We present the first study of jet substructure modifications during the bottom-up evolution that describes the early stages of heavy-ion collisions. To this end, we study the bremsstrahlung radiation rate of soft gluons from a hard parton propagating through out-of-equilibrium QCD matter. The gluon spectrum is computed within the improved opacity expansion, which accounts for both multiple soft and single hard momentum exchanges between the hard probe and the medium. The background evolution is obtained from effective kinetic theory simulations that determine the jet quenching parameter, which in turn controls the radiation rate. We compute the radiation rate for initially underoccupied and overoccupied systems, as well as for an expanding system undergoing hydrodynamization, which typically represents the initial stages of heavy-ion collisions. The results for these dynamical backgrounds are compared to static and thermally matched scenarios, allowing to gauge the importance of bulk expansion in the evolution of the jet cascade. Our findings show that the early stages of the bulk matter evolution in heavy-ion collisions leave a sizable imprint on the radiation pattern inside jets. These results establish a basis for incorporating preequilibrium dynamics into realistic descriptions of jet quenching and hard-probe evolution.

CERN

João Barata

France

Austria

Kirill Boguslavski

United States

Florian Lindenbauer

Portugal

Spain

Andrey V. Sadofyev

Jet quenching in out-of-equilibrium QCD matter

We investigate thermal properties of a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> lattice gauge theory in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1</mn><mo>+</mo><mn>1</mn></math> dimensions through real-time simulations. We extract the spectral functions directly coupling to the pseudoscalar and scalar mesons, demonstrating the thermal modifications of these states with increasing temperatures. Introducing the notion of energy-flow operators, we quantify the temporal buildup of correlations in the energy flows across the lattice. We demonstrate that energy-energy correlators fail to factorize to products of energy flows, both in the vacuum and at nonzero temperature, indicating the presence of nontrivial correlations in the quantum states. Our results constitute a first real-time ab initio study of bound-state thermal broadening and finite temperature energy-flow correlations in a gauge theory, providing a benchmark for future studies of hadronic matter under extreme conditions.

David Frenklakh

Swagato Mukherjee

Thermal modifications of mesons and energy-energy correlators from real-time simulations of a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow></math> lattice gauge theory

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.

Russia

Matvey V. Kuzmin

José Guilherme Milhano

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

In this Letter, we study the collinear limit of the energy-energy correlator in single-inclusive jet production in proton-proton and proton-nucleus collisions. We introduce a nonperturbative model that allows us to describe the energy-energy correlator in the entire angular region of the current experiments. Our results for proton-proton collisions show excellent agreement with CMS and ALICE data over a wide range of jet transverse momenta. For proton-nucleus collisions, we include modifications from the nuclear medium, and our predictions align well with the trends observed in recent ALICE measurements.

Zhong-Bo Kang

Xoán Mayo López

Jani Penttala

Energy-Energy Correlator for Jet Production in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mi>p</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mi>A</mi></math> Collisions

Hadronic vacuum polarization (HVP) and light-by-light scattering (HLBL) are crucial for evaluating the Standard Model predictions concerning the muon’s anomalous magnetic moment. However, direct first-principle lattice gauge theory-based calculations of these observables in the timelike region remain challenging. Discrepancies persist between lattice quantum chromodynamics (QCD) calculations in the spacelike region and dispersive approaches relying on experimental data parametrization from the timelike region. Here, we introduce a methodology employing 1+1-dimensional quantum electrodynamics (QED), i.e. the Schwinger Model, to investigate the HVP and HLBL. To that end, we use both tensor network techniques, specifically matrix product states, and classical emulators of digital quantum computers. Demonstrating feasibility in a simplified model, our approach sets the stage for future endeavors leveraging digital quantum computers.

Kazuki Ikeda

Jonathan Raghoonanan

Towards a real-time computation of timelike hadronic vacuum polarization and light-by-light scattering: Schwinger Model tests

Motivated by recent developments in the application of light-ray operators (LROs) in high energy physics, we propose a new strategy to study correlation functions of LROs through real-time quantum simulations. We argue that quantum simulators provide an ideal laboratory to explore the properties LROs in lower-dimensional quantum field theories. This is exemplified in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>1</mn><mo>+</mo><mn>1</mn></mrow></math>-d Schwinger model, employing tensor network methods, focusing on the calculation of energy and charge correlators. Despite some challenges in extracting the necessary correlation functions from the lattice, the methodology used can be extended to real quantum devices.

