[Theory-seminars] Theory Seminar Today

[Joseph Karpie]

Hey everyone,

Today we will be having a remote seminar by Yang Li (USTC) at 1pm. Please join us in F224-225 or on Zoom https://jlab-org.zoomgov.com/j/1612031893?pwd=Q4KRMVsR2m27YbCgvPrZzbfClwqSLD.1

Title: Revisiting Hadron Structure in Three Dimensions: From Electromagnetic to Mechanical Densities

Abstract:
Understanding the three-dimensional structure of hadrons — such as the proton and pion — has become a central focus in modern nuclear and particle physics. Probes via electroweak currents (vector and axial) and the energy-momentum tensor have spurred significant theoretical and experimental advances. Nevertheless, fundamental challenges remain, particularly in defining relativistic charge and mechanical distributions. A key issue is the breakdown of the traditional Sachs formalism, which relies on non-relativistic assumptions ill-suited for hadrons whose size is comparable to their Compton wavelength (r ∼ λ). This limitation echoes a century-old problem in relativistic electromagnetism — the proper description of electromagnetic observables for moving bodies — studied by Lorentz, Einstein, and Minkowski.

In this talk, I will show how a covariant formulation based on relativistic multipole expansions naturally resolves this issue, revealing that various proposed hadron density distributions emerge as multipole moment densities in a covariant macroscopic electrodynamics framework. This approach not only clarifies the physical meaning of charge distributions in relativistic systems but also provides a unified foundation for interpreting experimental data from elastic and deep exclusive scattering. I will further extend this framework to the mechanical structure of hadrons, including pressure and shear force distributions derived from the energy-momentum tensor, offering new insights into the internal forces that bind quarks and gluons.  This perspective bridges historical foundations in relativity with contemporary hadronic physics, paving the way for a consistent and observable-based understanding of hadron structure in three dimensions.


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Joe Karpie
Pronouns: He/His/Him
Postdoctoral Fellow
Theoretical and Computational Physics Center
Jefferson Lab

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