October 15 (in person)

Sooraj Radhakrishnan (Kent State University/LBNL)

Host: Zhenyu Ye

Title: Measurement of jet $v_1$ to study path length dependent jet energy loss in heavy-ion collisions at $sqrt{s_{NN}}$ = 200 GeV at RHIC

Abstract: Studying the path length dependence of parton energy loss in the Quark Gluon Plasma (QGP) is cruical towards understanding the parton energy loss mechanism in the QGP. A path length asymmetry along impact parameter direction ($x$) exists for hard probes in heavy-ion collisions at finite rapidity, as the QGP bulk is tilted in reaction plane, while the hard scattering profile is not~\cite{ref1}. Jet $v_1$ ($\langle p_x/p_T \rangle$) measurements provide access to path length dependent energy loss of partons in the QGP. Such a measurement has several advantages. Energy loss can be accessed without measurements in p+p collisions. Event-by-event fluctuations in geometry do not contribute to $v_1$, unlike to the elliptic anisotropy, $v_2$. Another advantage is that the bulk $v_1$ is two orders of magnitude smaller than the bulk $v_2$, making the evolution of the first order anisotropy much weaker during the system evolution. 

In this talk we present the first measurement of charged jet and high $p_T$ charged hadron $v_1$ and $\langle p_x \rangle$ in heavy-ion collisions using Au+Au and Ru+Ru, Zr+Zr collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV by STAR. Centrality, jet $p_{\mathrm{T}}$ and radius dependence of the observables will be shown. We will also present an evaluation of the initial path length asymmetry utilizing models describing the measured pseudorapidity dependence of particle production in asymmetric collisions. We will also discuss future opportunities with event shape engineering for the first order anisotropy to further study the path length dependence of energy loss in the QGP. 

[1] PRL 123, 162301 (2019); PRL 120, 192301 (2018); PRC 72, 034907 (2005).

November 05 (in person)

Tom Reichert (Frankfurt Institute for Advanced Studies (FIAS))

Host: Volker Koch

Title: Frisco3 - A three-fluid model with continuous freeze-out for intermediate energy heavy-ion collisions

Abstract: Frisco3 is a novel 3-fluid hydrodynamical simulation model of relativistic heavy-ion reactions with a continuous transition from the fluid phase to the regime of free streaming hadrons. The time evolution of the reaction is consistently described in terms of three coupled ideal fluids, resembling the two nuclei and the fireball respectively, and a drain term, which is determined by the local thermal emissivity and the survival probability of produced hadrons given by their momentum differential, temperature and density dependent scattering rate in the medium. The drain from the fluid phase is irrecoverably transferred into the observable hadronic final state. Here, first benchmarking results on hadron multiplicities, rapidity and transverse momentum spectra and freeze-out properties related to the phase diagram are presented for Au+Au collisions ranging from 3 GeV to 20 GeV center of mass energy.

November 21 (in-person)

David d'Enterria (CERN) 

Host: Spencer Klein

Title: Photon-photon physics at the LHC 

Abstract: The CERN LHC is not only the current energy-frontier collider for parton-parton collisions, but has proven a powerful photon collider providing photon-photon (γγ) collisions at center-of-mass energies and luminosities never reached before. By exploiting the large fluxes of quasireal photons emitted by the accelerated protons and heavy-ions, a very rich and unique range of SM and BSM measurements is open to study. Such γγ processes can be examined in particularly clean conditions in the so-called ultraperipheral collisions (UPCs) where the colliding hadrons interact without hadronic overlap, and survive their purely electromagnetic interaction. I will review the latest experimental and theoretical photon-photon physics results at the LHC including measurement of the anomalous tau lepton magnetic moment via γγ->tau+tau- production, observation of light-by-light scattering, and searches for axion- and graviton-like particles, among others. 

December 03 (in person)

Ben Gilbert (Livermore)

Host: Spencer Klein

Title: Measurement of photonuclear dijet production in ultra-peripheral Pb+Pb collisions at the LHC with the ATLAS Detector

Abstract:  In ultra-relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Photon-induced processes are the dominant interaction mechanism when the colliding nuclei have an impact parameter larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus, analogous to deep inelastic scattering. This talk presents a measurement of jet production in UPCs performed with the ATLAS detector using high-statistics 2018 Pb+Pb data. Events are selected using requirements on jet production, rapidity gaps, and forward neutron emission to identify inclusive photo-nuclear hard-scattering processes. These measurements also include detailed studies of rapidity gap distributions and nuclear break-up effects, allowing for precise comparisons between data and theory for inclusive photo-nuclear processes. The measured cross-sections are compared to theoretical models in phase-space regions where significant nuclear PDF modifications are expected but not well constrained by world data, demonstrating the potential of these data to provide a strong new constraint on nPDF effects.

December 10 (in person)

Ivan Kisel (Goethe University Frankfurt)

Host: Nu Xu

Title: The High Level Trigger and Express Data Production at STAR

Abstract: In In this talk, I will present the design, functionality, and operational impact of the High Level Trigger (HLT) system developed for the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The HLT system plays a pivotal role in enhancing data collection efficiency by enabling real-time selection and monitoring of events through sophisticated tracking algorithms integrated directly into the STAR data acquisition framework. By identifying events of interest on-the-fly, the HLT significantly reduces data volumes, making data handling and storage more efficient without compromising the quality or integrity of the information collected.

  A core component of the HLT system is the Express Data Production scheme, a framework designed to complement the HLT by rapidly processing a subset of data during data-taking periods. This capability proved instrumental during Beam Energy Scan II program of the STAR experiment, where it enabled immediate feedback on physics phenomena, such as particle correlations and event topologies, essential for optimizing experimental configurations and data quality. I will discuss how this real-time processing pipeline was integrated within the STAR framework and demonstrate its impact on physics analysis by presenting select results from online reconstruction achieved with the HLT.

January 21 or 28 2025 (online)

Mi Ke (CCNU)

Host: Nu Xu

Title: Recent results on Baryon correlations at RHIC-STAR

Abstract: In high-energy nuclear collisions, the measurements of two-particle femtoscopy is a powerful and unique method for extracting information about the femtoscopic spatio-temporal properties of the source and characterising the final state interactions (FSI). However, measurements of baryon correlations are scarce. Understanding the strong interactions between baryons, especially nucleon-nucleon (N-N), hyperon-nucleon (Y-N) and hyperon-hyperon (Y-Y) interactions, are crucial for comprehending the equation-of-state (EoS) of the nuclear matter and inner structure of neutron star. Furthermore, baryon correlations involving light nuclei, which are loosely bound objects, are critical for understanding many-body interactions and the production mechanisms of light nuclei.

    In this talk, we will present recent results on baryon correlations measured with RHIC-STAR experiment, including p-p,  p-d, d-d, p-\Lambda, p-Xi^-, and d-\Lambda. Extracted source size parameters, driven by collision dynamics, and FSI parameterization, determined by the nature of the particle pairs under study, will be discussed within the framework of lattice calculations (interaction potentials) and hadronic transport model calculations.