Dilepton production from a nonequilibrium quark-gluon plasma in ultrarelativistic nucleus-nucleus collisions

Dilepton production from a nonequilibrium quark-gluon plasma produced in ultrarelativistic nucleus-nucleus collisions is studied, based on the flux-tube model which has been formulated in terms of a model relativistic Boltzmann-Vlasov equation with a boost-invariant particle source term. This model kinetic equation is solved with the collision term in the relaxation-time approximation. In the collisionless limit, the solution of the kinetic equation becomes oscillatory, indicating the spontaneous excitation of the plasma oscillation. We study how such a nonequilibrium evolution of the system is reflected in the dilepton spectrum. It is shown that the dileptons emitted during such a nonequilibrium stage of matter evolution does not necessarily lead to a spectrum which interpolates smoothly the spectrum from thermal emission and that from the Drell-Yan mechanism. Our results show, on the contrary, that the dilepton spectrum is softened significantly in the absence of thermalization. $M_T$ scaling, which is expected in scaling one-dimensional hydrodynamic evolution, is also broken due to the anisotropy in the phase-space distribution of quarks.