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

We report the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a polarized ¹³⁹La target. Using the explicit forward scattering amplitude for ¹³⁹La, 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 χ² fit treating p-wave resonance energy Eₚ 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 |Δσ_TP| < 1.2 × 10²~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.