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arXiv · 2509.06542

First Constraint on P-odd/T-odd Cross Section in Polarized Neutron Transmission through Transversely Polarized $^{139}$La

Rintaro Nakabe·Clayton J. Auton·Shunsuke Endo·Hiroyuki Fujioka·Vladimir Gudkov·Katsuya Hirota·Ikuo Ide·Takashi Ino·Motoyuki Ishikado·Wataru Kambara·Shiori Kawamura·Atsushi Kimura·Masaaki Kitaguchi·Ryuju Kobayashi·Takahiro Okamura·Takayuki Oku·Takuya Okudaira·Mao Okuizumi·J. G. Otero Munoz·Joseph D. Parker·Kenji Sakai·Tatsushi Shima·Hirohiko M. Shimizu·Takenao Shinohara·William M. Snow·Shusuke Takada·Ryuta Takahashi·Shingo Takahashi·Yusuke Tsuchikawa·Tamaki Yoshioka
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Abstract

We report the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a transversely polarized $^{139}$La target. We formulate the transmission asymmetry within the density matrix formalism, explicitly incorporating the forward scattering amplitude of $^{139}$La including tensor polarization terms up to third-rank. The formalism is applied to existing transmission data originally obtained to measure the spin-dependent cross section near the $0.75$~eV $p$-wave resonance. Since these data were not optimized for P-odd/T-odd observables, the attainable sensitivity is intrinsically limited; nevertheless, they provide a useful test of the formalism on real experimental data. No statistically significant TRIV signal is observed. By analyzing the global $χ^2$ structure in the parameter space, we obtain an upper limit of $|W_T|<15~\mathrm{eV}$ at the 90\% confidence level. This corresponds to an upper limit on the resonance-averaged TRIV cross section of $|Δσ_{\not{T}\not{P}}|<8.3\times10^2~\mathrm{b}$. These results validate the present theoretical framework and provide guidance for future dedicated TRIV searches in polarized neutron transmission experiments.

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Rintaro Nakabe, Clayton J. Auton, Shunsuke Endo, Hiroyuki Fujioka, Vladimir Gudkov, Katsuya Hirota, Ikuo Ide, Takashi Ino, Motoyuki Ishikado, Wataru Kambara, Shiori Kawamura, Atsushi Kimura, Masaaki Kitaguchi, Ryuju Kobayashi, Takahiro Okamura, Takayuki Oku, Takuya Okudaira, Mao Okuizumi, J. G. Otero Munoz, Joseph D. Parker, Kenji Sakai, Tatsushi Shima, Hirohiko M. Shimizu, Takenao Shinohara, William M. Snow, Shusuke Takada, Ryuta Takahashi, Shingo Takahashi, Yusuke Tsuchikawa, Tamaki Yoshioka. 2026-04-22. First Constraint on P-odd/T-odd Cross Section in Polarized Neutron Transmission through Transversely Polarized $^{139}$La. https://doi.org/10.1093/ptep%2Fptag088

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