PREX II and CREX: Precision Parity Violating Measurement of the Neutron Skin of Heavy Nuclei at Jefferson Laboratory

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One of the most fundamental properties that a nuclear structure model could predict is the spatial extent of the nucleus, that is, "What is the distribution of protons and neutrons within the nucleus?". The full answer to this question has truly far-reaching consequences. The difference between the radius of neutrons and protons in a nucleus has direct consequences for the structure of neutron stars, affecting how big they can be, how thick their crusts are and how long they will take to cool. This information will also help us predict whether a star will collapse into a neutron star or a black hole. What we traditionally call the size of a nucleus is really, in most cases, the charge distribution. This has been measured extremely well using electron scattering: the cross section in electron-nucleus scattering is measured at different kinematics and then used to obtain the charge radius. That is, by measuring the form factor of the nucleus at different four-momentum transfer, and then taking the Fourier transform, you can obtain the density of charge within the nucleus as a function of radius. What this tells you is the distribution of protons within the nucleus, because the proton is what gives the nucleus its charge. But the question remains whether the distribution of neutrons is the same as that of the protons. North America's premier electron scattering facility, Jefferson Laboratory, is the ideal place to find the answer to this question. The Pb Radius Experiment (PREX) and the Ca Radius Experiment (CREX) will measure the neutron radius of specific isotopes of both lead (Pb) and calcium (Ca) using the quality of parity-violation in the weak interaction to measure the weak charge of the nucleus. This effect is analogous to the scattering being different when it is observed in a mirror. Because the proton has almost no weak charge, while the neutron has maximum weak charge, measuring the weak charge distribution tells you the neutron distribution. The PREX and CREX collaborations consist of over 20 institutions world-wide with over 50 collaborators. Our Canadian group will play a key role in these experiments through design, construction, testing and deployment of key pieces of equipment.