Neutron Skins: Weak Elastic Scattering and Neutron Stars

The recently completed PREX-2 campaign - which measured the weak form factor of lead at an optimal momentum transfer - has confirmed that the neutron skin of lead is relatively large and has provided a precise determination of the interior baryon density of a heavy nucleus. In turn, the measured form factor can be related to various nuclear and neutron-star properties. Astrophysical observations by the NICER mission have benefited from improvements in flux, energy resolution, and notably, timing resolution. NICER has the capability to measure pulse profile data, which enables simultaneous mass-radius determinations. During the next decade, measurements in astrophysics, gravitational wave astronomy, and nuclear physics are expected to provide a wealth of more precise data. In this review we provide an overview of the current state of neutron skin measurements and offer insights into the prospects for the future.Comment: submitted to Annual Reviews of Nuclear and Particle Scienc

A spallation target at TRIUMF for fundamental neutron physics

Ultracold neutrons (UCNs) are a powerful tool for probing the Standard Model at high precision. The TRIUMF Ultracold Advanced Neutron (TUCAN) collaboration is building a new UCN source to provide unprecedented densities of UCNs for experiments. This source will use a tantalum-clad tungsten spallation target, receiving up to 40 µA of 480-MeV protons from TRIUMF’s main cyclotron. The beamline and target were constructed from 2014 to 2016 and operated at beam currents up to 10 µA from 2017 to 2019 as part of a prototype UCN source. We describe the design choices for the target and target-handling system, as well as our benchmarking of the target performance using UCN production measurements

The P2 experiment

This article describes the future P2 parity-violating electron scattering facility at the upcoming MESA accelerator in Mainz. The physics program of the facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries stemming from two-photon exchange and a possible future extension to the measurement of hadronic parity violation. The first measurement of the P2 experiment aims for a high precision determination of the weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 = 4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z pole. It comprises a sensitive test of the standard model up to a mass scale of 50 TeV, extendable to 70 TeV. This requires a measurement of the parity violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150 \micro A polarized electron beam impinging on a 60 cm liquid H_2 target allowing for an extraction of the weak charge of the proton which is directly connected to the weak mixing angle. Contributions from gamma Z-box graphs become small at the small beam energy of 155 MeV. The size of the asymmetry is the smallest asymmetry ever measured in electron scattering with an unprecedented goal for the accuracy. We report here on the conceptual design of the P2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. There has been substantial theory work done in preparation of the determination of the weak mixing angle. The further physics program in particle and nuclear physics is described as well.Comment: Invited EPJ A Manuscript, many figures, large file siz