High sensitivity of a future search for effects of $P$-odd/$T$-odd interactions on the 0.75 eV $p$-wave resonance in $\vec{n}+{}^{139}\overrightarrow{\mathrm{La}}$ forward transmission determined using a 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 ${}^{139}\mathrm{La}+n$ using both $(n,\gamma )$ spectroscopy and ($\vec{n}+{}^{139}\overrightarrow{\mathrm{La}}$) transmission. This result quantifies for the first time the high sensitivity of the ${}^{139}\mathrm{La}$ 0.75-eV $p$-wave resonance in a future search for effects of $P$-odd/$T$-odd interactions in ($\vec{n}+{}^{139}\overrightarrow{\mathrm{La}}$) forward transmission.

Figure 1

Visualization of the value of $\varphi$ on the $(x,y)$ plane. The red, blue, green, and purple filled areas represent the neutron transmission, $a_1$, $a_2$, and $a_3$ results with a $1\sigma$ region. The corresponding solid lines indicate the central values.

Figure 2

PDFs for $\varphi$ on the unit circle. The red, shaded, and gray filled areas represent the PDFs of Eq. (3), the product of PDFs of Eqs. (5, 6, 7), and the product of PDFs of Eqs. (3) and (5)–(7), respectively.