Minimally flavored colored scalar in $ \overline{B}\to {D^{{\left( * \right)}}}\tau \overline{\nu} $ and the mass matrices constraints

Abstract

The presence of a colored scalar that is a weak doublet with fractional electric charges of |Q| = 2/3 and |Q| = 5/3 with mass below 1 TeV can provide an explanation of the observed branching ratios in B → D ( * ) τν decays. The required combination of scalar and tensor operators in the effective Hamiltonian for b → cτν is generated through the t-channel exchange. We focus on a scenario with a minimal set of Yukawa couplings that can address a semitauonic puzzle and show that its resolution puts a nontrivial bound on the product of the scalar couplings toτ b andcν. We also derive additional constraints posed by Z → bb, muon magnetic moment, lepton flavor violating decays µ → eγ, τ → µγ, τ → eγ, and τ electric dipole moment. The minimal set of Yukawa couplings is not only compatible with the mass generation in an SU (5) unification framework, a natural environment for colored scalars, but specifies all matter mixing parameters except for one angle in the up-type quark sector. We accordingly spell out predictions for the proton decay signatures through gauge boson exchange and show that p → π 0 e + is suppressed with respect to p → K +ν and even p → K 0 e + in some parts of available parameter space. Impact of the colored scalar embedding in 45-dimensional representation of SU (5) on lowenergy phenomenology is also presented. Finally, we make predictions for rare top and charm decays where presence of this scalar can be tested independently.ArXiv ePrint: 1306.6493( 1.2) Both results are consistent with previous measurements performed by Belle [2] but are larger than the SM values of R * ,SM τ / = 0.252(3) and R SM τ / = 0.296(16) with 3.4 σ significance when the two observables are combined (see Ref. [3]). If confirmed these results might point to NP in (semi)tauonic b → c transitions. We accordingly consider a plausible scenario where the presence of one light color triplet, weak doublet, scalar leptoquark (LQ) state addresses the aforementioned discrepancy through a minimal set of couplings to matter. The minimality is motivated by the fact that the presence of this state was shown to be too severely constrained by rare decays of charm and strange mesons and of tau lepton to be able to affect c → s + ν significantly [4,5]. It also renders our analysis more manageable.The colored scalar we consider appears in various studies of possible extensions of the SM [5-10]. In particular, it can be embedded in 45-dimensional scalar representation of SU (5) [11][12][13][14][15] to help provide viable unification of the SM interactions. Inclusion of additional scalar representations can result in two important improvements over the original SU (5) setup [16]. Namely, the mass relations between down-type quarks and charged leptons can be corrected [11] and strong, weak and electromagnetic interactions can unify [12][13][14][15]. We thus investigate whether the minimal set of Yukawa couplings we posit can be made compatible with the mass generation mechanism within a simple SU (5) setup that relies on the use of 45-dimensional...

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