The recent light-ion collision programme at RHIC and the LHC provides a unique opportunity to investigate the onset of quark-gluon plasma formation and parton energy loss in small systems. A quantitative interpretation of emerging jet quenching measurements requires precise control over cold nuclear matter (CNM) effects, which modify hard-process cross sections independently of any hot-medium dynamics. In this work, we present a comprehensive set of perturbative QCD baseline calculations for nuclear modification factors ( $$R_\textrm{AA}$$ ) in proton-oxygen (pO), oxygen-oxygen (OO) and neon-neon (NeNe) collisions at LHC energies. The study includes charged hadron, neutral pion, prompt photon, and electroweak-boson production computed at next-to-leading order using a broad set of recent nuclear parton distribution functions (nPDFs). We demonstrate that CNM effects alone can induce sizeable suppressions in light-ion systems, with large associated nPDF uncertainties that currently limit the quantitative extraction of parton energy loss. To address this limitation, we explore a range of multi-cross-section ratios in which CNM effects and their uncertainties largely cancel. In particular, ratios of neutral pion $$R_\textrm{OO}$$ to prompt photon $$R_\textrm{OO}$$ or charged hadron $$R_\textrm{OO}$$ to $$R_\textrm{pO} ^2$$ provide theoretically robust observables with substantially reduced nPDF uncertainties, thereby enhancing sensitivity to possible energy-loss signatures.

Switzerland

Florian Jonas

United States

Constantin Loizides

Germany

Aleksas Mazeliauskas

Petja Paakkinen

Nicolas Strangmann

A compendium of cold-nuclear matter baseline predictions in light-ion collisions

We apply weakly coupled QCD kinetic theory to investigate the thermalization of high-momentum on-shell partons (minijets) in a Quark-Gluon Plasma (QGP). Our approach incorporates isotropic hard thermal loop screening to model soft quark and gluon exchanges, allowing us to verify consistency with established analytic results of jet transport coefficients. We perform kinetic simulations of minijets propagating through a thermal gluon plasma, incorporating both collinear radiation and elastic scatterings. The resulting evolution is compared to predictions from jet transport coefficients, including the longitudinal and transverse jet-quenching parameters $$ \hat{q} $$ , energy loss, and the drag coefficient. We find that standard definitions of jet transport coefficients neglect the contributions from recoiling medium particles. Including these contributions restores consistency with the kinetic evolution. Finally, we show that the minijet thermalization time scales remarkably well with $$ \hat{q} $$ and we produce a phenomenological estimate of the minijet quenching time in heavy-ion collisions.

France

Austria

Kirill Boguslavski

Florian Lindenbauer

Adam Takacs

Fabian Zhou

Minijet thermalization and jet transport coefficients in QCD kinetic theory

I present predictions for inclusive charged hadron spectra in minimum-bias proton-proton and oxygen-oxygen collisions at a centre-of-mass energy of <math altimg='si4.svg'><mrow><msqrt><msub><mi>s</mi><mtext>NN</mtext></msub></msqrt><mo linebreak='goodbreak'>=</mo><mn>5.36</mn><mspace width='0.16em' /><mtext>TeV</mtext></mrow></math>, assuming no final-state interactions. Using next-to-leading order perturbative QCD matrix elements, along with state-of-the-art (nuclear) parton distribution and fragmentation functions, I establish a baseline for the nuclear modification factor <math altimg='si5.svg'><msubsup><mi>R</mi><mtext>AA</mtext><mi>h</mi></msubsup></math> in oxygen-oxygen collisions in the absence of quenching. Theoretical uncertainties in this baseline are found to be substantial for transverse momenta below 20&#8201;GeV. In the intermediate range <math altimg='si6.svg'><mrow><mn>20</mn><mspace width='0.16em' /><mtext>GeV</mtext><mo>&#8818;</mo><msubsup><mi>p</mi><mi>T</mi><mi>h</mi></msubsup><mo>&#8818;</mo><mn>70</mn><mspace width='0.16em' /><mtext>GeV</mtext></mrow></math>, these uncertainties are significantly reduced to approximately 5%. At higher momenta ( <math altimg='si7.svg'><mrow><msubsup><mi>p</mi><mi>T</mi><mi>h</mi></msubsup><mo>&#8819;</mo><mn>70</mn><mspace width='0.16em' /><mtext>GeV</mtext></mrow></math>), however, predictions exhibit a marked spread due to differences between fragmentation functions, reflecting varying assumptions about isospin symmetry. Finally, I show that considering neon-neon collisions in the initial state or neutral pions in the final state does not appreciably change the nuclear modification factor.

Energy loss baseline for light hadrons in oxygen-oxygen collisions at <math altimg='si4.svg'><mrow><msqrt><msub><mi>s</mi><mtext>NN</mtext></msub></msqrt><mo linebreak='goodbreak'>=</mo><mn>5.36</mn><mspace width='0.16em' /><mtext>TeV</mtext></mrow></math>

<p>We present a detailed numerical and analytical study of the out-of-equilibrium dynamics of Model G, the dynamical universality class relevant to the chiral phase transition. We perform numerical 3D stochastic (Langevin) simulations of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>O</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></mrow></math> critical point for large lattices in the chiral limit. We quench the system from the high-temperature unbroken phase to the broken phase and study the nonequilibrium dynamics of pion fields. Strikingly, the nonequilibrium evolution of the two-point functions exhibits a regime of growth, a parametrically large enhancement, and a subsequent slow relaxation to equilibrium. We analyze our numerical results using dynamic critical scaling and mean-field theory. The growth of the two-point functions is determined by the nonlinear dynamics of an ideal non-Abelian superfluid, which is a limit of Model G that reflects the broken chiral symmetry. We also relate the nonequilibrium two-point functions to a long-lived parametric enhancement of soft pion yields relative to thermal equilibrium following a quench.</p>

Adrien Florio

Italy

Eduardo Grossi

Slovenia

Alexander Soloviev

Derek Teaney

Quenching through the QCD chiral phase transition

<p>We discuss a universal nonequilibrium enhancement of long-wavelength Goldstone bosons induced by quenches to the broken phase in Model G—the dynamical universality class of an <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>O</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></math> antiferromagnet and the chiral phase transition in QCD. Scaling arguments for the coarsening dynamics describing the formation of the chiral condensate predict a parametric enhancement in the infrared spectra of Goldstone bosons, a prediction confirmed by stochastic simulations of the transition. The details of the enhancement are determined by the nonlinear dynamics of a superfluid effective theory, which is a limit of Model G reflecting the broken <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>O</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></math> symmetry. Our results translate to a parametric enhancement of low-momentum pions in heavy-ion collisions at the LHC, which are underpredicted in current hydrodynamic models without critical dynamics.</p>

Supercooled Goldstone Bosons at the QCD Chiral Phase Transition

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.

Jannis Gebhard

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.

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

Gian Michele Innocenti

Sohyun Park

Wilke van der Schee

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

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

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

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