Contribution invited talk
Some Recent Theoretical Developments in Small Systems
While proton-proton collisions and heavy-ion collisions can be well-described by their own theoretical frameworks – factorization and hydrodynamics, respectively – the collisions of intermediate “small systems” pose unique challenges and opportunities. The “dilute-dilute” limit associated with proton-proton collisions is characterized by the dominance of a single partonic hard scattering event and can be described theoretically in terms of collinear or transverse-momentum-dependent factorization theorems. In contrast, the “dense-dense” limit associated with heavy-ion collisions is characterized by abundant multi-parton interactions and multiple rescattering and can be described theoretically as a resummation of these effects, leading to the geometry-driven collective flow of soft particles and the quenching or modification of hard particle production. But the intermediate regime, which might be characterized as “semi-dilute”, lies in a regime in which neither description is sufficient. Multi-parton interactions are enhanced and can play an important role in particle production, while the small system size poses a challenge for any in-equilibrium description in hydrodynamics. As such, the collisions of small nuclei with each other and in asymmetric collisions with dilute or dense probes provides a fertile ground for exploring these mechanisms. In this talk, I will present a few new results applicable for particle production in small systems. One growing niche is the extension of the saturation framework, typically applied to small-x kinematics and heavy nuclei, to describe particle production and correlations in asymmetric “heavy-light ion collisions.” Recent calculations in this framework can describe correlations among several produced quarks and gluons, highlighting the interesting role of Bose versus Fermi statistics. Another developing area is the study of collisions of nuclei which are smaller or deformed. This thrust has the potential to use high-energy hadronic collisions to obtain new constraints on the shape parameters associated with these nuclei and to study the properties of the quark-gluon plasma which could be produced in such collisions, which would be smaller and hotter than the corresponding plasmas achieved in large systems. Finally, I will present a new application which combines these two areas: the calculation of spatial correlations among quarks and antiquarks in the initial stages of heavy-light ion collisions, which can be used to initialize and study the evolution of conserved charges through the hydrodynamic phase.