nab

Nab, nPDGamma, and nHe3: decay correlation and parity-violating asymmetry experiments with cold neutrons

Location: Oak Ridge
Participating Canadian institutions: Manitoba, TRIUMF, Winnipeg
International partners: USA

The Nab experiment at the Spallation Neutron Source (SNS) at Oak Ridge is gearing up to follow in the footsteps of two efforts that recently have provided the most stringent constraints on the 6 weak nucleon-nucleon coupling constant to date. The nPDGamma experiment carried out the world’s first measurement of parity violation in the neutron-proton system with a first nonzero observation of the corresponding asymmetry \(A_{PV} = 3.0 \pm 1.4 {\rm (stat)} \pm 0.2{\rm(sys)} \times 10^{-8}\) ¹. The result confirms in a simpler system measurements in \(^{18}\)F of the smallness of the isovector versus isoscalar weak NN interaction, nicely consistent with several recent theory approaches. Likewise, the n3He experiment carried out the first measurement of parity-violation in the neutron-\(^3\)He system, which is the next simplest nuclear system, aside from the technically very difficult measurements on deuterium. This measurement was a first and observed an asymmetry \(A_{PV} = 1.58 \pm 0.97{\rm(stat)} \pm 0.24{\rm(sys)} \times 10^{-8}\) ². Future improvements of these measurements will be technically very challenging and are a long way off, requiring the construction of new facilities, such as the European Neutron Spallation Source.

Figure 1: Left: Schematic illustration of the 8 meter long Nab spectrometer and detector assembly with electric and magnetic field configuration. Right: Assembly and testing of the Nab large area silicon detectors at the University of Manitoba. — Figure 1: Left: Schematic illustration of the 8 meter long *Nab* spectrometer and detector assembly with electric and magnetic field configuration. Right: Assembly and testing of the *Nab* large area silicon detectors at the University of Manitoba.

The focus in the coming years will be the Nab experiment. It measures the correlation parameter \(a\) and the Fierz interference term \(b\) in un-polarized cold neutron beta decay. The heart of the apparatus is an \(\approx 4\) m tall solenoidal spectrometer with segmented Si detectors on either end to measure the energy of the decayed electrons and momentum of the decayed protons, shown on the left in Figure 1. The Canadian group’s primary contribution to the experiment is the development of a 30 keV proton accelerator at the University of Manitoba that will be used to characterize the large area Si detectors used in the experiment (shown on the right in Figure 1). Nab will begin collecting data in 2021 and continue until 2024, after which spin-polarized cold neutrons will be used with the same apparatus to measure the neutron beta decay correlation coefficients \(A\) and \(B\).