Sorting Out Quenched Jets.

We introduce a new "quantile" analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum (p_{T}). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower p_{T}, making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet p_{T}. In this strategy, jets in heavy-ion collisions ordered by p_{T} are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet p_{T} is a natural observable (Q_{AA}). Furthermore, despite nonmonotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the p_{T} of the parton that initiates a heavy-ion jet and the p_{T} of the vacuum jet which corresponds to it via the quantile procedure (p_{T}^{quant}). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of p_{T} migration in heavy-ion collisions.

Probing the Time Structure of the Quark-Gluon Plasma with Top Quarks.

The tiny droplets of quark gluon plasma (QGP) created in high-energy nuclear collisions experience fast expansion and cooling with a lifetime of a few fm/c. Despite the information provided by probes such as jet quenching and quarkonium suppression, and the excellent description by hydrodynamical models, direct access to the time evolution of the system remains elusive. We point out that the study of hadronically decaying W bosons, notably in events with a top-antitop quark pair, can provide key novel insight into the time structure of the QGP. This is because of a unique feature, namely a time delay between the moment of the collision and that when the W-boson decay products start interacting with the medium. Furthermore, the length of the time delay can be constrained by selecting specific reconstructed top-quark momenta. We carry out a Monte Carlo feasibility study and find that the LHC has the potential to bring first limited information on the time structure of the QGP. Substantially increased LHC heavy-ion luminosities or future higher-energy colliders would open opportunities for more extensive studies.