Development and Application of an eV Neutron Polarization for Parity Violation Studies at CSNS Back-n Beamline

TL;DR

This paper reports the development of a polarized neutron system at CSNS for parity violation studies, enabling precise measurements of PV asymmetries crucial for understanding fundamental symmetries in physics.

Contribution

The paper introduces a novel in-situ polarized neutron source using SEOP $^3$He at CSNS, with successful measurement of PV asymmetry at a specific resonance, validating its application for symmetry violation experiments.

Findings

Measured PV asymmetry of 7.8% with uncertainties at a specific resonance.

Validated the neutron polarization system by comparing with previous results.

Demonstrated system's capability for future parity violation experiments.

Abstract

The dynamic enhancement of symmetry-breaking effects in neutron-nucleus resonances provides a sensitive testing ground for Time-Reversal Invariance Violation (TRIV). Exploiting this mechanism, the Neutron Optics Parity and Time Reversal Experiment (NOPTREX) seeks to elucidate the origin of the universe's baryon asymmetry. Critical to this effort is the precise measurement of Parity Violation (PV) asymmetries, which is essential to calibrate the nuclear parameters required for future TRIV experiments. To facilitate these studies, we developed an eV polarized neutron at the Back-n white neutron beamline of the China Spallation Neutron Source (CSNS). Neutron polarization is generated by an in-situ Spin-Exchange Optical Pumping (SEOP) $^3$He filter. Spin manipulation is performed by an adiabatic spin flipper, while spin polarization is preserved over the flight path by a vacuum transport system equipped with a solenoidal guide field. Experiments successfully measured an asymmetry of approximately $7.8 \pm 2.4$ (stat.) $\pm 0.3$ (sys.) % at the 0.747 eV p-wave resonance of $^{139}$La. These results are in agreement with previous results on this resonance and validate the system's capability for PV measurements.