In this work, we study for the first time jet-medium interactions in heavy-ion collisions with introduction of saturation and Sudakov effects with parameters tuned for upcoming forward calorimeter acceptances in experiments, in particular the ALICE FoCal detector. We focus on $$\gamma +$$ jet correlations by taking into account in-medium parton evolution using the BDIM equation that describes jet interactions with the quark-gluon plasma (QGP) combined with vacuum-like emissions (VLE). We systematically introduce the early time gluon saturation dynamics through the small- Improved Transverse Momentum Dependent factorization (ITMD). For our purpose, we use Monte Carlo programs and to generate hard events and in-medium parton evolution, respectively. We present results of azimuthal correlations and nuclear modification ratios to gauge the impact of the gluon saturation effects at early time for the in-medium jet energy loss.

Poland

Czechia

Souvik Adhya

Krzysztof Kutak

Wiesław Płaczek

Martin Rohrmoser

Norway

Konrad Tywoniuk

Predictions for photon-jet correlations at forward rapidities in heavy-ion collisions

Azimuthal anisotropies of high-<math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></mrow></math> particles produced in heavy-ion collisions are understood as an effect of a geometrical selection bias. Particles oriented in the direction in which the QCD medium formed in these collisions is shorter suffer less energy loss, and thus, are over-represented in the final ensemble compared to those oriented in the direction in which the medium is longer. In this work we present the first semianalytical predictions, including propagation through a realistic, hydrodynamical background, of the elliptic azimuthal anisotropy for jets, obtaining a quantitative agreement with available experimental data as a function of the jet <math><msub><mi>p</mi><mi>T</mi></msub></math>, its cone size <math><mi>R</mi></math>, and the collisions centrality. Jets are multipartonic, extended objects and their energy loss is sensitive to substructure fluctuations. This sensitivity is determined by the physics of color coherence that relates to the ability of the medium to resolve those partonic fluctuations. Specifically, color dipoles with an angular separation smaller than a critical angle, <math><msub><mi>θ</mi><mi>c</mi></msub></math>, are not resolved by the medium and they effectively act as a coherent source of energy loss. We find that elliptic jet azimuthal anisotropy has a specially strong dependence on coherence physics due to the marked length dependence of <math><msub><mi>θ</mi><mi>c</mi></msub></math>. By combining our predictions for the collision systems and center-of-mass energies studied at RHIC and the LHC, covering a wide range of typical values of <math><msub><mi>θ</mi><mi>c</mi></msub></math>, we show that the relative size of elliptic jet azimuthal anisotropies for jets with different cone sizes <math><mi>R</mi></math> follows a universal trend that indicates a transition from a coherent regime of jet quenching to a decoherent regime. These results suggest a way forward to reveal the role played by the physics of jet color decoherence in probing deconfined QCD matter.

United States

Yacine Mehtar-Tani

Spain

Daniel Pablos

Jet suppression and azimuthal anisotropy from RHIC to LHC

We study jet fragmentation via final-state parton splittings in the medium. These processes are usually calculated theoretically by invoking the large-N  c  limit. In this paper we perform the first computation of a 1 → 2 parton splitting in a thermal medium at finite numbers of colors N  c , for arbitrary momentum-sharing fraction z and with full transverse dynamics. We show how the problem can be transformed into a system of coupled Schrödinger equations, that we solve numerically. The novel numerical results are used to estimate the accuracy of several widely used approximations. We check the error introduced while going from finite N  c  (i.e. N  c  = 3) to the large-N  c  limit, which we find to be small. For unbalanced splittings, e.g. when z → 0, only one of the partons is affected by transverse momentum exchanges with the medium. The emission process then separates into a term responsible for the 1 → 2 splitting and the subsequent independent broadening of the daughter partons. This is also referred to as the factorizable term. For finite z, further contributions arise that are responsible for the coherent color dynamics of the two-parton system, and these are referred to as non-factorizable terms. These were argued to be small for soft (unbalanced) splittings and for large media. In this work we therefore determine the accuracy of keeping only the factorizable term of the large-N  c  solution. We find that the error is insignificant at a small splitting fraction z ~ 0, but can be sizable in a more balanced splitting with z ~ 0.5. Finally, we also examine the eikonal approximation, which amounts to approximating the partons’ paths through the medium as straight lines. We find that it is associated with a substantial error for the parameter values we explored in this work.

