Impact of $$b \rightarrow s \ell \ell $$ anomalies on rare charm decays in non-universal $$Z'$$ models

In this work, we study the impact of $$b \rightarrow s \ell \ell $$ b → s ℓ ℓ , $$B_s - \bar{B_s}$$ B s - B s ¯ mixing and neutrino trident measurements on observables in decays induced by $$c \rightarrow u $$ c → u transition in the context of a non-universal $$Z'$$ Z ′ model which generates $$C^{\mathrm{NP}}_{9} <0$$ C 9 NP < 0 and $$C^{\mathrm{NP}}_9 = - \,C^{\mathrm{NP}}_{10} $$ C 9 NP = - C 10 NP new physics scenarios at the tree level. We inspect the effects on $$D^0 \rightarrow \pi ^0 \nu {\bar{\nu }}$$ D 0 → π 0 ν ν ¯ , $$D^+ \rightarrow \pi ^+ \nu {\bar{\nu }}$$ D + → π + ν ν ¯ and $$B_c \rightarrow B^+ \nu {\bar{\nu }} $$ B c → B + ν ν ¯ decays which are induced by the quark level transition $$c \rightarrow u \nu {\bar{\nu }}$$ c → u ν ν ¯ . The fact that the branching ratios of these decays are negligible in the standard model (SM) and the long distance effects are relatively smaller in comparison to their charged dileptons counterparts, they are considered to provide genuine null-tests of SM. Therefore the observation of these modes at the level of current as well as planned experimental sensitivities would imply unambiguous signature of new physics. Using the constraints on $$Z'$$ Z ′ couplings coming from a combined fit to $$b \rightarrow s \ell \ell $$ b → s ℓ ℓ , $$\varDelta M_s$$ Δ M s and neutrino trident data, we find that any meaningful enhancement over the SM value is ruled out in the considered framework. The same is true for $$D - {\bar{D}}$$ D - D ¯ mixing observable $$\varDelta M_D$$ Δ M D along with $$D^0 \rightarrow \mu ^+ \mu ^-$$ D 0 → μ + μ - and $$D^+ \rightarrow \pi ^+ \mu ^+ \mu ^-$$ D + → π + μ + μ - decay modes which are induced through $$c \rightarrow u \mu ^+ \mu ^-$$ c → u μ + μ - transition.

Probing for new physics with rare charm baryon (Λc, Ξc, Ωc) decays

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 $$ {C}_7^{\left(\prime \right)} $$ C 7 ′ , and to the right-handed 4-fermion vector coupling $$ {C}_9^{\left(\prime \right)} $$ C 9 ′ . 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, $$ {A}_{\mathrm{FB}}^{\mathrm{CP}} $$ A FB CP probes CP-violating phases in C10. Physics beyond the standard model can induce branching ratios of dineutrino modes Λc→ pν$$ \overline{\nu} $$ ν ¯ 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.

Theoretical and experimental status of rare charm decays

Rare charm decays offer the unique possibility to explore flavor-changing neutral-currents in the up-sector within the Standard Model and beyond. Due to the lack of effective methods to reliably describe its low energy dynamics, rare charm decays have been considered as less promising for long. However, this lack does not exclude the possibility to perform promising searches for New Physics per se, but a different philosophy of work is required. Exact or approximate symmetries of the Standard Model allow to construct clean null-test observables, yielding an excellent road to the discovery of New Physics, complementing the existing studies in the down-sector. In this review, we summarize the theoretical and experimental status of rare charm [Formula: see text] transitions, as well as opportunities for current and future experiments such as LHCb, Belle II, BES III, the FCC-ee and proposed tau-charm factories. We also use the most recent experimental results to report updated limits on lepton-flavor conserving and lepton-flavor violating Wilson coefficients.