe fell below ∼10 K as a result of cooling due to expansion, the quarks got confined inside the nucleons to reside there ever after—making them inaccessible to any direct observation in isolation. Natural objects like the core of the compact astrophysical objects may also contain deconfined quark matter. Therefore, creation of a deconfined phase of thermal quarks and gluons will be very important for understanding the evolution of the early universe and the physics of compact astrophysical objects like neutron star. Calculations based on lattice QCD indicate that hadronic matter at high temperatures and densities undergoes a phase transition to a thermalized system of deconfined quarks and gluons, called quark gluon plasma (QGP). It is expected that such high densities and temperatures may be achieved by colliding two nuclei at RHIC, LHC, or FAIR energies. The present volume addresses some of the experimental and theoretical issues related to quark-hadron phase transition in the relativistic heavy ion collisions by the world experts in the field. On the experimental sides it contains results from CERN’s Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) and Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). Comparison of experimental results from RHIC and LHC is extremely useful to gain insights into the properties of the matter formed in these experiments. The present volume offers a review on this subject. On the theoretical side the present volume contains papers on the space-time evolution of the system formed in heavy ion collisions including the effects of viscosities through hydrodynamic equations, issues of equilibration, electromagnetic and heavy quark probes of QGP, quarkonia suppression in QGP, ridge, and transverse correlations. Recent progress in the femtoscopy and particle production in strong electromagnetic fields in heavy ion collisions is also addressed adequately. In summary, the present volume brings out recent developments on almost all the relevant issues of heavy ion collisions like creation, evolution, and signals of QGP. We are sure that the volume will be useful for the current practitioners and young new entrants to this exciting field of research.

Particle Yields in Heavy Ion Collisions and the Influence of Strong Magnetic Fields

It is expected that the magnetic fields in heavy ion collisions are very high. In this work, we investigate the effects of a strong magnetic field on particle ratios within a thermal model of particle production. We model matter as a free gas of baryons and mesons under the influence of an external magnetic field varying from zero to30mπ2through an χ2fitting to some data sets of the STAR experiment. For this purpose, we use the Dirac, Rarita-Schwinger, Klein-Gordon, and Proca equations subject to magnetic fields in order to obtain the energy expressions and the degeneracy for spin 1/2, spin 3/2, spin 0, and spin 1 particles, respectively. Our results show that, if the magnetic field can be considered as slowly varying and leaves its signature on the particle yields, a field of the order of6mπ2produces an improved fitting to the experimental data as compared to the calculations without magnetic field.

Interplay of anisotropies of momentum distribution and mean field in heavy-ion collisions

Two important parametrizations of momentum-dependent nucleonic fields, proposed for the simulations of central heavy-ion collisions, one by Gale et al. and the other by Welke et al., suffer from practical limitations. The first gives rise to mean fields isotropic in momentum, even when underlying momentum distributions are anisotropic, making descriptions of early nonequilibrium stages of collisions unrealistic. The second parametrization gives rise to anisotropic mean fields, but is computationally expensive, because the mean field has to be computed separately for every location of a nucleon in phase space, through folding. Here we construct a parametrization of the nucleonic mean field that yields an anisotropic mean field for an anisotropic momentum distribution and is inexpensive computationally. To demonstrate the versatility of our parametrization, we take the case of results from the parametrization by Welke et al. and attempt to approximate them. In arriving at a suitable anisotropic mean-field potential, we draw, on one hand, from the idea behind the parametrization of Gale et al., of a separable expansion of the potential energy, and, on the other, from the idea of a parallel expansion of the energy and mean field in anisotropy. We show that using our novel parametrization we can qualitatively and partially quantitatively reproduce the features of the mean-field parametrization of Welke et al.. This opens up the possibility of exploring the effects of mean-field anisotropy in collisions, without the penalty of computational cost behind the folding parametrization.

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.

Anisotropy of Photon Production: Initial Eccentricity or Magnetic Field

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of Relativistic Heavy Ion Collider show that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this Letter, we propose a method to experimentally measure the degree to which a magnetic field in heavy-ion collisions is responsible for the observed anisotropy of photon production. The experimental test proposed in this Letter may potentially change our understanding of the nonequilibrium stage and possible thermalization in heavy-ion collisions.

Quantum chaos in the quantum fluid: Spectrum of initial fluctuations in the little bang

Abstract We outline how unstable quantum fluctuations decohere classical fields in heavy ion collisions, leading to an equation of state and hydrodynamics. Explicit numerical realization of this frame-work in a scalar ϕ 4 theory demonstrates that anomalously low values of η / s can be generated.

Photon azimuthal anizotropy and magnetic field in heavy-ion collisions

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of RHIC showed that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this talk, we consider a novel photon production mechanism stemming from the conformal anomaly of QCDxQED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. The comparison of the results to the data from the PHENIX collaboration, show that this mechanism might be the most responsible for the observed azimuthal anisotropy of photons.