Johannes Isaksen

Precise description of medium-induced emissions

In this work, we explore the features of gluonic cascades in static and Bjorken expanding media by numerically solving the full BDIM evolution equations in longitudinal momentum fraction x and transverse momentum  $${{\varvec{k}}}$$  <math> <mrow> <mi>k</mi> </mrow> </math>   using the Monte Carlo event generator MINCAS. Confirming the scaling of the energy spectra at low-x, discovered in earlier works, we use this insight to compare the amount of broadening in static and expanding media. We compare angular distributions for the in-cone radiation for different medium profiles with the effective scaling laws and conclude that the out-of-cone energy loss proceeds via the radiative break-up of hard fragments, followed by an angular broadening of soft fragments. While the dilution of the medium due to expansion significantly affects the broadening of the leading fragments, we provide evidence that in the low-x regime, which is responsible for most of the gluon multiplicity in the cascade, the angular distributions are very similar when comparing different medium profiles at an equivalent, effective in-medium path length. This is mainly due to the fact that in this regime, the broadening is dominated by multiple splittings. Finally, we discuss the impact of our results on the phenomenological description of the out-of-cone radiation and jet quenching.

Transverse momentum broadening of medium-induced cascades in expanding media

We revisit the picture of jets propagating in the quark-gluon plasma. In addition to vacuum radiation, partons scatter on the medium constituents resulting in induced emissions. Analytical approaches to including these interactions have traditionally dealt separately with multiple, soft, or rare, hard scatterings. A full description has so far only been available using numerical methods. We achieve full analytical control of the relevant scales and map out the dominant physical processes in the full phase space. To this aim, we extend existing expansion schemes for the medium-induced spectrum to the Bethe-Heitler regime. This covers the whole phase space from early to late times, and from hard splittings to emissions below the thermal scale. Based on the separation of scales, a space-time picture naturally emerges: at early times, induced emissions start to build from rare scatterings with the medium. At a later stage, induced emissions due to multiple soft scatterings result in a turbulent cascade that rapidly degrades energy down to, and including, the Bethe-Heitler regime. We quantify the impact of such an improved picture, compared to the current state-of-the-art factorization that includes only soft scatterings, by both analytical and numerical methods for the medium-induced energy distribution function. Our work serves to improve our understanding of jet quenching from small to large systems and for future upgrades of Monte Carlo generators.

Adam Takacs

A unified picture of medium-induced radiation

Going beyond the simplified gluonic cascades, we introduce both gluon and quark degrees of freedom for partonic cascades inside the medium. We then solve the set of coupled evolution equations numerically with splitting kernels calculated for static, exponential, and Bjorken expanding media to arrive at medium-modified parton spectra for quark and gluon initiated jets. Using these, we calculate the inclusive jet  $$R_\mathrm {AA}$$  <math> <msub> <mi>R</mi> <mi>AA</mi> </msub> </math>   where the phenomenologically driven combinations of quark and gluon jet fractions are included. Then, the rapidity dependence of the jet  $$R_\mathrm {AA}$$  <math> <msub> <mi>R</mi> <mi>AA</mi> </msub> </math>   is examined. We also study the path-length dependence of jet quenching for different types of expanding media by calculating the jet  $$v_2$$  <math> <msub> <mi>v</mi> <mn>2</mn> </msub> </math>  . Additionally, we study the sensitivity of observables on effects from nuclear modification of parton distribution functions, vacuum-like emissions in the plasma, and the time of the onset of the quenching. All calculations are compared with recently measured data.

