[Paper Review] Hot QCD White Paper

M. Arslandok, S. A. Bass+172 more|arXiv (Cornell University)|Mar 30, 2023

High-Energy Particle Collisions Research4 citations

TL;DR

This Hot QCD White Paper outlines the current state and future directions of quark-gluon plasma (QGP) research at RHIC and the LHC, emphasizing experimental and theoretical advances in understanding QGP properties under extreme temperature and density. It advocates for sustained support of RHIC and LHC upgrades, enhanced data analysis with machine learning, and expanded theory-experiment collaboration to probe transport coefficients, collectivity, and penetrating probes with higher precision.

ABSTRACT

Hot QCD physics studies the nuclear strong force under extreme temperature and densities. Experimentally these conditions are achieved via high-energy collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). In the past decade, a unique and substantial suite of data was collected at RHIC and the LHC, probing hydrodynamics at the nucleon scale, the temperature dependence of the transport properties of quark-gluon plasma, the phase diagram of nuclear matter, the interaction of quarks and gluons at different scales and much more. This document, as part of the 2023 nuclear science long range planning process, was written to review the progress in hot QCD since the 2015 Long Range Plan for Nuclear Science, as well as highlight the realization of previous recommendations, and present opportunities for the next decade, building on the accomplishments and investments made in theoretical developments and the construction of new detectors. Furthermore, this document provides additional context to support the recommendations voted on at the Joint Hot and Cold QCD Town Hall Meeting, which are reported in a separate document.

Motivation & Objective

• To summarize the progress made in hot QCD physics since the 2015 Long Range Plan in Nuclear Science.
• To identify key open questions in the study of quark-gluon plasma (QGP) formation, thermalization, and transport properties.
• To advocate for sustained experimental operations at RHIC and LHC, including detector upgrades like sPHENIX and ALICE 3.
• To strengthen theory-experiment collaboration, especially in developing tools like Bayesian inference and machine learning for data analysis.
• To promote workforce diversity and inclusion to support long-term scientific leadership in nuclear physics.

Proposed method

• Leveraging high-luminosity data from RHIC and LHC heavy-ion collisions to probe QGP properties across a wide range of energy and density.
• Utilizing multi-differential measurements of dilepton and photon pairs (e.g., γγ → e⁺e⁻, μ⁺μ⁻) to study final-state interactions and medium modifications.
• Applying k⊥-factorization and transverse momentum-dependent (TMD) formalisms to model photon polarization and its impact on cos⁴φ modulations.
• Employing advanced computational techniques, including machine learning and Bayesian inference, to extract transport coefficients and viscosity from complex experimental data.
• Integrating results from multiple detectors (STAR, ALICE, CMS, ATLAS) and theoretical frameworks to constrain QGP dynamics.
• Projecting future experimental capabilities, such as ALICE 3’s extended acceptance (|η| < 4) and improved luminosity, to enable high-precision studies of ultra-peripheral collisions.

Experimental results

Research questions

• RQ1 How do transport coefficients like shear viscosity and electrical conductivity evolve with temperature and density in the quark-gluon plasma?
• RQ2 To what extent do final-state interactions wash out the initial cos⁴φ modulation in dilepton production, and what does this reveal about QGP dynamics?
• RQ3 Can future high-precision measurements of e⁺e⁻ and μ⁺μ⁻ pair transverse momentum spectra confirm or rule out final-state broadening effects in QGP?
• RQ4 How do photon polarization effects in ultra-peripheral collisions constrain the validity of k⊥-factorization and TMD treatments in QED processes?
• RQ5 What role will future detectors like sPHENIX and ALICE 3 play in enabling multi-differential, high-statistics analyses of QGP properties?

Key findings

• The sPHENIX detector at RHIC and upgraded STAR are expected to enable significantly higher precision measurements of dilepton transverse momentum spectra and cos⁴φ modulations.
• Future data from STAR will allow multi-differential analysis of cos⁴φ modulation strength versus pair pT, providing stronger constraints on final-state broadening effects.
• The ALICE 3 detector, planned for mid-to-late 2030s, will offer full acceptance for charged and neutral particles over |η| < 4, greatly enhancing the study of complex ultra-peripheral collision final states.
• LHC experiments (ALICE, CMS, ATLAS) will achieve improved measurements of γγ → e⁺e⁻ and γγ → μ⁺μ⁻ processes with higher Lorentz-boost factors, enabling better constraints on photon kinematic distributions.
• Differential measurements of dilepton kinematics in hadronic-overlap events may reveal medium-induced modifications, offering new insights into QGP effects on electromagnetic probes.
• The integration of machine learning and Bayesian inference tools is expected to enhance the extraction of transport coefficients and viscosity from multi-dimensional experimental data.