__Overview__

The on-line seminar series on “RHIC Beam Energy Scan: Theory and Experiment” will be held online weekly using ZOOM.The topics of this online seminar series cover broad aspects of hot and dense QCD matter, with an emphasis on physics related to RHIC beam energy scan (BES) program.

The VIII-th season of the online seminar series is scheduled on every other Tuesday and the first talk will start on Feb. 27, 2024. Considering that our participants and speakers come from all parts of the world, we have selected the following time slot for the seminar: 6:00am (San Francisco), 9:00am (New York), 3:00pm (Frankfurt), 6:30pm(New Delhi), 10:00pm (Beijing), 11:00pm (Tokyo) Each talk will be one hour long, with a limited number of questions allowed during the talk, followed by an additional 30 minutes for discussion.

__Organizers: __

Brandenburg, Daniel(OSU) | Brandenburg.89@osu.edu |

Heinz, Ulrich (OSU) | heinz.9@osu.edu |

Luo, Xiaofeng (CCNU) | xfluo@ccnu.edu.cn |

Ratti, Claudia (UH) | cratti@central.uh.edu |

Song, Huichao (PKU) | huichaosong@pku.edu.cn |

Xu, Nu (LBNL) | nxu@lbl.gov |

Yao, Xiaojun(UW) | xjyao@uw.edu |

Yin, Yi (IMP) | yiyin@impcas.ac.cn |

__Schedule__

ZOOM Connection information:

https://osu.zoom.us/j/93447823744?pwd=aWdaTEZ2MnE4VDJuTXN6RER5cnFyQT09

Meeting ID: 934 4782 3744

Password: 567449

Seminar PPT and Videos Downloading:

February 27th 2024

Title: **Probing the speed of sound and phases of QCD matter with ultra-central nuclear collisions**

Speaker: **Prof. Wei Li (Rice University)**

Abstract:

Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, this talk will present an extraction of the speed of sound in this medium created using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions, using data collected by the CMS experiment at the LHC. Results are compared with state-of-the-art hydrodynamics simulations and lattice chromodynamic (QCD) calculations of the equation of state. Future prospects for understanding the nature QCD phase transitions and searches for a possible critical end point utilizing the analysis technique established will also be discussed.

March 12th, 2024:

Title: **"****Heavy-ion physics at the LHC: status and future prospects****"**

Speaker:** Prof. Marco Leeuwen (Utrecht University)**

Abstract:

Collisions of heavy nuclei at high energy provide the opportunity to study the Quark-Gluon Plasma, a state of matter where quarks and gluons are effectively free to move over larger distances, instead of being confined in hadrons, in the laboratory. In this presentation I will review some of the key results from the LHC physics program, with emphasis on the implications for multi-body QCD physics and the QGP. I also present a brief outlook on the plans for the ongoing and future runs of the LHC.

March 26th, 2024:

Title: **"****Bayesian techniques for the quantification of QGP properties****"**

Speaker:** Prof. ****Jean-Francois Paquet**** (Vanderbilt)**

Abstract:

Understanding the collective properties of quarks and gluons is possible by comparing models of heavy-ion collisions to measurements of the distribution of particles produced in the collisions. The complexity of modeling heavy-ion collisions and the large amount of experimental data pose a challenge to accurate model-to-data comparisons. Bayesian inference provides a rigorous statistical framework to constrain the properties of nuclear matter by systematically comparing models and measurements and propagating their respective uncertainties.

I will review applications of Bayesian methods to heavy-ion physics, including recent developments. As an example, I will use a comparison of seven recent Bayesian analyses that studied the shear and bulk viscosities of QCD at zero chemical potential, discussing how seemingly similar descriptions of heavy-ion collisions can yield dissimilar constraints on the viscosities.

April 16th, 2024:

Title: **"Baryon Correlations and Scattering Parameters from RHIC Beam Energy Scan"**

Speaker: **Dr. Xin Dong (LBNL)**

Abstract:

At the high baryon density region, collisions from the RHIC Beam Energy Scan phase II (BES-II) program offer a unique laboratory environment to investigate the nature of interactions between various baryons. These interactions between nucleons/nuclei/hyperons are crucial for the understanding of the structure of light nuclei and hypernuclei. They are also key ingredients to the Equation-of-State of high density QCD matter which may offer significant insights to the understanding of the inner structure of compact stars.

