Rare charm hadron decays offer clean null tests of the standard model that can reach branching ratios up to 10^{-3} for dark photons and 10^{-4} for ALPs in unconstrained parameter spaces.
Form Factors for Lambda_b -> Lambda Transitions in SCET
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abstract
We present a systematic discussion of Lambda_b -> Lambda transition form factors in the framework of soft-collinear effective theory (SCET). The universal soft form factor, which enters the symmetry relations in the limit of large recoil energy, is calculated from a sum-rule analysis of a suitable SCET correlation function. The same method is applied to derive the leading corrections from hard-collinear gluon exchange at first order in the strong coupling constant. We present numerical estimates for form factors and form-factor ratios and their impact on decay observables in Lambda_b -> Lambda mu^+ mu^- decays.
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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.
The work computes differential and total branching ratios plus forward-backward asymmetries for Λ_b → Λ ℓ⁺ℓ⁻, Σ_b → Σ ℓ⁺ℓ⁻ and Ξ_b → Ξ ℓ⁺ℓ⁻ in 2HDM Type III and contrasts them with SM predictions.
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Probing invisible particles with charm
Rare charm hadron decays offer clean null tests of the standard model that can reach branching ratios up to 10^{-3} for dark photons and 10^{-4} for ALPs in unconstrained parameter spaces.
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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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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
The work computes differential and total branching ratios plus forward-backward asymmetries for Λ_b → Λ ℓ⁺ℓ⁻, Σ_b → Σ ℓ⁺ℓ⁻ and Ξ_b → Ξ ℓ⁺ℓ⁻ in 2HDM Type III and contrasts them with SM predictions.