Lifting Effective-Field-Theory Degeneracies in Semileptonic Heavy-Baryon Decays

Abstract

Semileptonic heavy-baryon decays provide a sensitive probe of the helicity structure underlying possible lepton-flavor universality violation in $b\to c\tau {\bar{\nu }}_{\tau }$ transitions. We perform an effective-field-theory analysis of ${\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau }$ and related baryonic modes using lattice-QCD helicity form factors with full covariance propagation. Propagating meson-compatible EFT solutions into the baryonic observable space$(R_{{\Lambda }_c},P_{\tau },A_{\mathrm{FB}})$, we show that tau polarization provides the leading source of sensitivity for lifting EFT degeneracies that remain unresolved in current measurements of $R(D)$ and $R(D^{\ast })$. Vector-like and tensor-like solutions remain clustered near theStandard-Model prediction, whereas scalar-containing directions produce large polarization displacements and characteristic low-$q^2$ deformations of the normalized differential spectrum. A covariance-aware analysis demonstrates that baryonic polarization and differential observables provide complementary and independent information on the Lorentz structure of semileptonic new physics. $P_{\tau }({\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau })$ and the low-$q^2$ spectrum as particularly powerful probes for future tests of semitauonic flavor anomalies at LHCb and future flavor facilities.

Comments: 17 pages, 6 figures. Minor updates and supplementary numerical material may be included in future versions