High sensitivity of a future search for P-odd/T-odd interactions on the 0.75 eV $p$-wave resonance in $\vec{n}{+}^{139}\overrightarrow{\mathrm{L}\mathrm{a}}$ forward transmission determined using pulsed neutron beam

Neutron transmission experiments can offer a new type of highly sensitive search for time-reversal invariance violating (TRIV) effects in nucleon-nucleon interactions via the same enhancement mechanism observed for large parity violating (PV) effects in neutron-induced compound nuclear processes. In these compound processes, the TRIV cross-section is given as the product of the PV cross-section, a spin-factor $\kappa$, and a ratio of TRIV and PV matrix elements. We determined $\kappa$ to be $0.59\pm 0.05$ for ${}^{139}$La+$n$ using both $(n,\gamma )$ spectroscopy and ($\vec{n}{+}^{139}\overrightarrow{\mathrm{L}\mathrm{a}}$) transmission. This result quantifies for the first time the high sensitivity of the ${}^{139}$La 0.75~eV $p$-wave resonance in a future search for P-odd/T-odd interactions in ($\vec{n}{+}^{139}\overrightarrow{\mathrm{L}\mathrm{a}}$) forward transmission.