Session 3WBA: Future of Hot and Cold QCD Physics at High-energy Colliders and their Synergies I

Dynamics of high-energy nuclear collisions: present and future

In this talk, I first summarize the current understanding of dynamics of high-energy nuclear collisions from a hydrodynamic point of view. State-of-the-art dynamical models have been utilized towards comprehensive understanding of the deconfined nuclear matter, the quark gluon plasma (QGP). Bayesian parameter estimation is a well-known powerful tool to constrain the transport properties of the QGP such as shear and bulk viscosity. However, this can be valid only within an applicability of the model itself. In particular, given a fact that non-equilibrium initial stages just after the collisions are not well understood yet, one needs to keep it in mind in interpretation of the results from Baysian parameter estimation.

I discuss to include a non-equilibrium effect based on the core-corona picture into the dynamical approach for complehensive description of the dynamics from small to large colliding systems. At extremely large multiplicity in A+A collisions, the system is dominated by the equilibrated matter. While at low multiplicity in p+p collisions, pQCD based approach would be more desirable to describe the reaction. This suggests importance of the interpolation of these two approaches in a unified manner. I show some results from the core-corona picture in this presentation.

Relativistic hydrodynamics has been successful in description of space-time evolution in high-energy nuclear collisions. However, it is not at all trivial at which stage one can apply hydrodynamics to the description of dynamics. In particular, at the very early stage, non-equilibrium effects play an essential role before the system is close to the local equilibrium. On the other hand, at the very late stage, the system is too diluted to maintain the local equilibrium. From the viewpoint of the causality, I discuss the applicability of relativistic viscous hydrodynamics and constrain the initial conditions of fluids in a simle case. This turns out that, when the system is far from the local equilibirium, the causality is violated in relativistic hydrodynamics.

We finally present the future of dynamical approaches in high-energy nuclear collisions.    

Recent highlights of cold QCD physics at RHIC

The exploration of the fundamental structure of strongly interacting matter has always thrived on the complementarity of lepton scattering and purely hadronic probes. As the community eagerly anticipates a future electron-ion collider (EIC) in the U.S., an outstanding scientific opportunity remains to complete “must-do” measurements in p+p and p+A physics in the years preceding the EIC. The unique capability of Relativistic Heavy Ion Collider (RHIC) to provide both longitudinally and transversely polarized hadronic collisions at various energies has opened new avenues for studying the internal structure of the proton with unprecedented depth and precision. A remarkable breadth of physics results has been produced from the RHIC Cold QCD program over the past two decades.

In this talk, I will present the highlights of longitudinal and transverse spin, as well as the unpolarized physics conducted at RHIC to date. These measurements are essential for fully realizing the scientific missions of the EIC by providing a comprehensive set of measurements in hadronic collisions. By combining the data from RHIC and the EIC, we can establish the validity and limits of factorization and universality. Additionally, while much of the physics in this program is unique to p+p and p+A collisions and offers discovery potential on its own, when combined with data from the EIC, it will provide a broad foundation for a deeper understanding of fundamental QCD.   

Recent QCD studies and future directions using UPCs

We will review recent QCD results using photon-induced physics processes using ultra-peripheral heavy-ion collisions at both RHIC and the LHC, 

describing some of the open questions and future directions in the coming years.