Tau polarization in baryonic semileptonic decays supplies the leading sensitivity to lift EFT degeneracies unresolved by meson measurements in b to c tau nu transitions.
Angular Analysis of the Decay $\Lambda_b \to \Lambda (\to N \pi) \ell^+\ell^-$
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abstract
We study the differential decay rate for the rare $\Lambda_b \to \Lambda (\to N \pi)\ell^+\ell^-$ transition, including a determination of the complete angular distribution, assuming unpolarized $\Lambda_b$ baryons. On the basis of a properly chosen parametrization of the various helicity amplitudes, we provide expressions for the angular observables within the Standard Model and a subset of new physics models with chirality-flipped operators. Hadronic effects at low recoil are estimated by combining information from lattice QCD with (improved) form-factor relations in Heavy Quark Effective Theory. Our estimates for large hadronic recoil -- at this stage -- are still rather uncertain because the baryonic input functions are not so well known, and non-factorizable spectator effects have not been worked out systematically so far. Still, our phenomenological analysis of decay asymmetries and angular observables for $\Lambda_b \to \Lambda (\to N \pi)\ell^+\ell^-$ reveals that this decay mode can provide new and complementary constraints on the Wilson coefficients in radiative and semileptonic $b \to s$ transitions compared to the corresponding mesonic modes.
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Perturbative QCD calculations find that branching fractions for Ξ_b to Ξ lepton-pair decays lie within LHCb reach and that angular observables plus a specific ratio can constrain Wilson coefficients and extract |V_td/V_ts| independently of mesonic modes.
The LFU ratio R_Λ^{τ/μ} for Λ_b → Λ τ⁺τ⁻ is predicted in the SM with below-10% uncertainty and can be enhanced by several orders of magnitude in EFT models motivated by b→cτν and b→sμμ anomalies.
Predictions for Br(Λ_b → Λ^{(*)} ν ν̄) are 2.07 times the SM value with new physics scale constrained to 2.04–11.76 TeV at 1σ, plus a sum rule linking baryonic and mesonic modes.
citing papers explorer
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Lifting Effective-Field-Theory Degeneracies in Semileptonic Heavy-Baryon Decays
Tau polarization in baryonic semileptonic decays supplies the leading sensitivity to lift EFT degeneracies unresolved by meson measurements in b to c tau nu transitions.
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Semileptonic neutral current decays of $\Xi_b$ with dileptons or dineutrinos in the final state
Perturbative QCD calculations find that branching fractions for Ξ_b to Ξ lepton-pair decays lie within LHCb reach and that angular observables plus a specific ratio can constrain Wilson coefficients and extract |V_td/V_ts| independently of mesonic modes.
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Probing Lepton Flavour Universality with $\Lambda_b$ decays to $\tau^+\tau^-$ final states
The LFU ratio R_Λ^{τ/μ} for Λ_b → Λ τ⁺τ⁻ is predicted in the SM with below-10% uncertainty and can be enhanced by several orders of magnitude in EFT models motivated by b→cτν and b→sμμ anomalies.
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$\Lambda_b\to\Lambda^{(*)}\nu{\bar\nu}$ and $b\to s$ $B$ decays
Predictions for Br(Λ_b → Λ^{(*)} ν ν̄) are 2.07 times the SM value with new physics scale constrained to 2.04–11.76 TeV at 1σ, plus a sum rule linking baryonic and mesonic modes.
- Comprehensive analyses of rare $ \Lambda_b \rightarrow \Lambda \ell^+ \ell^-$, $\Sigma_b \rightarrow \Sigma \ell^+ \ell^-$ and $\Xi_b \rightarrow \Xi \ell^+ \ell^-$ decays in 2HDM