A white paper recently published in the journal Nuclear Physics A presented an overview of future opportunities in hot quantum chromodynamics (QCD) and cold QCD.
Study: Charting Future Frontiers in Quantum Chromodynamics. Image Credit: ra2studio/Shutterstock.com
QCD Theory
In this paper, the authors delve into the promising avenues of research within both hot and cold QCD in the near future. However, applying QCD theory to understand the intricacies of both hot and cold QCD physics poses a considerable challenge due to the nonperturbative nature of strong interaction phenomena. Consequently, researchers must resort to approximations to reconcile experimental observations, either through the construction of a robust numerical framework or via QCD factorization employing suitable power counting methodologies.
At the heart of this endeavor lies the strong force, a fundamental force in the natural world governing the interactions between gluons and quarks. These interactions are responsible for the vast majority of visible mass in the universe. QCD, a non-Abelian gauge theory, elegantly describes the mathematical framework behind the strong force.
Despite its successes, QCD encounters hurdles when it comes to explaining the construction of nuclei and nucleons from quarks, as well as elucidating the behavior of gluons and quarks across all energy scales. These challenges, compounded by the pressing need to comprehend visible matter at its most fundamental level, have thrust the study of QCD into the forefront of nuclear science research.
Opportunities in Hot QCD
According to the researchers, many questions of fundamental importance are being addressed by hot QCD research, including the determination of nuclear matter's phase structure, understanding the mechanisms that result in the emergence of fluid behavior of dense and hot nuclear matter, quantifying the quark-gluon plasma's (QGP’s) dynamic properties, and utilizing the heavy ion collisions' broad physics reach.
While determining the nuclear matter's phase structure, the phase diagram has to be pinned down as a function of net-conserved charges and temperature, including the possible QCD critical point determination, which requires theoretical study and experimental measurement of collisions with varying collision energy.
Specifically, the chiral symmetry restoration and deconfinement transition must be understood, and the nuclear equation of state, for which neutron stars and heavy ion collisions can offer complementary input, must be determined. The understanding of fluid behavior emergence mechanisms requires investigating the QGP at short distance scales by employing hard probes, including quarkonia, heavy flavor hadrons, and jets.
The researchers believe that further insight can be gained by more effectively constraining the initial state from complementary experiments and theory and extending the scope to small collision systems. Electromagnetic probes carry more information on the system's time evolution.
QGP's transport properties, including its bulk and shear viscosity and its interaction with high and heavy momentum probes, must be determined as functions of densities and temperature of conserved charges and understood within QCD/effective theories. Additionally, spin-related transport properties can be accessed by probing the fluid flow fields' vortical structure.
Heavy ion collisions generate a wealth of data that facilitates physics studies extending beyond QCD and QGP when this information is meticulously isolated. Utilizing ultra-peripheral collisions and photo-nuclear events, researchers can probe extremely low x-values and investigate quantum electrodynamic phenomena. These studies involve several processes that are sensitive to phenomena beyond the Standard Model, offering insights into previously unexplored aspects of particle physics.
Heavy ion collisions also offer a unique opportunity to explore quantum anomalies via the chiral magnetic effect. Furthermore, several observables are highly sensitive to the intricate nuclear structure of the colliding nuclei. Data collected from the far-forward regions in heavy ion collisions can also provide valuable insights into cosmic ray physics, enhancing our understanding of these high-energy phenomena.
Cold QCD in the Next Decade
The study shows that substantial progress has been made by the hadron physics community in answering fundamental questions regarding the universe's building blocks, like the tomographic imaging of partons within the hadrons, the spin and mass origins of the nucleon, and the encoded nucleon many-body interactions in partonic structures inside the nucleus.
Since 2015, the advancements in these areas have demonstrated the robust capabilities of hadron physics facilities to reveal the underlying QCD dynamics and the related non-perturbative structure of nuclei and nucleons. Moving forward, a deeper understanding of these questions will require a focused examination of multiple aspects.
These aspects are nucleon properties such as:
- Proton charge radius and the nucleon's generalized polarizabilities
- Precision measurements of unpolarized and polarized quark distributions within the large-x region, specifically when x → 1
- Unprecedented mapping of the quark distributions' three-dimensional (3D) tomography inside nucleons
- Revealing the nucleon's mass and spin origins, specifically for the quark orbital angular momentum contribution and trace anomaly contribution to the proton spin and proton mass, respectively
- Parton distributions' nuclear modification in the valence region and short-range nucleon-nucleon correlations in nuclei
- Unraveling the structure and spectrum of exotic and conventional hadrons using precision meson and baryon spectroscopy
- Parity-violation measurements and connections to other fields.
Overall, the future opportunities for both hot QCD and cold QCD outlined in this white paper effectively set the priorities for QCD research for the coming decades.
Journal Reference
Achenbach, P., Adhikari, D., Afanasev, A., Afzal, F., Aidala, C. et al. (2024). The present and future of QCD. Nuclear Physics A, 1047, 122874. https://doi.org/10.1016/j.nuclphysa.2024.122874, https://www.sciencedirect.com/science/article/pii/S0375947424000563
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