Session C01: Ultra-High Energy Nuclear Physics - Experiment

Measurement of global spin polarization in relativistic Au+Au collisions

Heavy-Ion Collisions (HICs) at high energies create energy densities sufficient to create a new state of matter, the Quark-Gluon Plasma (QGP), which is characterized by the deconfinement of quarks and gluons; these partons which normally exist in bound states (protons and neutrons) are free for a very short period of time (\mathcal{O}(1 fm/c)) in the QGP. Studying correlations of particles generated by the QGP created in HICs is the only feasible way to experimentally study the strong nuclear force; therefore, experiments such as the Solenoid Tracker at the Relativistic Heavy-Ion Collider (STAR) have generated much interest in the community and have yielded extremely useful measurements. Substantial fluid vorticity leading to a spin alignment of emitted hadrons with the angular momentum of the nucleus-nucleus collision system, in the paradigm of hydrodynamics, was predicted theoretically. The STAR Collaboration has recently measured non-zero global spin polarization and found the associated fluid vorticity to be the largest in any known system by many orders of magnitude. While substantial progress has been made, there is still much to understand about vorticity-driving mechanisms within the QGP formed in HICs. In this talk, I address the exciting physics in these measurements.   

$\gamma$+Jet Measurements at the RHIC Facility

Jets recoiling from a direct-photon have long been seen as a golden probe of the quark gluon plasma created in relativistic heavy ion collisions, due to the ability to tightly constrain the initial hard scattering kinematics. Until recently, the ability to measure this channel and the ensuing observables at RHIC were largely statistics-limited, owing to the small cross-section of direct photon production compared to, for example, the more abundant di-jet cross-section. In this talk, we will discuss the details of gamma-jet measurements at RHIC, with a brief review of measurements from PHENIX and STAR and a view towards sPHENIX.   

Centrality determination with a forward detector in the RHIC Beam Energy Scan

Higher order moments of net-proton distributions have been identified as potential markers of the critical point between QGP and hadronic phases of QCD matter. Analyses of these moments can potentially suffer from autocorrelation effects should centrality determination and particle selection both be performed at mid-rapidity. This talk will focus on a determination of the centrality in the forward region (with UrQMD simulated collisions), which would avoid autocorrelation with particles selected at midrapidity, using the event plane detector (EPD) in the STAR experiment as the potential candidate for a forward centrality detector. We will also discuss the challenge of accounting for spectator hadrons in the forward region for the collision energy range covered by the RHIC beam energy scan and how to leverage the inclusion of these hadrons in the EPD centrality determination.   

$v_{n}$ measurement in Au+Au collisions at $\sqrt{s_{NN}} =$ 27 GeV with the Event Plane Detector from STAR

The measurement of pseudorapidity ($\eta$) dependence of $v_{n}$ at lower collision energies can provide unique constraints on the three-dimensional initial conditions, baryon transport, shear viscosity over entropy density as well as its dependence on temperature and baryon chemical potential. The combination of the Event Plane Detector (EPD, $2.1<|\eta|<5.1$) installed in the year 2018, STAR time projection chamber (TPC, $|\eta|<1$) and high statistics Beam Energy Scan phase-II data enables us to perform precise measurements of $v_{n}(\eta)$. In this presentation,I will talk about the measurement of directed flow ($v_{1}(\eta)$) and elliptic flow ($v_{2}(\eta)$) of charged hadrons.   

Two--particle correlations for studying jets at RHIC and LHC

In this talk I will present of summary of some recent two-particle correlation results for RHIC experiments including some outlook for the new sPHENIX experiment.   

Feasibility Study of QGP-like Jet Suppression Observable in RHIC p$+$p 500 GeV Dataset

Some signals of QGP have been observed in special high multiplicity p$+$p collisions at LHC energies but to date, none of these effects have been observed at RHIC energies. The highest RHIC p$+$p energy of 500 GeV presents an interesting candidate to look for such effects. Especially for the QGP-like jet quenching-like observables which have been hard to observe if seen at all even in larger (but still small numbers of nucleon) systems like p$+$A, where small effects would be expected, an observable which can reduce the systematics is desirable. This talk will discuss the possibility of using such an observable which has fortunately recently been applied with some success to some other RHIC data for the RHIC 500 GeV p$+$p runs, in terms of statistical feasibility, detector outlook for certain detectors such as PHENIX and sPHENIX, and importantly expectations for non-QGP-like physics in various version of the PYTHIA event generator.   

Potential Calorimeter Calibration Techniques for the sPHENIX Experiment at RHIC

I will discuss some recently developed techniques that could be used to calibrate EM and hadronic calorimeters such as for the new sPHENIX Experiment at RHIC.