Minimally flavored colored scalar in $\bar B \to D^{(*)} τ\bar ν$ 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 $B \to D^{(*)} τ\bar ν$ decays. The required combination of scalar and tensor operators in the effective Hamiltonian for $b \to c τ\bar ν$ 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 to $\bar τb$ and $\bar c ν$. We also derive additional constraints posed by $Z \to b\bar b$, muon magnetic moment, lepton flavor violating decays $μ\to e γ$, $τ\to μγ$, $τ\to e γ$, 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 $p \rightarrow π^0 e^+$ is suppressed with respect to $p \rightarrow K^+ \barν$ and even $p \rightarrow K^0 e^+$ in some parts of available parameter space. Impact of the colored scalar embedding in 45-dimensional representation of SU(5) on low-energy phenomenology is also presented. Finally, we make predictions for rare top and charm decays where presence of this scalar can be tested independently.