Abstract
The production of dileptons with an invariant mass in the range 1GeV<M<5 GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory. We argue that the dilepton mass spectrum provides a measure of the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors.
Highlights
Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) produce a rapidly-expanding plasma of quarks and gluons
We focus on dilepton production with M > 1 GeV, significantly larger than the highest temperature achieved in a central Pb+Pb collision at the LHC, which typically does not exceed 400 MeV [10]
We conclude that intermediate mass dilepton production is sensitive to the very early stages of heavy-ion collisions
Summary
Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) produce a rapidly-expanding plasma of quarks and gluons. This attractor behaviour, which was discovered in the context of strong-coupling calculations, was identified in the weak-coupling limit (kinetic theory), first in the relaxation-time approximation [19, 20, 21] and in QCD kinetic theory [22, 23] These different modelizations lead to very similar attractors [24], which paves the way to a robust modeling of the pre-equilibrium dynamics, where the information about the thermalization is encoded into a single parameter, typically the viscosity over entropy ratio η/s.
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