In this work, we perform computations of inclusive jet, and semi-inclusive jet-hadron cross sections for minimum bias oxygen-oxygen collisions at RHIC and LHC collision energies. We compute the no-quenching baseline for the jet nuclear modification factor $\textit{R}$ and jet-, and hadron-triggered semi-inclusive nuclear modification factors $\textit{I}$ . We do this with state-of-the-art nuclear parton distribution functions (nPDFs), next-to-leading-order matrix elements, parton shower, and hadronization. We observe deviations from unity due to cold-nuclear matter effects, even without quenching. We demonstrate that the parton distribution uncertainties constitute a significant obstacle in detecting energy loss in small collision systems. Hadron-triggered observables are particularly sensitive to uncertainties due to correlations between the trigger and analyzed particles. For jet-triggered $\textit{I}$ , there exists a kinematic window in which nPDF and scale uncertainties cancel dramatically while showing little sensitivity to parton shower and hadronization models, addressing a major limiting factor for energy loss discovery in small systems.

Germany

Jannis Gebhard

Aleksas Mazeliauskas

Adam Takacs

No-quenching baseline for energy loss signals in oxygen-oxygen collisions

We study the energy deposition and thermalisation of high-momentum on-shell partons (minijets) travelling through a non-equilibrium Quark-Gluon Plasma using QCD kinetic theory. For thermal backgrounds, we show that the parton energy first flows to the soft sector by collinear cascade and then isotropises via elastic scatterings. In contrast, the momentum deposition from a minijet reaches the equilibrium distribution directly. For expanding non-equilibrium QGP, we study the time for a minijet perturbation to lose memory of its initial conditions, namely, the hydrodynamisation time. We show that the minijet evolution scales well with the relaxation time τ  R  ∝ η/s/T(τ), where T(τ) is the effective temperature and η/s is the viscosity over entropy ratio.

Fabian Zhou

United Kingdom

Jasmine Brewer

Minijet quenching in non-equilibrium quark-gluon plasma

We show that the same QCD formalism that accounts for the suppression of high-<math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></mrow></math> hadron and jet spectra in heavy-ion collisions predicts medium-enhanced production of <math><mi>c</mi><mover><mi>c</mi><mo>¯</mo></mover></math> pairs in jets. We demonstrate that this phenomenon, which cannot be accessed by traditional jet-quenching observables, can be directly observed using <math><mrow><msup><mrow><mi>D</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mover><mrow><mi>D</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mn>0</mn></mrow></msup></mrow></math>-tagged jets in nuclear collisions.

Netherlands

CERN

Maximilian Attems

United States

Gian Michele Innocenti

Sohyun Park

Wilke van der Schee

France

Gregory Soyez

Urs Achim Wiedemann

Medium-Enhanced <math><mi>c</mi><mover><mi>c</mi><mo>¯</mo></mover></math> Radiation

This paper is a write-up of the ideas that were presented, developed and discussed at the fourth International Workshop on QCD Challenges from pp to AA, which took place in February 2023 in Padua, Italy. The goal of the workshop was to focus on some of the open questions in the field of high-energy heavy-ion physics and to stimulate the formulation of concrete suggestions for making progresses on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

Australia

Javira Altmann

Carlota Andres

Anton Andronic

Italy

Federico Antinori

Pietro Antonioli

Andrea Beraudo

Eugenio Berti

Livio Bianchi

Thomas Boettcher

Lorenzo Capriotti

Sweden

Peter Christiansen

Czechia

Jesus Contreras Nuño

Leticia Cunqueiro Mendez

Cesar Silva

Andrea Dainese

Hans Dembinski

Brazil

Austria

David Dobrigkeit Chinellato

Andrea Dubla

Mattia Faggin

Chris Flett

Vincenzo Greco

Ilia Grishmanovskii

Jack Holguin

Finland

Yuuka Kanakubo

Dong Kim

Ramona Lea

South Korea

Su Lee

Saverio Mariani

Poland

Adam Matyja

Vincenzo Minissale

Andreas Morsch

Lucia Oliva

Luca Orusa

Petja Paakkinen

Spain

Daniel Pablos

Mexico

Guy Paić

Tanguy Pierog

Salvatore Plumari

Francesco Prino

Andrea Rossi

Lorenzo Sestini

Peter Skands

Olga Soloveva

Francesca Soramel

Alba Soto Ontoso

Martin Spousta

Andre Stahl Leiton

Jiayin Sun

Norway

Stefano Trogolo

Rosario Turrisi

Marta Verweij

Vytautas Vislavicius

Jing Wang

Klaus Werner

Valentina Zaccolo

China

Mingyu Zhang

Jianhui Zhu

Davide Zuliani

QCD challenges from pp to AA collisions: 4th edition

We use QCD kinetic theory to compute photon production in the chemically equilibrating Quark-Gluon Plasma created in the early stages of high-energy heavy-ion collisions. We do a detailed comparison of pre-equilibrium photon rates to the thermal photon production. We show that the photon spectrum radiated from a hydrodynamic attractor evolution satisfies a simple scaling form in terms of the specific shear viscosity η/s and entropy density dS/dζ ∼ (Tτ 1/3)3/2 ∞. We confirm the analytical predictions with numerical kinetic theory simulations. We use the extracted scaling function to compute the pre-equilibrium photon contribution in   <math> <msqrt> <msub> <mi>s</mi> <mi>NN</mi> </msub> </msqrt> </math>  $$ \sqrt{s_{NN}} $$  = 2.76 TeV 0–20% PbPb collisions. We demonstrate that our matching procedure allows for a smooth switching from pre-equilibrium kinetic to thermal hydrodynamic photon production. Finally, our publicly available implementation can be straightforwardly added to existing heavy ion models.