Conformal Anomaly as a Source of Soft Photons in Heavy Ion Collisions

We introduce a novel photon production mechanism stemming from the conformal anomaly of QCD×QED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. This mechanism also provides a significant positive contribution to the azimuthal anisotropy of photons, v(2), as well as to the radial "flow." We compare our results to the data from the PHENIX Collaboration.

Event-by-event gluon multiplicity, energy density, and eccentricities in ultrarelativistic heavy-ion collisions

The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments ${\ensuremath{\epsilon}}_{n}$ (up to $n=6$) at early times in heavy-ion collisions at both the BNL Relativistic Heavy Ion Collider (RHIC) ($\sqrt{s}=200\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$) and the CERN Large Hardron Collider (LHC) ($\sqrt{s}=2.76\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$) are computed in the IP-Glasma model. This framework combines the impact parameter dependent saturation model (IP-Sat) for nucleon parton distributions (constrained by HERA deeply inelastic scattering data) with an event-by-event classical Yang-Mills description of early-time gluon fields in heavy-ion collisions. The model produces multiplicity distributions that are convolutions of negative binomial distributions without further assumptions or parameters. In the limit of large dense systems, the $n$-particle gluon distribution predicted by the Glasma-flux tube model is demonstrated to be nonperturbatively robust. In the general case, the effect of additional geometrical fluctuations is quantified. The eccentricity moments are compared to the MC-KLN model; a noteworthy feature is that fluctuation dominated odd moments are consistently larger than in the MC-KLN model.

Probing strongly interacting matter with heavy resonances in Pb+Pb collisions at LHC energies

With the recent Pb+Pb collisions experiments at LHC, a new era has began in the field of heavy ion collisions. The heavy hadronic resonances such as J/ψ, Υ and their states are produced in plenty at LHC and can be tackled in perturbative QCD. These resonances along with other low mass resonances can be very well reconstructed with advanced detector technologies and are used to study the detailed tomography of the matter which would not have been possible at lower energies. In addition to the hot matter effects one also has to address the questions such as modification of parton distributions of protons when placed inside a nucleus. This is known as initial state effect and can be well addressed at LHC by measuring the Z boson. In addition, there are (cold) nuclear matter effects which can modify the probes and are planned to be studied in future p+Pb collisions. In this paper, we review the recent LHC results with emphasis on quarkonia. We devote a section on modeling of quarkonia suppression. The new calculation done in the case of Υ and its states are given.

Event-by-event generation of electromagnetic fields in heavy-ion collisions

We compute the electromagnetic fields generated in heavy-ion collisions by using the HIJING model. Although after averaging over many events only the magnetic field perpendicular to the reaction plane is sizable, we find very strong magnetic and electric fields both parallel and perpendicular to the reaction plane on the event-by-event basis. We study the time evolution and the spatial distribution of these fields. Especially, the electromagnetic response of the quark-gluon plasma can give non-trivial evolution of the electromagnetic fields. The implications of the strong electromagnetic fields on the hadronic observables are also discussed.

Event-by-event fluctuations of magnetic and electric fields in heavy ion collisions

We show that fluctuating proton positions in the colliding nuclei generate, on the event-by-event basis, very strong magnetic and electric fields in the direction both parallel and perpendicular to the reaction plane. The magnitude of E and B fields in each event is of the order of mπ2≈1018 Gauss. Implications on the observation of electric dipole in heavy ion collisions is discussed, and the possibility of measuring the electric conductivity of the hot medium is pointed out.

Anomalous baryon production and its interplay with jet energy loss at RHIC and LHC energies

Hadron production consists traditionally of two main channels in the intermediate-pT region at RHIC and LHC energies: parton coalescence/recombination and jet fragmentation. We have studied a new channel, namely hadron production connected to the appearance of strong time-dependent coherent field in heavy ion collisions. In the momentum window of 5 GeV/c <pT < 25 GeV/c, this channel could yield an anomalous baryon enhancement. This extra yield is able to explain the measured structure of nuclear modification factors for identified hadrons at RHIC energies. We give predictions for LHC energies.

Erratum: Synchrotron radiation by fast fermions in heavy-ion collisions [Phys. Rev. C82, 034904 (2010)

We study the synchrotron radiation of gluons by fast quarks in strong magnetic field produced by colliding relativistic heavy-ions. We argue that due to high electric conductivity of plasma, time variation of the magnetic field is slow and estimate its relaxation time. We calculate the energy loss due to synchrotron radiation of gluons by fast quarks. We find that the typical energy loss per unit length for a light quark at LHC is a few GeV per fm. This effect alone predicts quenching of jets with $p_\bot$ up to about 20 GeV. We also show that the spin-flip transition effect accompanying the synchrotron radiation leads to a strong polarization of quarks and leptons with respect to the direction of the magnetic field. Observation of the lepton polarization may provide a direct evidence of existence of strong magnetic field in heavy-ion collisions.