Spin dependence in the $p$-wave resonance of $\overrightarrow{{}^{139}\mathrm{La}}+\vec{n}$

We measured the spin-dependent cross section at a neutron-induced $p$-wave resonance using a polarized epithermal neutron beam and a polarized nuclear target. Our study focuses on the 0.75 eV $p$-wave resonance of ${}^{139}\mathrm{La}+n$, where previous experiments have shown a large enhancement of parity violation. The spin dependence of the neutron absorption cross section of polarized ${}^{139}\mathrm{La}$ and polarized neutrons determines the partial neutron width of the $p$-wave resonance. This partial width is a key parameter needed to determine the sensitivity of this system to possible parity-odd and time reversal-odd interactions beyond the standard model. Our findings therefore serve as a foundation for a future search for time reversal violation in this compound nuclear system.

Figure 1

Experimental setup. The neutron spin after the ${}^3\mathrm{He}$ spin filter adiabatically tilts from the longitudinal direction towards the nuclear polarization direction, following the magnetic field on the beam path.

Figure 2

Configuration around the La target. The thermometer was installed in the cold head.

Figure 3

Neutron TOF spectra of the transmitted neutrons (top panel) and the spin-dependent asymmetries (bottom panel). Black and white points in the bottom panel denote the asymmetries in the conditions (a) and (b), respectively, in the bottom panel.

Figure 4

Ratio of the counts of transmitted neutrons for the polarized to the unpolarized ${}^3\mathrm{He}$ spin filters. The curved line shows the best fit.

Figure 5

${}^3\mathrm{He}$ polarization versus elapsed time from the beginning of the measurement. The curved line shows the result of fitting to an exponential function. The measurement was not conducted from 16 h to 22 h due to a liquid He transfer for the superconducting magnet.

Figure 6

Neutron polarization obtained from the averaged ${}^3\mathrm{He}$ polarization.

Figure 7

(a) Neutron TOF dependence of $P_I{\sigma }_{\mathrm{S}}^{\exp }$. The curved line is the best fit of the slowly varying component derived from the negative $s$-wave resonance. (b) Resonance component of spin-dependent cross section. (c) $p$ value for $P_I{\sigma }_{\mathrm{S},r}^{\exp }$. The dotted line shows 99.7% confidence level.

Figure 8

Visualization of the $\varphi$ values on the $xy$ plane. The curved lines, shaded areas, and dashed lines show Eq. (18) and its $1\sigma$ region of the statistical error.

Figure 9

Visualization of the $\varphi$ values obtained using Terilzzi et al.'s resonance parameters on the $xy$ plane. The curved lines and shaded area shows Eq. (18) and its $1\sigma$ region of the statistic error.