Peripheral Relativistic Heavy Ion Collisions Research Articles

Two-gluon one-photon vertex in a magnetic field and its explicit one-loop approximation in the intermediate field strength regime

We find the general structure for the two-gluon one-photon vertex in the presence of a constant magnetic field. We show that, when accounting for the symmetries satisfied by the strong and electromagnetic interactions under parity, charge conjugation and gluon interchange, and for gluons and photons on mass-shell, there exist only three possible tensor structures that span the vertex. These correspond to external products of the polarization vectors for each of the particles in the vertex. We also explicitly compute the one-loop approximation to this vertex in the intermediate field strength regime, which is the most appropriate one to describe possible effects of the presence of a magnetic field to enhance photon emission during preequilibrium in peripheral relativistic heavy-ion collisions. We show that the most favored direction for the photon to propagate is in the plane transverse to the field, which is consistent with a positive contribution to ν2 and may help to understand the larger than expected elliptic flow coefficient measured in this kind of reaction. Published by the American Physical Society 2024

Challenges to the Good-Walker paradigm in coherent and incoherent photoproduction

High-energy vector meson photoproduction is an important tool for studying the partonic structure of matter at low Bjorken$-x$. In the Good-Walker (GW) paradigm, the cross-section $d\sigma/dt$ for coherent production of vector mesons or other final states, depends the average transverse distribution of gluons, while the incoherent cross-section depends on fluctuations in the nuclear structure, due to variations in nucleon positions, and/or gluonic hot spots. However, predictions of the the GW paradigm seemingly conflict with data from multiple experiments which observe coherent production of vector mesons accompanied by nuclear excitation, or in peripheral relativistic heavy-ion collisions. These data are consistent with a simpler, semi-classical approach. We will discuss this contradiction and explore how and why GW fails. We will also contrast the significant differences in incoherent photoproduction on $^{197}$Au and $^{208}$Pb targets in the GW approach with the much smaller expected differences in their low$-x$ gluon content.

Lepton pair photoproduction in peripheral relativistic heavy-ion collisions

We study the lepton pair photoproduction in peripheral heavy-ion collisions based on the formalism in our previous work [R.-j. Wang, S. Pu, and Q. Wang, Phys. Rev. D 104, 056011 (2021)]. We present the numerical results for the distributions of the transverse momentum, azimuthal angle and invariant mass for ${e}^{+}{e}^{\ensuremath{-}}$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ pairs as functions of the impact parameter and other kinematic variables in $\mathrm{Au}+\mathrm{Au}$ collisions. Our calculation incorporates the information on the transverse momentum and polarization of photons which is essential to describe the experimental data. We observe a broadening effect in the transverse momentum for lepton pairs with and without smear effects. We also observe a significant enhancement in the distribution of $\mathrm{cos}(2\ensuremath{\varphi})$ for ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ pairs. Our results provide a baseline for future studies of other higher order corrections beyond Born approximation and medium effects in the lepton pair production.

Nonequilibrium Dynamics of the Chiral Quark Condensate under a Strong Magnetic Field

Strong magnetic fields impact quantum-chromodynamics (QCD) properties in several situations; examples include the early universe, magnetars, and heavy-ion collisions. These examples share a common trait—time evolution. A prominent QCD property impacted by a strong magnetic field is the quark condensate, an approximate order parameter of the QCD transition between a high-temperature quark-gluon phase and a low-temperature hadronic phase. We use the linear sigma model with quarks to address the quark condensate time evolution under a strong magnetic field. We use the closed time path formalism of nonequilibrium quantum field theory to integrate out the quarks and obtain a mean-field Langevin equation for the condensate. The Langevin equation features dissipation and noise kernels controlled by a damping coefficient. We compute the damping coefficient for magnetic field and temperature values achieved in peripheral relativistic heavy-ion collisions and solve the Langevin equation for a temperature quench scenario. The magnetic field changes the dissipation and noise pattern by increasing the damping coefficient compared to the zero-field case. An increased damping coefficient increases fluctuations and time scales controlling condensate’s short-time evolution, a feature that can impact hadron formation at the QCD transition. The formalism developed here can be extended to include other order parameters, hydrodynamic modes, and system’s expansion to address magnetic field effects in complex settings as heavy-ion collisions, the early universe, and magnetars.

Bound-free pair production with a correction term in relativistic heavy-ion collisions

In our previous work, we calculated bound-free electron-positron pair production cross section without a correction term. In this work, bound-free electron-positron pair production with a correction term is considered to calculate the cross section for peripheral relativistic heavy ion collisions. The Dirac wave functions have been used for the leptons and first order corrections are included. It is seen that the results for the production cross sections are considerably smaller than those of the previous calculations. We applied the same method for the calculations of the antihydrogen production cross sections as well.

Unitary Limit of Two-Nucleon Interactions in Strong Magnetic Fields.

