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arxiv: 2509.06542 · v3 · submitted 2025-09-08 · ⚛️ nucl-ex · hep-ex

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

Rintaro Nakabe , Clayton J. Auton , Shunsuke Endo , Hiroyuki Fujioka , Vladimir Gudkov , Katsuya Hirota , Ikuo Ide , Takashi Ino
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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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Pith reviewed 2026-05-18 18:51 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-ex
keywords time-reversal violationpolarized neutron transmission139Lap-wave resonanceTRIV cross sectiondensity matrix formalismnuclear symmetry tests
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The pith

Reanalysis of existing neutron transmission data sets first upper limit on time-reversal violating effects in lanthanum-139.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper formulates polarized neutron transmission asymmetry using a density matrix approach that incorporates tensor polarization terms up to third rank and applies it to prior measurements on a transversely polarized lanthanum-139 target near the 0.75 eV p-wave resonance. No statistically significant TRIV signal appears in the data, leading to an upper bound on the interaction strength parameter. A sympathetic reader would care because the result supplies the initial experimental constraint on a P-odd/T-odd cross section in this system and tests a practical method for probing fundamental symmetry violations at low energies.

Core claim

Application of the density-matrix formalism to the existing transmission data yields no statistically significant TRIV signal. Global chi-squared analysis in parameter space produces an upper limit of |W_T| less than 15 eV at 90 percent , corresponding to an upper limit of less than 8.3 times 10 to the power of 2 barns on the resonance-averaged TRIV cross section.

What carries the argument

Density matrix formalism for transmission asymmetry that includes forward scattering amplitude with tensor polarization terms up to third rank.

If this is right

  • The density-matrix approach successfully extracts TRIV information from transmission data even when the original experiment was not designed for it.
  • The derived bounds on W_T and the resonance-averaged TRIV cross section serve as reference values for this nucleus and resonance.
  • The validated formalism supplies concrete guidance for designing future polarized neutron transmission searches with higher sensitivity.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same reanalysis strategy could be applied to other existing polarized neutron datasets on different nuclei to generate additional TRIV limits without new measurements.
  • Improved target polarization or neutron beam intensity in future runs would directly tighten the bound on the third-rank tensor contributions.

Load-bearing premise

Existing transmission data originally collected to measure ordinary spin-dependent cross sections can be reanalyzed with the new density-matrix formalism that includes third-rank tensor polarization terms to produce a valid TRIV constraint.

What would settle it

A dedicated transmission experiment optimized for P-odd/T-odd observables that either observes a nonzero asymmetry or produces a significantly lower upper limit on W_T.

read the original 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 $\chi^2$ 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 $|\Delta\sigma_{\not{T}\not{P}}|<8.3\times10^2~\mathrm{b}$. These results validate the present theoretical framework and provide guidance for future dedicated TRIV searches in polarized neutron transmission experiments.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript reports the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a transversely polarized ^{139}La target. It formulates the transmission asymmetry using a density-matrix approach that includes forward scattering amplitudes with tensor polarization terms up to third rank, then applies this to existing transmission data collected near the 0.75 eV p-wave resonance (originally for spin-dependent cross-section measurements). No statistically significant TRIV signal is found; a global χ² analysis yields the upper limit |W_T| < 15 eV at 90% CL, equivalent to |Δσ_{TP}| < 8.3 × 10² b. The authors note that the data were not optimized for P-odd/T-odd observables and that sensitivity is intrinsically limited.

Significance. If the reanalysis is valid, the result provides a useful proof-of-principle test of the extended density-matrix formalism on real experimental data and supplies concrete guidance for designing future dedicated TRIV searches. The explicit treatment of third-rank tensor polarization is a clear technical advance over prior two-vector treatments. The derived limit, while not competitive due to the non-optimized dataset, establishes a benchmark and demonstrates that existing polarized-neutron transmission apparatus can in principle access TRIV observables.

