### Conveners

#### Parallel Session T01: Initial state physics and approach to thermal equilibrium: I

- Michael Strickland (Kent State University)

#### Parallel Session T01: Initial state physics and approach to thermal equilibrium: II

- Steffen A. Bass (Duke University)

Isolated photons and dijets measurements in small collision systems probe the initial state of the collision, providing the opportunity to constrain PDFs, test pQCD predictions, and probe cold nuclear matter effects. In addition, dijet measurements are sensitive to interactions of partons with the medium produced in Pb-Pb collisions that induce modifications in jet properties. Measurements in...

Beams of relativistic heavy ions are accompanied by a large flux of equivalent photons, and thus photon-induced reactions are the dominant interaction mechanism in heavy-ion collisions when the colliding nuclei have transverse separation larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs) the photon can provide a clean probe of the partonic structure of the nucleus...

Over the last decades, many of the available theoretical jet quenching formalisms have been extended to account for the medium’s finite longitudinal extension and expansion. However, only recently a first-principle approach has been developed that allows to study jet evolution in anisotropic media in the dilute limit. In this talk, we show how to extend some of the previous results to the...

Angular correlations present in dijet photoproduction are studied, for the first time, using ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The second moment of the angular distribution, $\langle \cos(2\Phi) \rangle$, where $\Phi$ is the angle between the vector sum $\vec{Q}_\mathrm{T}$ and the vector difference $\vec{P}_\mathrm{T}$ of the...

Within the framework of a 3+1D Boltzmann transport approach at fixed $\eta/s$ with the full collision integral, we investigate the existence of far-from equilibrium attractors in momentum moments of the one particle distribution function. We first compare our results for a conformal and non conformal gas for different values of $\eta/s$ and $m$, in order to investigate the role of the equation...

We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the...

The relativistic generalization of the relaxation time approximation proposed by Anderson and Witting [1] is widely used in several fields of physics and, in particular, in the study of the hydrodynamization of the matter produced in ultrarelativistic heavy ion collisions. We demonstrate that the approximation proposed by Anderson and Witting contains basic flaws, not being consistent with...

Measurements of anisotropic flow in heavy-ion collisions are key to extract properties of the quark-gluon plasma (QGP). The combination of different flow harmonics relative to the participant and spectator planes provides a unique insight into the initial conditions and the space-time evolution of such collisions. In particular, the spectator plane provides novel information about the...

RHIC's capability to perform relativistic collisions of various ion species provides a unique opportunity to explore and constrain neutron skin thickness and deformation parameters of nuclei.

The study of neutron skin thickness $\Delta r_{np}$ of nuclei can help us directly infer nuclear symmetry energy. Such information is of critical importance to the equation of state of dense...

The far-from-equilibrium non-abelian plasma is created in the early stage of heavy-ion collision. The “bottom-up” mechanism is the well-studied phenomenological description of its approaching to thermal equilibrium, but has been restricted to leading order coupling within kinetic theory calculation. In our recent work, we provide a next-to-leading-order (NLO) weak-coupling description of the...

We employ an effective kinetic description to study the space-time dynamics and development of transverse flow of small and large collision systems. By combining analytical insights in the few interactions limit with numerical simulations at higher opacity, we are able to describe the development of transverse flow from very small to very large opacities, realised in small and large collision...

Determining the multi dimensional structure of protons and nuclei at high energy is one central goal of the future Electron-Ion Collider. This fundamental information is a crucial input for models describing the initial state of heavy ion collisions. In particular the event-by-event fluctuating proton geometry should have a strong effect on the flow and multiplicity distribution in high...

In Bayesian analyses of heavy ion collisions up to now one usually uses the TRENTo prescription for the initial state, followed by a free streaming initial stage. In this work, we extend this picture in two ways. Firstly, we generalize the TRENTo formula so that it is able to describe binary scaling. This introduces a parameter which we subsequently use to determine whether binary scaling is...