<p>The observation of the medium response generated by the propagation of high energy partons in the quark gluon plasma produced in heavy-ion collisions would provide clear and unmistakable evidence for the hydrodynamic behavior of the bulk. Recently, it has been argued that the features of the medium’s backreaction to the jet could be cleanly imprinted in the correlations of asymptotic energy flows, in principle, allowing one to isolate this signal from other uncorrelated physical processes. Nonetheless, the current limited theoretical understanding of these jet observables in heavy-ion collisions constrains their applicability as probes of the medium (hydro)dynamics. In this work, we provide an analytic picture for the medium backreaction’s effect on the energy flux and two point energy correlator. We show that the medium response leads to the emergence of a universal classical scaling law, competing with the perturbative QCD contribution at large angles. Comparing the associated correlator to recent experimental measurements, we find that the observed large angle features can be qualitatively described by a purely hydrodynamically driven response and its interplay with the hard jet component.</p>

CERN

João Barata

Russia

Matvey V. Kuzmin

Portugal

José Guilherme Milhano

Andrey V. Sadofyev

Giving wake to energy-energy correlators: Hydrodynamic response on the celestial sphere

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 (<math><msub><mi>p</mi><mi>T</mi></msub></math>). 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 <math><msub><mi>p</mi><mi>T</mi></msub></math>, 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 <math><msub><mi>p</mi><mi>T</mi></msub></math>. In this strategy, jets in heavy-ion collisions ordered by <math><msub><mi>p</mi><mi>T</mi></msub></math> 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 <math><msub><mi>p</mi><mi>T</mi></msub></math> is a natural observable (<math><msub><mi>Q</mi><mrow><mi>A</mi><mi>A</mi></mrow></msub></math>). Furthermore, despite nonmonotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the <math><msub><mi>p</mi><mi>T</mi></msub></math> of the parton that initiates a heavy-ion jet and the <math><msub><mi>p</mi><mi>T</mi></msub></math> of the vacuum jet which corresponds to it via the quantile procedure (<math><msubsup><mi>p</mi><mi>T</mi><mtext>quant</mtext></msubsup></math>). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of <math><msub><mi>p</mi><mi>T</mi></msub></math> migration in heavy-ion collisions.

United States

Jasmine Brewer

Jesse Thaler

Sorting Out Quenched Jets

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 <math><mi>fm</mi><mo>/</mo><mi>c</mi></math>. 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 <math><mi>W</mi></math> 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 <math><mi>W</mi></math>-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.

Liliana Apolinário

Gavin P. Salam

Spain

Carlos A. Salgado

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

The di-jet asymmetry—the measure of the momentum imbalance in a di-jet system—is a key jet quenching observable. Using the event generator Jewel we show that the di-jet asymmetry is dominated by fluctuations both in proton–proton and in heavy-ion collisions. We discuss how in proton–proton collisions the asymmetry is generated through recoil and out-of-cone radiation. In heavy-ion collisions two additional sources can contribute to the asymmetry, namely energy loss fluctuations and differences in path length. The latter is shown to be a sub-leading effect. We discuss the implications of our results for the interpretation of this observable.

Korinna Christine Zapp

Origins of the di-jet asymmetry in heavy-ion collisions

Centrality selection has been observed to have a large effect on jet observables in pPb collisions at the Large Hadron Collider, stronger than that predicted by the nuclear modification of parton densities. We study to which extent simple considerations of energy–momentum conservation which link the hard process with the underlying event that provides the centrality estimator, affect jets observables in such collisions. We develop a simplistic approach that considers first the production of jets in a pp collision as described by PYTHIA. From each pp collision, the value of the energy of the parton from the proton participating in the hard scattering is extracted. Then, the underlying event is generated simulating a pPb collision through HIJING, but with the energy of the proton decreased according to the value extracted in the previous step, and both collisions are added. This model is able to capture the bulk of the centrality effect for central to semicentral collisions, for the two available sets of data: dijets from the CMS Collaboration and single jets from the ATLAS Collaboration. As expected, the model fails for peripheral collisions where very few nucleons from Pb participate.

Néstor Armesto

Doğa Can Gülhan

Kinematic bias on centrality selection of jet events in pPb collisions at the LHC