Abstract
Driven by the recent experimental hints of lepton-flavor-universality violation in the bottom-quark sector, we consider a simple extension of the Standard Model (SM) with an additional vector leptoquark $V_{\rm LQ}({\bf 3},{\bf 1},2/3)$ and a scalar diquark $S_{\rm DQ}({\bf 6},{\bf 1},4/3)$ under the SM gauge group $SU(3)_c\times SU(2)_L\times U(1)_Y$, in order to simultaneously explain the $b \to s \ell^+ \ell^-$ (with $\ell=e,\mu$) and $b \to c l^- \bar \nu_l$ (with $l=e,\mu,\tau$) flavor anomalies, as well as to generate small neutrino masses through a two-loop radiative mechanism. We perform a global fit to all the relevant and up-to-date $b \to s \ell^+ \ell^-$ and $b \to c l^- \bar \nu_l$ data under the assumption that the leptoquark couples predominantly to second and third-generation SM fermions. We then look over the implications of the allowed parameter space on lepton-flavor-violating $B$ and $\tau$ decay modes, such as $B_s \to l^+_i l^-_j, \ B \to K^{(*)} l^+_i l^-_j, \ B_s \to \phi l^+_i l^-_j$, $\Upsilon(nS) \to \mu \tau$ and $\tau \to \mu \gamma$, $\tau \to \mu \phi (\eta^{(\prime)})$, respectively. Minimally extending this model by adding a fermion singlet $\chi({\bf 1},{\bf 1},0)$ also explains the ANITA anomalous upgoing events. Furthermore, we provide complementary constraints on leptoquark and diquark couplings from high-energy collider and other low-energy experiments to test this model.
Highlights
Driven by the recent experimental hints of lepton-flavor-universality violation in the bottom-quark sector, we consider a simple extension of the Standard Model (SM) with an additional vector leptoquark VLQð3; 1; 2=3Þ and a scalar diquark SDQð6; 1; 4=3Þ under the SM gauge group SUð3Þc × SUð2ÞL × Uð1ÞY, in order to simultaneously explain the b → slþl− and b → cl−νl flavor anomalies, as well as to generate small neutrino masses through a two-loop radiative mechanism
With the aim of understanding the experimental observations linked with both types of processes in a common framework, here we consider a simple extension of the SM by adding a single vector leptoquark (VLQ) VLQ which transforms as ð3; 1; 2=3Þ under the SM gauge group SUð3Þc × SUð2ÞL × Uð1ÞY
There is a two-loop contribution to light neutrino masses, similar to a colored variant of the Zee-Babu model [149,150], where the lepton doublet is replaced by up-type quark while the singly and doubly charged scalars are replaced by VLQ and SDQ, respectively
Summary
Over the last few years, several B-physics experiments, such as the LHCb [1,2,3,4,5,6,7,8,9,10], as well as the B factories BABAR [11,12] and Belle [13,14,15,16,17], have reported a number of deviations from the Standard Model (SM) expectations at the level of ð2–4Þσ [18] in the rare flavor-changing neutralcurrent (NC) and charged-current (CC) semileptonic B-meson decays involving the quark-level transitions b → slþl− (with l 1⁄4 e, μ) and b → cl−νl (with l1⁄4e, μ, τ), respectively, which provide intriguing hints of new physics (NP) beyond the SM (BSM). The flavor anomalies have been investigated in the VLQ scenario [20,22,23,24,27,28,29,30,35,38,39,40,42,45, 46,57,62,81,82,83,84,85,86,87,88,89,90,91] We update this discussion with the latest experimental data and minimally extend the VLQ model by introducing a scalar diquark (SDQ) SDQð6; 1; 4=3Þ, to explain the light neutrino mass generation through a two-loop radiative mechanism. The loop functions for τ → μγ are provided in Appendix D
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