December 17 2024 (in-person)

Chen, Yi (Luna) (Vanderbilt University)

Host: Peter Jacobs

Title: Evidence of the medium response to hard probes with Z-hadron correlations in PbPb and pp collisions

Abstract: In ultrarelativistic heavy-ion collisions at RHIC and LHC, a new state of matter known as the quark-gluon plasma (QGP) is believed to be created. As high-energy quarks or gluons traverse the plasma, their interactions with the QGP reduce their energy while simultaneously injecting energy into the QGP, which then propagates. This interaction and energy propagation result in intriguing phenomena, including the diffusion wake, observed in several phenomenological models.

In this seminar, I will present a recent measurement from the CMS collaboration that successfully isolates some of these wake effects for the first time. The measurement analyzes the charged hadron distribution in azimuthal angle and pseudorapidity associated with a Z boson in PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV with the CMS experiment. A stronger modification of charged hadron distributions near the Z boson is observed in PbPb collisions compared to the pp reference. This result provides new insights into the interactions within the QGP and contributes to a deeper understanding of the jet quenching phenomenon.

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.

Postponed (Date To be determined)

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.