Remote - Physics Division Lunch Seminar

Virtual Physics Division Lunch Seminar

Kent Paschke - University of Virginia

New Measurement of the Neutral Weak Form-Factor of the Pb-208: PREX-2

Abstract: Electromagnetic probes of nuclear matter have along served nuclear physics by mapping the distribution of electromagnetic charge. Direct studies of the neutron distribution, however, have been confounded by strong interaction dynamics or other complications to interpretation. As such, there is strong interest in measuring an observable which could be robustly interpreted to illuminate the distribution of neutrons in the nuclei.

The violation of the parity symmetry in elastic electron-nuclear scattering provides such an observable. Parity-violation is the hallmark of the weak interaction, and parity-violation in elastic scattering provides a measurement of the distribution of the weak charge of the nucleus, analogous to that of the electric charge in electromagnetic scattering. The neutron weak charge is large relative to that of the proton, so the neutron distribution is implied by such a measurement. For specific kinematics, this measurement would be in particular sensitive to the thickness of the neutron skin which is thought to form in neutron-rich nuclei.

The PREX-2 collaboration has now realized the potential of a measurement first proposed for Jefferson Lab’s Hall A in 2001, completing the measurement of the parity-violating asymmetry in elastic scattering of longitudinally polarized electrons from an isotopically pure lead-208 target. This result provides a robust determination of the neutron skin thickness in lead-208, corresponding to a constraint on the density dependence of the symmetry energy in the nuclear equation of state. This result is important in describing the nuclear equation of state, with implications for the central density of saturated nuclear matter and to astronomical observations of neutron stars. This new result will be presented and the experimental techniques that led to the success of this measurement will be described.