Neutron Spin Rotation in Polarised Xenon as a Bound on Spin-Torsion Coupling

This paper examines how neutron spin rotation through polarised xenon gas limits a spin-dependent interaction that would rotate the neutron spin plane. The system considered is cold neutrons traversing polarised xenon at a density near 5 x 10^26 per cubic metre, and the observable is spin rotation angle per unit length and its implied energy scale. Evaluating the ordinary behaviour of the system and the spin-torsion contribution under explicit assumptions about polarisation and uniformity, and inverting the comparison at the stated laboratory precision, gives a quantitative bound rather than a qualitative assertion of smallness. The computed record is: rotation_sensitivity_rad_per_m = 2.000e-07; polarised_density_per_m3 = 5.000e+26; implied_energy_scale_J = 1.687e-38; wavelength_m = 4.000e-10. The torsion-related contribution is far below the level that would alter the ordinary behaviour of the system at current precision, and the value that would have produced a visible residual is stated so that future improvements can be assessed against it directly. No new measurement is reported. This study is simulated and computed from stated assumptions, not measured.