Ongoing Experiment to Measure the Parity-Odd Asymmetry in the 0.73 eV Neutron Resonance of ¹³⁹La Using a Polarized ³He Neutron Spin Filter

Sensitive searches for new sources of time reversal violation in and among nucleons are one of the highest scientific priorities in nuclear/particle/astrophysics. The NOPTREX collaboration proposes a sensitive search for a time-reversal odd (T) and parity-odd (P) term in the neutron-nucleus forward scattering amplitude on the 0.734 eV p-wave resonance in ¹³⁹La. The 0.7 eV p-wave resonance in ¹³⁹La can amplify both P-odd and T-odd effects from mixing of s-wave and p-wave resonances [1], and a null test of T is possible for this observable [2]. In preparation for the P-odd T-odd experiment, we are currently conducting a precision measurement of the P-odd asymmetry on the 0.734 eV p-wave resonance in ¹³⁹La. While a measurement of the P-odd asymmetry has been recently completed to some degree [3], we have already found ways to improve the previous measurement by introducing a new ³He polarizer for much larger (>70%) neutron polarization and a NaI gamma detector array to be used simultaneously with our previous ⁶Li neutron transmission detector. We describe our experimental design and approach which can achieve a measurement of the P-odd asymmetry to <1%.

[1] V. P. Gudkov, Physics Reports 212, 77 (1992).

[2] J. D. Bowman and V. P. Gudkov, Phys. Rev. C 90, 065503 (2014).

[3] D.C. Schaper, et al, Nuclear Instruments and Methods in Physics Research Section A, 969 (2020).