In this seminar, we will discuss recent new measurements of nucleon/nuclei/hyperon two-particle correlations using the femtoscopy technique from the RHIC BES-II. The correlation functions are analyzed via the Lednicky-Lyuboshitz formalism to extract both the collision source dynamics as well as the strong interaction scattering parameters. We compare the extracted scattering parameters between different particles and to various effective theory model calculations. Implications on the light/hyper-nuclei structure will be discussed. Finally, we will discuss the prospects of these measurements from the full BES-II program as well as at future high density facilities.

April 23rd, 2024:

Title: **Classical and quantum computing of shear viscosity for 2+1D SU(2) gauge theory**

Speaker: **Prof. Xiaojun Yao (University of Washington)**

Abstract:

Relativistic hydrodynamics has been used to study collective behavior of light particles produced in heavy ion collisions. It has been shown that hydrodynamic calculations with a small shear viscosity give results that agree well with experimental data. Furthermore, a holographic calculation showed that the ratio of shear viscosity and entropy density is as small as 1/(4pi) for strongly coupled N=4 supersymmetric Yang-Mills theory, which is consistent with the value extracted from experimental data via hydrodynamic simulations. On the other hand, calculating shear viscosity in QCD is very challenging: Perturbative calculations are not applicable in the temperature range of interest and Euclidean lattice QCD calculations have uncontrolled systematic uncertainties caused by the ill-defined spectral reconstruction problem. In this talk, I will discuss the Hamiltonian lattice approach which enables real-time calculations. I will take the 2+1D SU(2) pure gauge theory as an example and show some results obtained on a small lattice. The calculations take into account the running coupling in the continuum limit and find the ratio of shear viscosity and entropy density is consistent with 1/(4pi). Finally, I will discuss a quantum algorithm to calculate the shear viscosity, which may help us to perform calculations on bigger lattices.

May 14th, 2024:

Topical Discussion on Spin-Hydrodynamics

**Speaker**: Prof. Shi Pu (USTC)

Title: Thermodynamic stability in relativistic viscous and spin hydrodynamics.

Abstract: We have applied thermodynamic stability analysis to derive the stability and causality conditions for conventional relativistic viscous hydrodynamics and spin hydrodynamics. We obtain the thermodynamic stability conditions for second-order relativistic hydrodynamics with shear and bulk viscous tensors, finding them identical to those derived from linear mode analysis. We then derive the thermodynamic stability conditions for minimal causal extended second-order spin hydrodynamics in canonical form, both with and without viscous tensors. Without viscous tensors, the constraints from thermodynamic stability exactly match those from linear mode analysis. In the presence of viscous tensors, the thermodynamic stability imposes more stringent constraints than those obtained from linear mode analysis. Our results suggest that conditions derived from thermodynamic stability analysis can guarantee both causality and stability in linear mode analysis.

**Speaker**: David Wagner (Johann Wolfgang Goethe–Universitat )

Title: Damping of spin waves

Abstract: We show that, in ideal-spin hydrodynamics, the components of the spintensor follow damped wave equations. The damping rate is related to nonlocal collisions of the particles in the fluid, which enter at first order in ℏ in a semi-classical expansion. This rate provides an estimate for the timescale of spinequilibration and is computed by considering a system of spin-1/2 fermions subject to a quartic self-interaction. It is found that the relaxation times of the components of the spin tensor can become very large compared to the usual dissipative timescales of the system. Our results suggest that the spin degrees of freedom in a heavy-ion collision may not be in equilibrium by the time of freeze-out, and thus should be treated dynamically.

