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Thursday, November 02, 2023
3:45 PM - 4:45 PM
Rosen Auditorium (TA-53, Bldg 1)

P/T Colloquium

What Makes the Proton Spin?

Prof. Carl Gagliardi
Texas A&M University

In the naïve quark model, the properties of the proton are determined by its three valence quarks. However, in the mid-1980s, polarized deep-inelastic scattering measurements by the European Muon Collaboration found that quark spins contribute very little to the spin of the proton. This proton “spin crisis” was the initial motivation to establish a spin physics program at the Brookhaven Relativistic Heavy Ion Collider (RHIC). During the past 20 years, the STAR and PHENIX experiments at RHIC have performed a sequence of measurements to determine the contributions of gluon and anti-quark spins to the spin of the proton. We have discovered that gluons carrying 5% or more of the proton momentum are polarized, and they make a larger contribution to the proton spin than quarks do. We have also discovered that there is an asymmetry between the 𝑢̅ and 𝑑̅ contributions to the proton spin that is opposite in sign to the famous 𝑑̅/𝑢̅ asymmetry in the unpolarized sector. This talk will describe how polarized proton collisions at RHIC elucidate the partonic origin of the proton spin, and what they have taught us.

Host: Ming Liu (mliu@lanl.gov)