Simulations of Neutron Spin Rotation Measurements by the NSR Collaboration

The Neutron Spin Rotation (NSR) collaboration has developed an apparatus capable of measuring the rotation of transversely polarized neutrons to the level of a few times $10^{-7}$ rad/m to place limits on coupling constants of the hadronic weak interaction (HWI) and long-range exotic fifth forces [RSI 86, 055101 (2015)]. The NSR measurement of $d\phi/dz = \left[ +1.7 \pm 9.1 (stat.)\pm1.4(sys.)\right] \times 10^{-7}$ rad/m [PRC 83, 022501(R) (2011)] for neutrons in liquid-He constrains both HWI coupling constants and the strength of possible exotic parity-odd long-range interactions [PRL 110, 082003 (2013)]. Given the dependence of possible systematic effects on the neutron phase space, neutron transport, and interactions with the target, a Monte-Carlo transport code was developed to examine the size of possible false rotations and their dependence on beam/apparatus characteristics. In addition, the code has been adapted to determine the sensitivity of measurements with a new target designed to measure a parity-conserving rotation due to possible long-range interactions. Simulation results and a status of the current experiment to place limits on the axial-axial coupling of such a long-range interaction and efforts to improve the HWI limits in liquid helium will be presented.