Two-nucleon systems are shown to exhibit large scattering lengths in strong magnetic fields at unphysical quark masses, and the trends toward the physical values indicate that such features may exist in nature. Lattice QCD calculations of the energies of one and two nucleons systems are performed at pion masses of m_{π}∼450 and 806MeV in uniform, time-independent magnetic fields of strength |B|∼10^{19}-10^{20} G to determine the response of these hadronic systems to large magnetic fields. Fields of this strength may exist inside magnetars and in peripheral relativistic heavy ion collisions, and the unitary behavior at large scattering lengths may have important consequences for these systems.

The pseudo chiral magnetic effect in QED3

Chiral magnetic effect (CME) has been suggested to take place during peripheral relativistic heavy ion collisions. However, signals of its realization are not yet independent of ambiguities and thus probing the non-trivial topological vacua of quantum chromodynamics (QCD) is still an open issue. Weyl materials, particularly graphene, on the other hand, are effectively described at low energies by the degrees of freedom of quantum electrodynamics in two spatial dimensions, QED3. This theory shares with QCD some interesting features, like confinement and chiral symmetry breaking and also possesses a non-trivial vacuum structure. In this regard, an analog of the CME is proposed to take place in graphene under the influence of an in-plane magnetic field in which the pseudo-spin or flavor label of charge carriers is participant of the effect, rather than the actual spin. In this contribution, we review the parallelisms and differences between the CME and the so-called pseudo chiral magnetic effect, PCME.

Physics of Ultra-Peripheral Collisions at RHIC and LHC

The creation of electron-positron pairs from the vacuum by strong electromagnetic fields is a fundamental and one of the most fascinating processes of relativistic quantum mechanics. Fields sufficiently strong to induce pair creation can be produced, for example, in collisions of bare ions. The process has been studied intensively both in theory and experiment since the 1980s. In peripheral relativistic heavy-ion collisions electromagnetic fields are very strong and interact with each other for a very short time. In these strong fields, probabilities of various electromagnetic processes such as free pair production, bound-free pair production and excitation are large and increase with the Lorentz factor γ. Measurement of cross sections and other relevant distributions are very important for understanding the strong field effect.

Coherent photon-photon interactions in very peripheral relativistic heavy ion collisions

Heavy ions at high velocities provide very strong electromagnetic fields for a very short time. The main characteristics of ultraperipheral relativistic heavy ion collisions are reviewed, characteristic parameters are identified. The main interest in ultraperipheral heavy ion collisions at relativistic ion colliders like the LHC is the interactions of very high energy (equivalent) photons with the countermoving (equivalent) photons and hadrons (protons/ions). The physics of these interactions is quite different from and complementary to the physics of the strong fields achieved with current and future lasers.

Lepton-pair production from deep inelastic scatteringin peripheral relativistic heavy ion collisions

We calculate the inelastic electron- and muon-pair production in peripheral relativistic heavy ion collisions in the region of large Q 2 of one of the photons. This offers a possibility to study the quark distribution functions in ions in "ultraperipheral heavy ion collisions". The calculations are compared with those making use of the equivalent photon and the equivalent lepton approximation. We compare the results for Pb-Pb and Pb-p collisions at RHIC (γ ≈ 100) and LHC (γ ≈ 3000) energies. Furthermore we include nuclear modifications to the parton distribution functions in our calculations to study their effect on the cross sections.

Transient effects in fission from new experimental signatures.

A new experimental approach is introduced to investigate the relaxation of the nuclear deformation degrees of freedom. Highly excited fissioning systems with compact shapes and low angular momenta are produced in peripheral relativistic heavy-ion collisions. Both fission fragments are identified in atomic number. Fission cross sections and fission-fragment element distributions are determined as a function of the fissioning element. From the comparison of these new observables with a nuclear-reaction code a value for the transient time is deduced.

Interference in exclusive vector meson production in heavy-Ion collisions

Vector mesons are produced copiously in peripheral relativistic heavy-ion collisions. Virtual photons from one ion can fluctuate into quark-antiquark pairs and scatter from the second ion, emerging as vector mesons. The emitter and target are indistinguishable, so emission from the two ions will interfere. Vector mesons have negative parity so the interference is destructive, reducing the production of mesons with small transverse momentum. The mesons are short lived, and decay before emission from the two ions can overlap. However, the decay-product wave functions overlap and interfere since they are produced in an entangled state, providing an example of the Einstein-Podolsky-Rosen paradox.

Bremsstrahlung from electrons and positrons in peripheral relativistic heavy-ion collisions

We study the spectrum of the bremsstrahlung photons coming from the electrons and positrons, which are produced in the strong electromagnetic fields present in peripheral relativistic heavy ion collisions. We compare different approaches, making use of the exact pair production cross section in heavy ion collisions as well as the double equivalent photon approximation.