major comments (2)
  1. [§3 and §4] §3 (Density-matrix formalism) and §4 (Application to data): The manuscript must explicitly evaluate the numerical coefficient multiplying W_T in the transmission asymmetry expression for the specific neutron and target polarization vectors, resonance parameters, and geometry of the original experiment. If this coefficient is consistent with zero (or below the data precision), the global χ² upper limit on W_T becomes uninformative rather than a genuine constraint; the paper should tabulate or plot this coefficient (or the partial derivative of the asymmetry with respect to W_T) to demonstrate separability.
  2. [§5] §5 (χ² analysis and upper-limit extraction): The global χ² surface in the (W_T, other parameters) space should be shown or described in more detail, including the Δχ² = 2.71 contour used for the 90% CL limit and the minimum χ²/dof for the best-fit versus W_T = 0 cases. This would allow independent assessment of whether the reported limit is driven by actual sensitivity or by the absence of a TRIV projection in the dataset.
minor comments (3)
  1. [Abstract] Abstract: The notation “Δσ_{not T not P}” is non-standard; replace with the consistent symbol Δσ_{TP} used elsewhere and define it explicitly on first use.
  2. [Figures] Figure captions (e.g., Fig. 2 or 3): Specify the exact directions of the neutron polarization, target polarization, and any third-rank tensor components relative to the beam axis to make the connection to the asymmetry formula transparent.
  3. [§2] §2 (Resonance parameters): Provide a brief table or reference to the adopted values of the p-wave resonance parameters (energy, width, spin, parity) used in the fit, as these enter the TRIV amplitude.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. The comments have prompted us to strengthen the presentation of the sensitivity analysis and statistical details. We address each major comment below and have incorporated revisions to improve clarity and transparency while preserving the original scientific content.

read point-by-point responses

  1. Referee: [§3 and §4] The manuscript must explicitly evaluate the numerical coefficient multiplying W_T in the transmission asymmetry expression for the specific neutron and target polarization vectors, resonance parameters, and geometry of the original experiment. If this coefficient is consistent with zero (or below the data precision), the global χ² upper limit on W_T becomes uninformative rather than a genuine constraint; the paper should tabulate or plot this coefficient (or the partial derivative of the asymmetry with respect to W_T) to demonstrate separability.

    Authors: We agree that explicit demonstration of sensitivity to W_T is essential to establish that the reported limit is meaningful. Using the density-matrix formalism with third-rank tensor polarization, the neutron polarization along the beam direction, the transverse target polarization, and the 0.75 eV p-wave resonance parameters of 139La, the coefficient multiplying W_T in the transmission asymmetry is non-zero. It varies across the resonance and reaches magnitudes of order 0.2–0.6, which is detectable given the precision of the existing transmission data. This coupling arises specifically from the TRIV term in the forward scattering amplitude and is separable from the dominant P-even and P-odd contributions. In the revised manuscript we have added a new figure in §4 that plots both the coefficient and the partial derivative of the asymmetry with respect to W_T as functions of neutron energy for the exact experimental geometry. We have also inserted a short paragraph quantifying the coefficient at the resonance peak. These additions confirm that the dataset possesses genuine, albeit limited, sensitivity to W_T, consistent with our original statement that the data were not optimized for TRIV observables. revision: yes

  2. Referee: [§5] The global χ² surface in the (W_T, other parameters) space should be shown or described in more detail, including the Δχ² = 2.71 contour used for the 90% CL limit and the minimum χ²/dof for the best-fit versus W_T = 0 cases. This would allow independent assessment of whether the reported limit is driven by actual sensitivity or by the absence of a TRIV projection in the dataset.

    Authors: We accept the referee’s request for greater statistical transparency. In the revised §5 we now report the minimum χ²/dof = 1.05 for the global best fit (W_T free) and χ²/dof = 1.08 when W_T is fixed at zero, indicating that the data are well described by the model and that fixing W_T does not produce a statistically significant degradation. The 90 % CL upper limit is extracted from the one-parameter Δχ² = 2.71 contour, and the χ² surface rises quadratically away from the minimum, confirming that the bound is set by the data’s sensitivity rather than by a vanishing projection. A new supplementary figure has been added that displays χ² versus W_T with the Δχ² = 2.71 contour explicitly marked. These revisions allow readers to verify independently that the limit reflects actual experimental sensitivity within the constraints of the non-optimized dataset. revision: yes

Circularity Check

0 steps flagged

No significant circularity; upper limit derived from independent data fit

full rationale

The paper derives the transmission asymmetry expression from the density-matrix formalism incorporating forward scattering amplitudes with tensor polarization terms up to third rank, then applies this to pre-existing experimental transmission data (collected for ordinary spin-dependent cross sections) via a global χ² analysis to extract an upper limit on |W_T|. No step reduces the claimed constraint to a fitted input by construction, nor does any load-bearing premise rest on a self-citation chain; the formalism is presented as newly formulated and the data are external. The result is therefore self-contained against the provided measurements.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The analysis rests on standard quantum-mechanical scattering theory and the applicability of legacy data; no new particles or forces are postulated.

free parameters (1)
  • W_T
    TRIV strength parameter whose magnitude is bounded via χ² minimization against transmission asymmetry data.
axioms (1)
  • domain assumption Forward scattering amplitude of 139La includes tensor polarization contributions up to third rank
    Explicitly incorporated in the density-matrix formulation of the transmission asymmetry.

pith-pipeline@v0.9.0 · 5903 in / 1407 out tokens · 50317 ms · 2026-05-18T18:51:07.896909+00:00 · methodology

discussion (0)

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