January 21 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.

January 28 2025 (in-person)

Youqi Song (Yale University)

Host:  Barbara Jacak

Title: Studying non-perturbative QCD with jet substructure measurements at colliders

Abstract: Understanding non-perturbative QCD effects is important for comprehension of QCD dynamics. To study the non-perturbative regime in detail, I have led two sets of jet substructure measurements in pp collisions at 200 GeV at STAR. In this talk, I will present results from these studies.

• Measurements of the correlation between CollinearDrop and SoftDrop observables provide access to study the interplay between different components of the parton shower. CollinearDrop observables are sensitive to early-stage soft radiation, while SoftDrop observables are sensitive to later-stage hard splittings. The results are fully corrected with a simultaneous unfolding of six observables utilizing MultiFold, a novel machine-learning based unfolding algorithm, which preserves the correlations in the multi-dimensional observable phase space.

• Measurement of the charge-correlator ratio (rc) can shed light on hadronization mechanisms. rc can probe for the contribution of string-like fragmentation by distinguishing the charge signs of leading and subleading charged particles within jets. The result shows that common Monte Carlo models over-predict such correlation in data.

Finally, I will discuss future opportunities for jet substructure measurements at RHIC and the LHC, and how they could further our understanding of QCD.

February 04 2025 (in-person)

Vassu Doomra (Stony Brook University)

Host:  Barbara Jacak

Title: From Photons to Parity: Developing High-Precision Tracking Across Two Distinct Frontiers of Nuclear Physics

Abstract: This talk highlights the development of advanced tracking techniques applied across diverse environments, from precision electron scattering to explore physics beyond the Standard Model with the MOLLER experiment at Jefferson Lab to complex heavy-ion collisions for studying the properties of the partonic matter with the PHENIX experiment at BNL.

In the first part, we will present the first PHENIX measurement of disentangling lepton pairs into heavy-flavor decays, prompt pairs, and background pairs using the distance of closest approach (DCA) to the interaction point using p+p data at 200 GeV taken in 2015. The DCA calculation is enabled by the PHENIX Silicon Vertex Tracker (VTX), which, with a total radiation length of 15%, introduces a significant photon conversion background. We will highlight how recent improvements in tracking techniques have enabled the development of effective conversion rejection methods that significantly suppress these backgrounds, enhancing the signal-to-background ratio by orders of magnitude. The resulting heavy-flavor contributions to the e+e− pair spectrum and its implications will be discussed.

Next, we will shift to the MOLLER experiment, expected to run in 2026/27, where precise tracking is essential to accurately measure the parity-violating asymmetry (APV) in longitudinally polarized e-e scattering using the 11 GeV electron beam of the CEBAF at Jefferson Lab. Achieving the desired level of accuracy is based primarily on our ability to precisely calculate the average kinematic factor and validate the acceptance of the toroidal spectrometer system. To meet these challenges, we have developed a sophisticated track reconstruction algorithm, which will be the focus of the second part of the talk.

February 18 2025 (in-person)

Ananya Rai (Yale University)

Host:  Barbara Jacak

Title: Probing QCD with projected N-Point Energy Correlators

Abstract: Jet substructure accesses the radiation history of a jet, providing a powerful tool for probing QCD across energy scales. In ultrarelativistic heavy-ion collisions, it helps investigate the microscopic properties of the Quark-Gluon Plasma (QGP). My research aims to answer the question “Can we systematically explore QCD energy scales to understand the emergence of various physical phenomena?” The projected N-Point Energy Correlators (ENCs) have emerged as a novel tool for probing hadronic collisions, offering a way to address this question. They are defined as energy-weighted correlations as a function of the largest angle between N particles. Jet evolution is imprinted in the slopes of the ENCs and they exhibit a distinct scaling behavior from perturbative to non-perturbative QCD. Moreover, ratios of higher-point correlators to the two-point correlator (ENC/E2C) provide insight into quantum scaling (anomalous dimensions), which is proportional to the strong coupling constant, αS. In this talk, I will present the measurement of the E2C, E3C, and the E3C/E2C ratio in proton-proton collisions at 13 TeV with the ALICE detector, highlighting their sensitivity to the running of αS. I will also discuss plans to extract αS from these measurements. Additionally, using the Hybrid Model, I showed that the E3C/E2C could be altered in the QGP due to the presence of jet wakes—soft hadrons produced when a traversing jet interacts with the strongly coupled QGP medium. This result has motivated the measurement of this ratio in ALICE Run 2 Pb-Pb data at 5.02 TeV, and I will discuss the ongoing work towards this goal. Finally, I will conclude by outlining prospective substructure measurements that will leverage the high-statistics data from Run 3 at the LHC. As we enter the era of jet tomography, these measurements will enable precise exploration of the microscopic properties of the QGP.

February 25 2025 (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.