First constraint on the P-odd/T-odd cross section in polarized neutron transmission through transversely polarized La

Nakabe, Rintaro; Endo, Shunsuke   ; Kambara, Wataru*; Kawamura, Shihori; Kimura, Atsushi   ; Kobayashi, Ryuju; Oku, Takayuki   ; Okudaira, Takuya*; Sakai, Kenji ; Shinohara, Takenao   ; Takada, Shusuke*; Takahashi, Ryuta*; Takahashi, Shingo*; Tsuchikawa, Yusuke ; 16 of others*

We report the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a transversely polarized La target. We formulate the transmission asymmetry within the density matrix formalism, explicitly incorporating the forward scattering amplitude of 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 ~eV -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 structure in the parameter space, we obtain an upper limit of at the 90% confidence level. This corresponds to an upper limit on the resonance-averaged TRIV cross section of . These results validate the present theoretical framework and provide guidance for future dedicated TRIV searches in polarized neutron transmission experiments.