Oscar Garcia-Montero

Philip Plaschke

Sören Schlichting

Pre-equilibrium photons from the early stages of heavy-ion collisions

We study how isotropic and homogeneous far-from-equilibrium quantum systems relax to nonthermal attractors, which are of interest for cold atoms and nuclear collisions. We demonstrate that a first-order ordinary differential equation governs the self-similar approach to nonthermal attractors, i.e., the prescaling. We also show that certain natural scaling-breaking terms induce logarithmically slow corrections that prevent the scaling exponents from reaching the constant values during the system’s lifetime. We propose that, analogously to hydrodynamic attractors, the appropriate mathematical structure to describe such dynamics is the transseries. We verify our analytic predictions with state-of-the-art 2PI simulations of the large-<math><mi>N</mi></math> vector model and QCD kinetic theory.

Belgium

Michal P. Heller

Thimo Preis

Prescaling Relaxation to Nonthermal Attractors

The equation of state of neutron-star cores can be constrained by requiring a consistent connection to the perturbative Quantum Chromodynamics (QCD) calculations at high densities. The constraining power of the QCD input depends on uncertainties from missing higher-order terms, the choice of the unphysical renormalization scale, and the reference density where QCD calculations are performed. Within a Bayesian approach, we discuss the convergence of the perturbative QCD series, quantify its uncertainties at high densities, and present a framework to systematically propagate the uncertainties down to neutron-star densities. We find that the effect of the QCD input on the neutron-star inference is insensitive to the various unphysical choices made in the uncertainty estimation.

Tyler Gorda

Oleg Komoltsev

Aleksi Kurkela

Bayesian uncertainty quantification of perturbative QCD input to the neutron-star equation of state

The formalism of Baier-Dokshitzer-Mueller-Peigné-Schiff and Zakharov determines the modifications of parton splittings in the QCD plasma that arise from medium-induced gluon radiation. Here, we study medium-modifications of the gluon splitting into a quark-anti-quark pair in this BDMPS-Z formalism. We derive a compact path-integral formulation that resums effects from an arbitrary number of interactions with the medium to leading order in the   <math> <mn>1</mn> <mo>/</mo> <msubsup> <mi>N</mi> <mi>c</mi> <mn>2</mn> </msubsup> </math>  $$ 1/{N}_c^2 $$  expansion. Analyses in the N = 1 opacity and the saddle point approximations reveal two phenomena: a medium-induced momentum broadening of the relative quark-anti-quark pair momentum that increases the invariant mass of quark-anti-quark pairs, and a medium-enhanced production of such pairs. We note that both effects are numerically sizeable if the average momentum transfer from the medium is comparable to the quark mass. In ultra-relativistic heavy-ion collisions, this condition is satisfied for charm quarks. We therefore focus our numerical analysis on the medium modification of   <math> <mi>g</mi> <mo>→</mo> <mi>c</mi> <mover> <mi>c</mi> <mo>¯</mo> </mover> </math>  $$ g\to c\overline{c} $$ , although our derivation applies equally well to   <math> <mi>g</mi> <mo>→</mo> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ g\to b\overline{b} $$  and to gluons splitting into light-flavoured quark-anti-quark pairs.

Gian Innocenti

Wilke Schee

Urs Wiedemann

The medium-modified   <math> <mi>g</mi> <mo>→</mo> <mi>c</mi> <mover> <mi>c</mi> <mo>¯</mo> </mover> </math>  $$ g\to c\overline{c} $$  splitting function in the BDMPS-Z formalism

We use the Hamiltonian formulation of kinetic theory to perform a stability analysis of nonthermal fixed points in a non-Abelian plasma. We construct a perturbative expansion of the Fokker-Planck collision kernel in an adiabatic approximation and show that the (next-to-)leading order solutions reproduce the known nonthermal fixed point scaling exponents. Working at next-to-leading order, we derive the stability equations for scaling exponents and find the relaxation rate to the nonthermal fixed point. This approach provides the basis for an understanding of the prescaling phenomena observed in QCD kinetic theory and nonrelativistic Bose gas systems.

Aleksandr N. Mikheev

Jürgen Berges

Stability analysis of nonthermal fixed points in longitudinally expanding kinetic theory

By including the recently introduced thermal shear term that contributes to the spin-polarization vector at local equilibrium, we determine longitudinal polarization of <math><mi>Λ</mi></math> hyperons emitted from a hot and rotating hadronic medium using the thermal model with single freeze-out. In our analysis, we consider the RHIC top energies and use the model parameters which were determined in the earlier analyses of particle spectra and elliptic flow. We confirm that, unlike the previous calculations done by using only the thermal vorticity, the thermal shear term alone leads to the correct sign of the quadrupole structure of the longitudinal component of the polarization three-vector measured in experiments. However, we find almost complete cancellation between thermal shear and vorticity terms, which eventually leads to disagreement with the data. To clarify the role played by velocity and temperature gradient terms, we present a systematic analysis of different contributions to the longitudinal polarization. Finally, replacing the mass of <math><mi>Λ</mi></math> hyperon with its constituent strange quark mass in the polarization formula we achieve agreement with the data.