Abstract
We analyze rare charm baryon decays within the standard model and beyond. We identify all null test observables in unpolarized Λc→ pℓ+ℓ−, ℓ = e, μ decays, and study the new physics sensitivities. We find that the longitudinal dilepton polarization fraction FL is sensitive to electromagnetic dipole couplings {C}_7^{left(prime right)} , and to the right-handed 4-fermion vector coupling {C}_9^{left(prime right)} . The forward-backward asymmetry, AFB, due to the GIM-suppression a standard model null test already, probes the left-handed axialvector 4-fermion coupling C10; its CP–asymmetry, {A}_{mathrm{FB}}^{mathrm{CP}} probes CP-violating phases in C10. Physics beyond the standard model can induce branching ratios of dineutrino modes Λc→ pν overline{nu} up to a few times 10−5, and one order of magnitude smaller if lepton flavor universality is assumed, while standard model rates are negligible. Charged lepton flavor violation allows for striking signals into e±μ∓ final states, up to 10−6 branching ratios model-independently, and up to order 10−8 in leptoquark models. Related three-body baryon decays Ξc→ Σℓℓ, Ξc→ Λℓℓ and Ωc→ Ξℓℓ offer similar opportunities to test the standard model with |∆c| = |∆u| = 1 transitions.
Highlights
Rare |∆c| = |∆u| = 1 processes are strongly Glashow-Iliopoulos-Maiani (GIM)-suppressed, a feature that blocks access to standard model (SM) short-distance contributions in simple observables such as branching ratios: either because of overwhelming resonance effects, or because branching ratios are too small [1]
We study Λc → p + −, = e, μ decays, and null test processes with charged lepton flavor violation Λc → pe±μ∓, and into dineutrinos Λc → pνν
The achievable upper limits on the branching ratios depend on charged lepton flavor, a search for rare charm dineutrino modes tests the SM, it tests at the same time lepton universality (LU), or charged lepton flavor conservation [14, 37]
Summary
Rare |∆c| = |∆u| = 1 processes are strongly Glashow-Iliopoulos-Maiani (GIM)-suppressed, a feature that blocks access to standard model (SM) short-distance contributions in simple observables such as branching ratios: either because of overwhelming resonance effects (semileptonic modes), or because branching ratios are too small (dineutrino modes) [1] These very characteristics, on the other hand, in addition to the approximate symmetries of the SM, give directions to form clean null test observables and to probe for a broad range of new physics (NP) phenomena. As for the Ξc, Ωc decays no comparable form factor computations are available we employ the Λc → p ones together with SU(3)F flavor symmetries, if applicable While this can be improved in the future the impact of this approximation on the present study is sub-dominant as we are mostly interested in null tests, and the SM-related uncertainties are dominated by resonance effects.
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