Calculated nuclide production yields in relativistic collisions of fissile nuclei

A model calculation is presented which predicts the complex nuclide distribution resulting from peripheral relativistic heavy-ion collisions involving fissile nuclei. The model is based on a modern version of the abrasion-ablation model which describes the formation of excited prefragments due to the nuclear collisions and their consecutive decay. The competition between the evaporation of different light particles and fission is computed with an evaporation code which takes dissipative effects and the emission of intermediate-mass fragments into account. The nuclide distribution resulting from fission processes is treated by a semi-empirical description which includes the excitation-energy dependent influence of nuclear shell effects and pairing correlatios. The calculations of collisions between 238U and different reaction partners reveal that a huge number of isotopes of all elements up to uranium is produced. The complex nuclide distribution shows the characteristics of fragmentation, mass-asymmetric low-energy fission and mass-symmetric high-energy fission. The yields of the different components for different reaction partners are studied. Consequences for technical applications are discussed.

Convergence of a lattice calculation for bound-free muon-pair production in peripheral relativistic heavy-ion collisions.

We have developed a nonperturbative treatment of bound-free lepton-pair production caused by the strong and sharply pulsed electromagnetic fields generated by heavy ions in peripheral, relativistic collisions based on the solution of the time-dependent Dirac equation using lattice techniques. In this paper, we discuss refinements and extensions of our numerical methods for this problem and demonstrate convergent calculations, with respect to the parameters of the lattice, for bound-free muon-pair production in collisions near grazing incidence of $^{238}\mathrm{U}^{92+}$${+}^{238}$${\mathrm{U}}^{92+}$ and beam kinetic energies of 30 GeV per nucleon in the fixed-target frame. \textcopyright{} 1996 The American Physical Society.

Neutron removal in peripheral relativistic heavy-ion collisions.

We investigate the relativistic Coulomb fragmentation of $^{197}\mathrm{Au}$ by heavy ions, leading to one-, two-, and three-neutron removal. To resolve the ambiguity connected with the choice of a specific minimum impact parameter in a semiclassical calculation, a microscopic approach is developed based on nucleon-nucleon collisions (``soft-spheres'' model). This approach is compared with experimental data for $^{197}\mathrm{Au}$ at 1 GeV/nucleon and with a calculation using the ``sharp-cutoff'' approximation. We find that the harmonic-oscillator model predicting a Poisson distribution of the excitation probabilities of multiphonon states gives a good agreement with one-neutron removal cross sections but is unable to reach an equally good agreement with three-neutron removal cross sections.

Nuclear coherent versus incoherent effects in peripheral RHI collisions

We derive simple and physically transparent expressions for the contribution of the strong interaction to one-nucleon-removal processes in peripheral relativistic heavy-ion collisions. The coherent contribution, i.e., the excitation of a giant dipole resonance via meson exchange, is shown to be negligible as well as the interference between Coulomb and nuclear excitation. The incoherent nucleon-knockout contribution is also derived suggesting the nature of the nuclear interaction in this class of processes. We also justify the simple formulae used to fit the data of the E814 Collaboration.

CORRELATED NUCLEON EMISSION IN PERIPHERAL RELATIVISTIC HEAVY ION COLLISIONS

The effects of short range correlations for two nucleon emission in peripheral relativistic heavy ion collisions are investigated. The cross section is calculated in Born approximation with properly orthogonalized initial and final states.

Nonperturbative electromagnetic lepton-pair production in peripheral relativistic heavy-ion collisions

We discuss a nonperturbative treatment of lepton-pair production caused by the strong and sharply pulsed electromagnetic fields generated in peripheral relativistic heavy-ion collisions with an emphasis on the capture process into the atomic K shell. We calculate, in a field-theoretical framework, impact-parameter-dependent probabilities and cross sections for such processes by solving the time-dependent Dirac equation on a three-dimensional Cartesian lattice using the basis-spline collocation method. We give a full discussion of the stationary states used in computing S-matrix elements. Use of the axial gauge for the electromagnetic potentials produces an interaction easier to implement on the lattice than the Lorentz gauge. Preliminary calculations are given for muon-pair production with capture into the K shell in collisions of $^{197}\mathrm{Au}$${+}^{197}$Au at collider energies per nucleon of 2 and 100 GeV.

SHORT RANGE CORRELATIONS AND NUCLEON EMISSION IN PERIPHERAL RELATIVISTIC HEAVY ION COLLISIONS

We investigate the effects of short range correlations on nucleon emission in peripheral relativistic heavy ion collisions, following an idea originally proposed by Feshbach and Zabek. Expressions for one- and two-nucleon emission cross sections are derived and calculations for typical heavy ion systems are performed. We show that momentum conservation in the phonon absorption process leads to pronounced ridges in the quadruple-differential cross section for two-nucleon emission. We suggest that the presence of such ridges in future exclusive experiments should be considered as a signature of correlated emission of a nucleon pair.