Probing celestial energy and charge correlations through real-time quantum simulations: Insights from the Schwinger model

We study the production of a quark-antiquark antenna in the presence of a dense and anisotropic QCD medium. We assume the antenna to originate from an unpolarized gluon state, and consider both massless and massive final states. The medium anisotropy is captured by allowing the jet quenching coefficient to take different magnitudes in orthogonal directions with respect to the jet axis. We find that the final particle distribution is sensitive to the medium anisotropy, and more importantly, that this effect couples directly to the helicity/spin of the final states. We propose to look into these effects by performing a Fourier decomposition of the particle distribution inside the jet. In our medium model, we find that the spin independent terms contribute to the even harmonics of the cosine series. The helicity/spin dependence enters only through the sine Fourier series. We further explore the spin dependence by examining the degree of polarization of the final states in different directions. Our results indicate that the anisotropies present in the QCD matter produced in heavy ion collisions can be probed by studying azimuthal and spin observables inside jets.

Carlos Salgado

João Silva

Gluon to <math> <mi>q</mi> <mover> <mi>q</mi> <mo>¯</mo> </mover> </math> $$ \textrm{q}\overline{\textrm{q}} $$ antenna in anisotropic QCD matter: spin-polarized and azimuthal jet observables

Inspired by the recent considerations of parton momentum broadening in the glasma phase, we study the medium-induced soft gluon radiation of jet partons at early times in heavy-ion collisions. The glasma state is assumed to be comprised of independent color domains with homogenous longitudinal fields that vary event by event, and we further complete this model with an event-averaging procedure accounting for the finite correlation length. Using this description, we evaluate the rate of medium-induced radiation from an energetic parton at midrapidity in the glasma phase. We mainly focus on SU(2) color fields for simplicity, also referring to the U(1) case and comparing with the Baier, Dokshitzer, Mueller, Peigné, Schiff, and Zakharov rate to gain further insight. Our results show that there is an intricate interplay of the synchrotron-like radiation in a single color domain with the destructive interference between different color domains, after the medium averaging is performed. Thus, we find that the emission rate is sensitive to the matter structure, decreasing for a glasma state populated by smaller color domains—i.e., for a glasma with a larger characteristic saturation scale. Our approach can be applied to more realistic backgrounds, and it sets the stage for the modeling of jet evolution in the early stages of heavy-ion collisions.

Sigtryggur Hauksson

Jet quenching in the glasma phase: Medium-induced radiation

We investigate the collinear limit of the energy-energy correlator (EEC) in a heavy-ion context. First, we revisit the leading-logarithmic (LL) resummation of this observable in vacuum following a $\textit{diagrammatic}$ approach. We argue that this route allows to naturally incorporate medium-induced effects into the all-orders structure systematically. As an example, we show how the phase-space constraints imposed by the static medium on vacuum-like emissions can be incorporated into the LL result by modifying the anomalous dimensions. On the fixed-order side, we calculate the $$ \mathcal{O} $$ ($\textit{α}$ ) expansion of the in-medium EEC for a $\textit{γ}$ → $$ q\overline{q} $$ splitting with arbitrary kinematics including, for the first time, subleading colour corrections. When comparing this result to previously used approximations in the literature, we find up to $$ \mathcal{O} $$ (1) deviations in the regime of interest for jet quenching signatures. Energy loss effects are also quantified and further suppress the EEC at large angles. These semi-analytic studies are complemented with a phenomenological study using the jet quenching Monte Carlo JetMed. Finally, we argue that the imprint of medium-induced effects in energy-energy correlators can be enhanced by using an alternative definition that takes as input Lund primary declusterings instead of particles.

Paul Caucal

Switzerland

Alba Soto-Ontoso

Robert Szafron

Advancing the understanding of energy-energy correlators in heavy-ion collisions

We study the modifications to the fragmentation pattern of partons into charged particles in the presence of a hot and dense quark gluon plasma. To this end, we analyze the perturbative renormalization group equations of the track functions, which describe the energy fraction carried by charged hadrons. Focusing on pure Yang-Mills theory, we compute the lowest-order moments of the medium-modified track functions, which are found to be sensitive to the reduced phase space for emissions in the medium and to energy loss. We use the extracted moments to calculate the energy energy correlator (EEC) on tracks in the collinear limit. The EEC on medium-evolved tracks does not differ qualitatively from the EEC on vacuum tracks despite being sensitive to the color decoherence transition and suppressing the distribution due to quenching, as seen in other jet observables.