| Summary: | Abstract We investigate the ability of the S 3 scalar leptoquark to address the recent hints of lepton universality violation in B meson decays. The S 3 leptoquark with quantum numbers 3 ¯ , 3 , 1 / 3 $$ \left(\overline{\mathbf{3}},\mathbf{3},1/3\right) $$ naturally emerges in the context of an SU(5) GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark S 3 with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both R K(∗) and R D(∗) . We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed SU(5) model calls for a second leptoquark state, i.e., R ˜ 2 $$ {\tilde{R}}_2 $$ with quantum numbers (3 , 2 , 1/6), if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate R ˜ 2 $$ {\tilde{R}}_2 $$ ’s ability to offset adverse effects of S 3 and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light S 3 to τ leptons are preferred. We furthermore identify B → K ∗ ν ¯ ν $$ B\to {K}^{\left(\ast \right)}\overline{\nu}\nu $$ as the most sensitive channel to probe the preferred region of parameter space. Large couplings of S 3 to τ leptons are finally confronted with the experimental searches for τ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass ττ finalstates.
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