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
Published: 2025/9/8
Abstract
We report the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a polarized $^{139}$La target. Using the explicit forward scattering amplitude for $^{139}$La, we derive analytic expressions for spin observables within the density matrix formalism and apply them to the existing transmission data. The experiment was not optimized for TRIV observables but was taken to measure the spin-dependent cross section around the $p$-wave resonance, which intrinsically limits the attainable sensitivity. To extract a robust TRIV constraint under these conditions, we performed a $\chi^2$ fit treating $p$-wave resonance energy $E_p$ as a Gaussian-constrained nuisance parameter. The fit showed no evidence for TRIV effects. We set an upper limit on the TRIV cross section of $|\Delta \sigma_{\not{T}\not{P}}| < 1.2 \times 10^2$~b averaged over the resonance region. These results validate the theoretical approach and demonstrate the feasibility of future dedicated TRIV searches using a polarized neutron beam and a polarized nuclear target.