Chiral enhancement in the vector-like fourth family: Case of $b \to s \gamma$

TL;DR

This paper shows that a vector-like fourth family of quarks can significantly enhance the $b o s \gamma$ decay rate through chiral effects, leading to observable deviations from the Standard Model predictions.

Contribution

It introduces a novel mechanism of chiral enhancement in $b o s \gamma$ decay due to vector-like quarks, which was not previously considered in the Standard Model context.

Findings

Chiral enhancement factor can reach up to 40 for moderate Yukawa couplings.

Deviations from SM in $b o s \\gamma$ can occur with TeV-scale vector-like quarks.

$Br(\\overline{B} o X_s \\gamma)$ constrains the model strongly.

Abstract

We demonstrate that a vector-like fourth family of quarks induces a genuine chiral enhancement in $b\to s\gamma$, which is absent in the Standard Model (SM). The coexistence of doublet and singlet states allows the chirality flip to occur inside the loop, leading to contributions proportional to the heavy vector-like mass. The resulting amplitude is enhanced by a factor $\overline{\lambda}_d v_H/m_b$, which can be as large as $\mathcal{O}(40)$ for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for $\mathcal{O}(\mathrm{TeV})$ vector-like quark masses and small mixing angles. We find that $\mathrm{Br}(\overline{B}\to X_s\gamma)$ provides the most stringent constraint on this scenario among a wide range of precision observables.