May 28th, 2024:

Speaker: Ashish Pandav

Title: **Precision Measurement of Net-proton Number Fluctuations in Au+Au Collisions at RHIC**

Chair: Mikhail Stephanov

June 11, 2024:

Topical Discussion on Theoretical Determination of the QCD Critical Point

Speaker: **Mauricio Hippert**

Title: Location of the QCD critical point predicted by holographic Bayesian analysis

Abstract: We present predictions for the QCD critical point from a Bayesian analysis within the holographic gauge/gravity correspondence, constrained by lattice QCD results at zero density. This analysis is performed for a Einstein-Maxwell-dilaton model capable of reproducing lattice QCD results at zero and finite baryon density. We find that constraints from lattice QCD strongly favor model realizations with critical points at chemical potentials of 550 − 630 MeV.

Speaker: **D****avid Clarke**

Title: Locating the critical point using lattice QCD

Abstract: We discuss our search for the QCD critical endpoint (CEP) using lattice data. Most recently we have employed multi-point Padé approximations to observables such as the net baryon number computed at pure imaginary chemical potential to find Lee-Yang edge (LYE) singularities of the QCD pressure in the plane of complex chemical potential. We follow an appropriate scaling ansatz for the LYEs to real chemical potential to estimate the CEP position on $N_\tau=6$ lattices. Additionally we discuss the results of an ordinary Padé approximation to $N_\tau=8$ HotQCD data generated at $\mu_B=0$.

Speaker: **Jan Pawlowski**

Title: Locating the CEP: Predictions, estimates & extrapolations and how to judge them

Abstract:Finding the location and determining experimental signatures and the nature of the critical

end point (CEP) in the QCD phase structure (or more generally, the onset of new phases) is

amongst the most pressing and important tasks in heavy ion physics at larger densities.

In this talk I would like to discuss the requirements for theoretical approaches that allow us to estimate or even predict the location of the CEP. It is also argued that the explicit

QCD computations and results support the use of (sufficiently sophisticated) low energy

effective theories for the computation of experimental signatures at large densities.

Chair: **Christian Schmidt-Sonntag (uni-bielefeld)**

June 25th, 2024

Speaker: Victor Ambrus

Title:Applicability of hydrodynamics in large and small systems

Abstract: The buildup of collective flow in relativistic heavy-ion collisions is routinely described using relativistic hydrodynamics. As an effective theory for high-multiplicity, near-equilibrium fluids, the applicability of hydrodynamics is questionable. By considering a comparison to relativistic kinetic theory, this talk aims to establish the applicability bounds of hydrodynamics in terms of a dimensionless parameter called opacity, which is a measure of the total interaction rate in the system. The preequilibrium dynamics play an important role in this comparison, as it is not described correctly in hydrodynamics. This leads to significant differences in late-time observables. At large opacity reached in Pb+Pb and central O+O collisions, agreement can be achieved by either altering the initial conditions in hydrodynamics or by using hybrid simulations, in which preequilibrium is modeled in kinetic theory. At small opacities characterizing e.g. p+p, p+A and non-central O+O, when hydrodynamics loses its applicability, we find significant discrepancies to kinetic theory in all previously-mentioned approaches.

[1] PRL 130 (2023) 152301. DOI: 10.1103/PhysRevLett.130.152301.

[2] PRD 107 (2023) 094013. DOI: 10.1103/PhysRevD.107.094013.

Chair: Urs Wiedemann

__ Code of Conduct __:

The organizers are committed to making this seminar series productive and enjoyable for everyone. Creating a supportive professional environment where open and frank discussion of ideas can take place, where everyone is treated with courtesy and respect, and in which diversity and inclusion are valued is the responsibility of all the participants. We will not tolerate harassment of attendees or others involved in the conference in any form. By joining the Zoom call, you agree to follow these guidelines:

1) Behave professionally in personal interactions. Harassment and sexist, racist, or exclusionary comments or jokes are not appropriate. Harassment includes sustained disruption of talks or scientific discussions, deliberate intimidation, stalking, offensive comments related to gender identity, sexual orientation, disability, physical appearance, body size, race, nationality, or the religion or non-religion of participants.

2) Be kind to others. Do not insult or put down the speaker or the other attendees. Scientific discussion and criticism is vital and should be conducted in this spirit.

3) All communication should be appropriate for a professional audience including people of many different backgrounds.

Should a participant be asked to stop any inappropriate behavior, they are expected to comply immediately. In serious cases, they may be muted or asked to leave the meeting at the sole discretion of the organizers.