Carlos Salgado

Martin Spousta

Multi-partonic medium induced cascades in expanding media

Deep learning techniques have the power to identify the degree of modification of high energy jets traversing deconfined QCD matter on a jet-by-jet basis. Such knowledge allows us to study jets based on their initial, rather than final, energy. We show how this new technique provides unique access to the genuine configuration profile of jets over the transverse plane of the nuclear collision, both with respect to their production point and their orientation. By effectively removing the selection biases induced by final-state interactions, one can analyze the potential azimuthal anisotropies of jet production associated to initial-state effects. Additionally, we demonstrate the capability of our new method to locate with precision the production point of a dijet pair in the nuclear overlap region, in what constitutes an important step forward toward the long term quest of using jets as tomographic probes of the quark-gluon plasma.

Yi-Lun Du

Italy

Jet Tomography in Heavy-Ion Collisions with Deep Learning

The strong suppression of high-<math><msub><mi>p</mi><mi>T</mi></msub></math> jets in heavy-ion collisions is a result of elastic and inelastic energy loss suffered by the jet multiprong collection of color charges that are resolved by medium interactions. Hence, quenching effects depend on the fluctuations of the jet substructure that are probed by the cone-size dependence of the spectrum. In this Letter, we present the first complete, analytic calculation of the inclusive <math><mi>R</mi></math>-dependent jet spectrum in PbPb collisions at LHC energies, including resummation of energy loss effects from hard, vacuumlike emissions occurring in the medium and modeling of soft energy flow and recovery at the jet cone. Both the geometry of the collision and the local medium properties, such as the temperature and fluid velocity, are given by a hydrodynamic evolution of the medium, leaving only the coupling constant in the medium as a free parameter. The calculation yields a good description of the centrality and <math><msub><mi>p</mi><mi>T</mi></msub></math> dependence of jet suppression for <math><mi>R</mi><mo>=</mo><mn>0.4</mn></math> together with a mild cone-size dependence, which is in agreement with recent experimental results. Gauging the theoretical uncertainties, we find that the largest sensitivity resides in the leading logarithmic approximation of the phase space of resolved splittings, which can be improved systematically, while nonperturbative modeling of the soft-gluon sector is of relatively minor importance up to large cone sizes.

Cone-Size Dependence of Jet Suppression in Heavy-Ion Collisions

We study hard 1 → 2 final-state parton splittings in the medium, and put special emphasis on calculating the Wilson line correlators that appear in these calculations. As partons go through the medium their color continuously rotates, an effect that is encapsulated in a Wilson line along their trajectory. When calculating observables, one typically has to calculate traces of two or more medium-averaged Wilson lines. These are usually dealt with in the literature by invoking the large-N  c  limit, but exact calculations have been lacking in many cases. In our work, we show how correlators of multiple Wilson lines appear, and develop a method to calculate them numerically to all orders in N  c . Initially, we focus on the trace of four Wilson lines, which we develop a differential equation for. We will then generalize this calculation to a product of an arbitrary number of Wilson lines, and show how to do the exact calculation numerically, and even analytically in the large-N  c  limit. Color sub-leading corrections, that are suppressed with a factor   <math> <msubsup> <mi>N</mi> <mi>c</mi> <mrow> <mo>−</mo> <mn>2</mn> </mrow> </msubsup> </math>  $$ {N}_c^{-2} $$  relative to the leading scaling, are calculated explicitly for the four-point correlator and we discuss how to extend this method to the general case. These results are relevant for high-p  T  jet processes and initial stage physics at the LHC.

Wilson line correlators beyond the large-N  c

The steeply falling jet spectrum induces a bias on the medium modifications of jet observables in heavy-ion collisions. To explore this effect, we develop a novel analytic framework to study the quenched jet spectrum and its cumulative. We include many energy-loss-related effects, such as soft and hard medium induced emissions, broadening, elastic scattering, jet fragmentation, cone size dependence, and coherence effects. We show that different observables, based on the jet spectrum, are connected, e.g., the nuclear modification, spectrum shift, and the quantile procedure. We present the first predictions for the nuclear modification factor and the quantile procedure with cone size dependence. As a concrete example, we compare dijet and boson+jet events to unfold the spectrum bias effects, and improve quark-, and gluon-jet classification using arguments based on the cumulative. Besides pointing out its flexibility, finally, we apply our framework to other energy loss models such as the hybrid weak/strong-coupling approach.