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 how big a nucleus should be, that is, "What is the distribution of protons and neutrons within the nucleus?". The full answer to this question has truly far-reaching consequences, both for a range of fields in physics and also, literally far-reaching - by telling us about neutron stars. 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 electric 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 electric 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 property of parity-violation in the weak interaction to measure the weak charge of the nucleus. Parity violation 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. Jefferson Lab's electron accelerator makes the electrons travel greater than 0.99999 times the speed of light. The detectors work because the electrons are actually traveling faster than light in the quartz that makes up the active area, causing them to emit Cherenkov radiation. The asymmetry we measure is smaller than the concentration of 4 drops of ink in a 55 gallon barrel of water! 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 the PREX and CREX experiments through design, construction, testing and deployment of key pieces of equipment including simulation of backgrounds and design of detectors used to measure the degree of polarization of the electron beam.