Session H02: Parallel Invited Session - Nuclear Physics III

An anomaly in an anomaly? First results from the muon g-2 experiment at FNAL

The Fermilab muon g-2 experiment recently released its first measurement of the positive muon magnetic moment anomaly, a_mu = (g_mu-2)/2 to an accuracy of 0.46 ppm. The anomaly a_mu is of interest since it can be predicted with impressive precision and its value is sensitive, via quantum corrections, to the interactions of the muon with the other particles of the Standard Model. Comparison of measurement results and theoretical predictions tests the completeness of the Standard Model, and a significant discrepancy would indicate the need for new physics. Details of the Fermilab experiment and its first result will be presented, along with a comparison with the theory and future prospects.   

Overview of spin-polarized nucleon structure studies in electron scattering in Hall A at Jefferson Lab

Polarization observables in electron-nucleon and electron-nucleus scattering have led to major discoveries that upended previously-held notions of nucleon structure. Polarized measurements are sensitive to details of nucleon structure that are otherwise inaccessible in unpolarized scattering experiments. Recent examples include the measurements of the proton and neutron electric/magnetic form factor ratios (FFR) at large momentum transfers using, respectively, polarization transfer and polarized beam-target asymmetry observables, and the discovery of the so-called "Collins" and "Sivers" effects giving rise to non-zero target single-spin asymmetries (SSA) in the Semi-Inclusive Deep Inelastic Scattering (SIDIS) process. For the proton, the polarization transfer measurements of the FFR yielded surprising results contradicting expectations based on decades of unpolarized cross section measurements, an apparent contradiction which is still not yet fully resolved more than two decades after its initial discovery. For the neutron, polarization measurements provide the only reliable way of accessing the FFR at large momentum transfers, since the magnetic scattering dominates the unpolarized cross section given the smallness of the neutron's electric form factor. SSA measurements in SIDIS have the potential to test fundamental QCD predictions and unravel the three-dimensional spin structure of the nucleon in momentum space. The Super BigBite Spectrometer (SBS) program in Jefferson Lab's Hall A consists of five major fully approved experiments totaling approximately 200 days of beam time, that will perform measurements of proton and neutron form factors and SIDIS SSAs at large momentum transfers, using CEBAF's polarized electron beam and a combination of polarization transfer and polarized target measurements. The SBS program started in Fall 2021, with the first two experiments focused on the neutron's magnetic form factor completed by February 2022. The 2nd experiment, focused on measuring the neutron electric form factor using a polarized Helium-3 target, has recently started in early October 2022. In this talk, I will give an overview of the SBS science program and its status, with an emphasis on upcoming measurements of polarization observables.