June 10 2025 (online)

Yushan Su (University of Maryland)

Host: Yuxun Guo

Title: Perturbative Accuracy for Calculating Parton Distributions under Large Momentum Effective Theory 

Abstract: Parton distribution functions (PDFs) are universal scaling functions essential for probing hadron structure as well as searching for New physics beyond the Standard Model (SM). The large momentum effective theory (LaMET) is a precision-controlled framework for directly calculating the x-dependence of PDFs, based on the first principles of Quantum Chromodynamics (QCD). In this talk, I introduce some of the efforts aimed at improving perturbative accuracy for the LaMET matching kernel, including DGLAP log resummation, threshold log resummation, and leading renormalon resummation. The main numerical effect is that the perturbative matching breaks down in the endpoint regions but remains reliable in the moderate x-range with controlled precision.

June 17 2025 (in-person)

Maciej Lewicki ( Institute of Nuclear Physics, Kraków)

Host: Spencer Klein

Title: Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei 

Abstract: Strong interactions preserve an approximate isospin symmetry between up (u) and down (d) quarks, part of the more general flavor symmetry. In the case of K meson production, if this isospin symmetry were exact, it would result in equal numbers of charged (K+ and K−) and neutral (K0 and K0bar) mesons produced in collisions of isospin-symmetric atomic nuclei.

In this talk, I will present recent experimental evidence of isospin-symmetry violation in high-energy argon-scandium collisions at √sₙₙ = 11.9A GeV, as observed by the NA61/SHINE collaboration. The measured K⁺, K⁻, and Kₛ⁰ yields at mid-rapidity show an unexpected ~18% excess of charged over neutral kaons, significantly exceeding model predictions based on isospin symmetry and neutron-proton asymmetry. The experimental setup, measurement methods, theoretical models, and implications of this discrepancy will be discussed.