Caption: (Left) Annihilation of a quark-antiquark pair from the colliding nuclei. In this process, called "Drell-Yan", the quark and antiquark are moving along the collision axis. (Right) Production of a lepton pair in the quark-gluon plasma before it reaches thermal equilibrium. The velocities of the quark and antiquark are preferentially transverse to the collision axis. The invariant mass of the lepton pair allows one to discriminate between the two processes, the one on the right becoming more likely below 4-5 GeV.
The LHC accelerator at CERN carries out collisions between lead nuclei at unprecedented energies. Each collision creates a state of matter named "quark-gluon plasma", which is known to rapidly reach local thermal equilibrium. In equilibrium, the velocities of quarks, antiquarks and gluons are distributed isotropically. The emergence of an isotropic distribution was one of the big surprises of the field, which has triggered intense theoretical activity in the last decade. There is consensus among theorists that before eqilibrium is reached, transverse velocities are on average larger than longitudinal velocities (right panel on figure), due to the rapid longitudinal expansion. But there is at present no direct experimental signature of this primordial anisotropy.
The present paper [1], written in collaboration between IRFU/DPhN and IPhT, proposes to detect it for the first time by measuring lepton-antilepton pairs (either electron-positron, or muon-antimuon) created by the annihilation of a quark-antiquark pair. The key idea is that leptons are emitted preferentially in the same direction as quarks. Once produced, lepton fly to the detector without undergoing further interactions, so that they can be used to probe the early stages of the quark-gluon plasma. This analysis should become possible in the next decade thanks to ongoing and future detector upgrades (of LHCb and ALICE in particular), which will allow the elimination of background leptons from weak decays of c and b quarks.
[1] Dilepton Polarization as a Signature of Plasma Anisotropy. Maurice Coquet, Michael Winn, Xiaojian Du, Jean-Yves Ollitrault, and Sören Schlichting. Phys. Rev. Lett. 132, 232301.
