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Null test BSM searches with rare charm baryon decays

Rare $\vert Δc \vert=\vert Δu \vert=1$ processes uniquely probe flavor in the Standard Model and beyond from the up-type quark sector. Opportunities to search for BSM physics in charm arise from the severe GIM suppression which kills SM contributions to leptonic axial-vector contributions and suppresses CP violation. Semileptonic decays of charmed hadrons offer a variety of clean null test observables, featuring new physics effects which are even enhanced by resonance contributions. In particular, angular observables in three- and self-analyzing four-body baryon decays, such as $Λ_c\to p \ell^+\ell^-$ and $Ξ_c^+\toΣ^+(\to pπ^0)\ell^+\ell^-$, $Ξ_c^0\toΛ^0\,(\to p π^-)\ell^+\ell^-$ and $Ω_c^0\toΞ^0\,(\to Λ^{0} π^0)\ell^+\ell^-$ disentangle possible new physics effects in electromagnetic dipole couplings $C_7^{(\prime)}$ and (axial-)vector 4-fermion ones $C^{(\prime)}_{9\,(10)}$. There is sensitivity to BSM couplings as small as $\sim 0.01$.