We calculate the medium dilepton yield from a quark-gluon plasma which has a time-dependent local momentum-space anisotropy. A phenomenological model for the hard momentum scale, $p_{\mathrm{hard}}(\tau )$, and plasma anisotropy parameter, $\xi (\tau )$, is constructed which interpolates between longitudinal free streaming at early times ($\tau \ll {\tau }_{\mathrm{iso}}$) and ideal hydrodynamic at late times ($\tau \gg {\tau }_{\mathrm{iso}}$). We show that high-energy dilepton production is sensitive to the plasma isotropization time, ${\tau }_{\mathrm{iso}}$, and can therefore be used to experimentally determine the time of onset for hydrodynamic expansion of a quark-gluon plasma and the magnitude of expected early-time momentum-space anisotropies.

• Received 4 October 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.102301