Minimally flavored colored scalar in $\bar{B}\to D^{(\ast )}\tau \bar{\nu }$ 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 $B\to D^{(\ast )}\tau \bar{\nu }$ decays. The required combination of scalar and tensor operators in the effective Hamiltonian for $b\to c\tau \bar{\nu }$ 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 $\bar{\tau }b$ and $\bar{c}\nu$. We also derive additional constraints posed by $Z\to b\bar{b}$, muon magnetic moment, lepton flavor violating decays $\mu \to e\gamma$, $\tau \to \mu \gamma$, $\tau \to e\gamma$, and $\tau$ 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 $p\to {\pi }^0{e}^{+}$ is suppressed with respect to $p\to K^{+}\bar{\nu }$ and even $p\to K^0{e}^{+}$ 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.

Comments: 30 pp, 6 figures; v2: includes additional constraints