Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams (1812.06772v2)

Abstract: The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated.

• The paper proposes a comprehensive roadmap for precision studies of QGP properties and parton dynamics using HL-LHC and HE-LHC upgrades.
• It details the use of enhanced detector technologies and varied collision systems, including Pb–Pb, p–Pb, Ar, and Kr, to achieve improved measurements.
• The paper demonstrates that refined observables like nuclear modification factors and elliptic flow can critically constrain theoretical QCD models.

An Analytical Perspective on Future High-Density QCD Studies at the LHC

The report titled "Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams" delineates a comprehensive research trajectory for exploiting the capabilities of the High-Luminosity LHC (HL-LHC) and proposes potential investigations at the High-Energy LHC (HE-LHC). The focal point of the discourse is the elucidation of high-density Quantum Chromodynamics (QCD) properties, leveraging the collision energies and luminosities anticipated in future LHC operations. This summary delineates the report's strategic framework, experimental rigors, and the scope for theoretical advancements within the field of high-density QCD research.

The report elaborates on four cardinal scientific goals, which together serve as the backbone of the proposed research portfolio:

1. Characterization of Quark-Gluon Plasma (QGP) Properties: The report underscores the imperative to attain an unprecedented level of precision in characterizing the macroscopic properties of the QGP, a state reached by deconfined quarks and gluons under extreme physical conditions akin to the early universe. Such characterization is anticipated through extensive experimental data acquiring from particularly Pb–Pb and p–Pb collisions at the LHC.
2. Investigation of Microscale Parton Dynamics: A critical exploration is proposed into the microscopic mechanisms underlying QGP properties, with an emphasis on the dynamics of constituent partons. This direction intends to distill insights into quasi-particle behaviors and transport phenomena influencing the QGP, supplemented by deploying upgraded detectors extending precision measurements.
3. Developing a Unified QCD Picture across System Sizes: The initiative aims to cultivate a cohesive understanding of particle production and collective QCD dynamics from small (pp) to large-scale systems (nucleus–nucleus collisions). The enhanced LHC capabilities are projected to facilitate nuanced exploration into these system-dependent QCD phenomena.
4. Probing Nuclear Parton Densities and Parton Saturation: The research identifies a deep analysis into parton densities within nuclei across a broad kinematic spectrum. This includes investigations into phenomena such as parton saturation, a significant aspect within high-density QCD dynamics potentially revealed through proposed high-luminosity light-ion collisions.

A pivotal aspect of the report is the discussion on expected experimental implementations and performance utilizing future LHC upgrades. The integration of upgraded tracking and timing detectors, enhanced collision luminosities, and more sophisticated data acquisition frameworks are posited to catalyze breakthroughs in both soft and hard QCD domains. The advent of heavy-ion collisions with lighter nuclei, such as Ar or Kr, is discussed as an augmentation for reaching higher statistics and refined observables subject to lower systematic uncertainties compared to Pb–Pb collisions.

Numerical projections and model simulations presented in the report illustrate a significant reduction of uncertainties in key observables, such as the nuclear modification factor and elliptic flow coefficients, over current capacities. These enhancements promise to resolve existing ambiguities, leading towards a more granular unpacking of phenomena like color screening and recombination processes within the QGP milieu.

Notably, discussions within the report anticipate substantial theoretical progress with enhanced data potentially feeding new or improved frameworks for QCD interactions. Experimental results from the LHC HL and HE configurations could enable greater constraints on theoretical models, reshaping our understanding of QCD at strong-coupling scenarios within the QGP.

In sum, while grounding in robust experimental methodologies and expected technological advances, this report persuasively frames a strategic roadmap for harnessing LHC operations to advance the frontier of high-density QCD research. Future outcomes from these initiatives could critically inform the broader theoretical landscape, offering rigorous tests and benchmarks for the quantum chromodynamic descriptions of extreme matter conditions.