Dilepton production as a measure of QGP thermalization time

We discuss dilepton production from a quark-gluon plasma which has a time-dependent anisotropy in momentum space. A phenomenological model for the hard momentum scale, p_hard(tau) and the plasma anisotropy parameter, xi(tau), is presented. The model interpolates between early-time 1+1 free streaming behavior (tau << tau_iso) and late-time ideal 1+1 hydrodynamical behavior (tau >> tau_iso). Using this model, we find that the dilepton rate arising from electromagnetic annihilations of quarks in the kinematic range 3 <p_T< 8 GeV is sensitive to the assumed isotropization time of the system, tau_iso. Therefore high-energy dilepton production can be used to probe the degree of momentum-space isotropy of a quark-gluon plasma produced in relativistic heavy ion collisions and the time of onset of hydrodynamic expansion of the QGP.