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Virtual photon polarization and dilepton anisotropy in relativistic nucleus–nucleus collisions
(2018)
The polarization of virtual photons produced in relativistic nucleus–nucleus collisions provides information on the conditions in the emitting medium. In a hydrodynamic framework, the resulting angular anisotropy of the dilepton final state depends on the flow as well as on the transverse momentum and invariant mass of the photon. We illustrate these effects in dilepton production from quark–antiquark annihilation in the QGP phase and π+π− annihilation in the hadronic phase for a static medium in global equilibrium and for a longitudinally expanding system.
Hadronic polarization and the related anisotropy of the dilepton angular distribution are studied for the reaction πN→Ne+e−. We employ consistent effective interactions for baryon resonances up to spin-5/2, where non-physical degrees of freedom are eliminated, to compute the anisotropy coefficients for isolated intermediate baryon resonances. It is shown that the spin and parity of the intermediate baryon resonance is reflected in the angular dependence of the anisotropy coefficient. We then compute the anisotropy coefficient including the N(1520) and N(1440) resonances, which are essential at the collision energy of the recent data obtained by the HADES Collaboration on this reaction. We conclude that the anisotropy coefficient provides useful constraints for unraveling the resonance contributions to this process.
A newly proposed framework of perfect-fluid relativistic hydrodynamics for particles with spin 1/2 is briefly reviewed. The hydrodynamic equations follow entirely from the conservation laws for energy, momentum, and angular momentum. The incorporation of the angular-momentum conservation requires that the spin polarization tensor ωμν is introduced. It plays a role of a Lagrange multiplier conjugated to the spin tensor Sλ,μν. The space-time evolution of the spin polarization tensor depends on the specific form chosen for the spin tensor.