A theoretical calculation predicts selected rare top-quark decays into mesons at tiny rates, with vector mesons overwhelmingly avoiding the right-handed helicity state. The work is designed to establish a Standard-Model baseline for these exclusive decays and provide a starting point for effective-field-theory interpretations of non-standard charged-current interactions.
The Ds+ channel is largest among the four displayed pseudoscalar cases, with a predicted branching ratio of 5.374 × 10−7. The corresponding figures are 1.605 × 10−7 for π+, 2.158 × 10−8 for D+, and 1.214 × 10−8 for K+. Even the largest predicted probability is below one decay in a million in this calculation.
The size of the prediction changes with the channel. The calculation reports partial widths of 3.784 × 10−7 GeV for Ds+, 1.13 × 10−7 GeV for π+, 1.52 × 10−8 GeV for D+, and 8.552 × 10−9 GeV for K+. The listed relative errors range from 0.3% for π+ to 10.3% for Ds+, with 1.1% for K+ and 6.1% for D+.
The channel matters
For vector mesons, the predicted branching ratios are also very small but vary substantially by channel. The Ds*+ mode is largest at 8.85245 × 10−7, followed by ρ+ at 4.12375 × 10−7, D*+ at 3.45154 × 10−8, and K*+ at 2.076664 × 10−8. Their calculated partial widths follow the same displayed ordering: 6.23412 × 10−7, 2.90405 × 10−7, 2.43066 × 10−8, and 1.46242 × 10−8 GeV, respectively.
The more distinctive result concerns how those vector mesons are polarized. In the massless-bottom limit used as an analytic check, the right-handed fraction, F+, goes to zero. The longitudinal fraction, F0, and the left-handed transverse fraction, F−, retain a dependence on the top-quark and meson masses.
Keeping the bottom-quark mass restores a nonzero right-handed component, but only at a highly suppressed level. The reported hierarchy is F0 approximately 1, F− much smaller than F0, and F+ much smaller than F−, with the right-handed fraction scaling as the square of the bottom-quark mass. Across the listed channels, F+ is 2.39047 × 10−8 for ρ+, 3.15962 × 10−8 for K*+, 1.59792 × 10−7 for D*+, and 1.59157 × 10−7 for Ds*+.
At the physical bottom-to-top mass ratio of about 0.0243, the study says F+ remains in the 10−8 to 10−7 range. The D*+ and Ds*+ values are slightly larger than those for ρ+ and K*+. For small mass ratios, the dependence is described as approximately quadratic, and the suppression is reported as stable under the quoted input variations.
Built as a baseline
The calculation treats separate t → bP and t → bV channels, using P for the selected pseudoscalar mesons and V for the vector mesons. It starts from the tree-level t → bW* → b(q q̄′) transition, uses factorized amplitudes, and keeps the exact meson-mass two-body phase space at leading order.
This is a channel calculation, not a participant study or an analysis of an observational dataset. Its listed inputs include a top-quark mass of 172.6 ± 0.27 GeV, a bottom-quark mass of 4.186 ± 0.0006 GeV, a displayed total top-quark width of 1.42 GeV with +19 and −15 annotations, and the Fermi constant, GF, given as 1.166 × 10−5 GeV−2. Uncertainties in quark and meson masses, meson decay constants, and CKM elements are propagated into the predicted widths and branching fractions.
The treatment also expands the off-shell W-boson propagator in an effective four-fermion description. The omitted terms are written as order q2/mW2 and are stated to be bounded by about 10−3 within that leading expansion. This sets the scale of one stated approximation, but does not replace a full calculation of higher-order effects.
A comparison, with caveats
As a cross-check, the authors compare the vector-channel results with another theoretical calculation. The reported relative deviations are 4.1% for ρ+, 1.4% for K*+, 10.2% for Ds*+, and 6.7% for D*+. The lighter channels therefore show closer agreement in the reported comparison, while the heavier channels show larger differences.
The comparison is presented as a normalization and order-of-magnitude check, including a proposed correction to a factor-of-four normalization discrepancy, rather than as evidence that the two calculations are numerically identical. The paper notes that the theoretical frameworks and higher-order treatments are not the same, which limits what can be inferred from the percentage differences.
The paper is therefore best treated as a theory baseline, not evidence that the predicted branching ratios have been observed or that anomalous tWb couplings have been measured or constrained. Its leading-order setup also does not provide a complete higher-order uncertainty estimate. The supplied front matter identifies the document as a preprint, arXiv:2608.25508v1, dated 26 August 2026.
The supplied text does not report funding or conflicts of interest.
Paper data and sources
Original title: Rare Exclusive Top Decays t , b M and Vector-Meson Helicity
Authors: M. Ahmadi, M. Monemzadeh, N. Tazimi
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text