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      Dilepton production rate in a hot and magnetized quark-gluon plasma

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          Abstract

          The differential multiplicity of dileptons in a hot and magnetized quark-gluon plasma, \(\Delta_{B}\equiv dN_{B}/d^{4}xd^{4}q\), is derived from first principles. The constant magnetic field \(B\) is assumed to be aligned in a fixed spatial direction. It is shown that the anisotropy induced by the \(B\) field is mainly reflected in the general structure of photon spectral density function. This is related to the imaginary part of the vacuum polarization tensor, \(\mbox{Im}[\Pi^{\mu\nu}]\), which is derived in a first order perturbative approximation. The final analytical expression for \(\Delta_{B}\) includes a trace over the product of a photonic part, \(\mbox{Im}[\Pi^{\mu\nu}]\), and a certain leptonic part, \({\cal{L}}_{\mu\nu}\). It is shown that \(\Delta_{B}\) consists of two parts, \(\Delta_{B}^{\|}\) and \(\Delta_{B}^{\perp}\), arising from the components \((\mu,\nu)=(\|,\|)\) and \((\mu,\nu)=(\perp,\perp)\) of \(\mbox{Im}[\Pi^{\mu\nu}]\) and \({\cal{L}}_{\mu\nu}\). Here, the transverse and longitudinal directions are defined with respect to the direction of the \(B\) field. Combining \(\Delta_{B}^{\|}\) and \(\Delta_{B}^{\perp}\), a novel anisotropy factor \(\nu_{B}\) is introduced. Using the final analytical expression of \(\Delta_{B}\), the possible interplay between the temperature \(T\) and the magnetic field strength \(eB\) on the ratio \(\Delta_{B}/\Delta_{0}\) and \(\nu_{B}\) is numerically studied. Here, \(\Delta_{0}\) is the Born approximated dilepton multiplicity in the absence of external magnetic fields. It is, in particular, shown that for each fixed \(T\) and \(B\), in the vicinity of certain threshold energies for dilepton production, \(\Delta_{B}\gg \Delta_{0}\) and \(\Delta_{B}^{\perp}\gg \Delta_{B}^{\|}\). The latter anisotropy may be interpreted as one of the microscopic sources of the macroscopic anisotropies, reflecting themselves, e.g., in the elliptic asymmetry factor \(v_{2}\) of dileptons.

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          Estimate of the magnetic field strength in heavy-ion collisions

          , , (2010)
          Magnetic fields created in the noncentral heavy-ion collision are studied within a microscopic transport model, namely the Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). Simulations were carried out for different impact parameters within the SPS energy range (\(E_{lab} = 10 - 158 A\) GeV) and for highest energies accessible for RHIC. We show that the magnetic field emerging in heavy-ion collisions has the magnitude of the order of \(eB_y \sim 10^{-1} m_\pi^2\) for the SPS energy range and \(eB_y \sim m_\pi^2\) for the RHIC energies. The estimated value of the magnetic field strength for the LHC energy amounts to \(eB_y \sim 15 m_\pi^2\).
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            Particle Production in Strong Electromagnetic Fields in Relativistic Heavy-Ion Collisions

            I review the origin and properties of electromagnetic fields produced in heavy-ion collisions. The field strength immediately after a collision is proportional to the collision energy and reaches ~mπ2at RHIC and ~10mπ2at LHC. I demonstrate by explicit analytical calculation that after dropping by about one-two orders of magnitude during the first fm/c of plasma expansion, it freezes out and lasts for as long as quark-gluon plasma lives as a consequence of finite electrical conductivity of the plasma. Magnetic field breaks spherical symmetry in the direction perpendicular to the reaction plane, and therefore all kinetic coefficients are anisotropic. I examine viscosity of QGP and show that magnetic field induces azimuthal anisotropy on plasma flow even in spherically symmetric geometry. Very strong electromagnetic field has an important impact on particle production. I discuss the problem of energy loss and polarization of fast fermions due to synchrotron radiation, consider photon decay induced by magnetic field, elucidateJ/ψdissociation via Lorentz ionization mechanism, and examine electromagnetic radiation by plasma. I conclude thatallprocesses in QGP are affected by strong electromagnetic field and call for experimental investigation.
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              Quantum field theory in a magnetic field: From quantum chromodynamics to graphene and Dirac semimetals

              A range of quantum field theoretical phenomena driven by external magnetic fields and their applications in relativistic systems and quasirelativistic condensed matter ones, such as graphene and Dirac/Weyl semimetals, are reviewed. We start by introducing the underlying physics of the magnetic catalysis. The dimensional reduction of the low-energy dynamics of relativistic fermions in an external magnetic field is explained and its role in catalyzing spontaneous symmetry breaking is emphasized. The general theoretical consideration is supplemented by the analysis of the magnetic catalysis in quantum electrodynamics, chromodynamics and quasirelativistic models relevant for condensed matter physics. By generalizing the ideas of the magnetic catalysis to the case of nonzero density and temperature, we argue that other interesting phenomena take place. The chiral magnetic and chiral separation effects are perhaps the most interesting among them. In addition to the general discussion of the physics underlying chiral magnetic and separation effects, we also review their possible phenomenological implications in heavy-ion collisions and compact stars. We also discuss the application of the magnetic catalysis ideas for the description of the quantum Hall effect in monolayer and bilayer graphene, and conclude that the generalized magnetic catalysis, including both the magnetic catalysis condensates and the quantum Hall ferromagnetic ones, lies at the basis of this phenomenon. We also consider how an external magnetic field affects the underlying physics in a class of three-dimensional quasirelativistic condensed matter systems, Dirac semimetals. While at sufficiently low temperatures and zero density of charge carriers, such semimetals are expected to reveal the regime of the magnetic catalysis, the regime of Weyl semimetals with chiral asymmetry is realized at nonzero density...
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                Author and article information

                Journal
                1601.04887

                High energy & Particle physics,Nuclear physics
                High energy & Particle physics, Nuclear physics

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