One Loop Calculations of Rare B-decays Beyond the Standard Model

Abstract

Rare $B$-meson decays, suppressed in the Standard Model (SM) by the GIM mechanism, are sensitive probes of new physics. In particular, loop-level $b\to s$ transitions can reveal effects of heavy virtual particles such as vector-like quarks (VLQs). VLQs, which do not require electroweak symmetry breaking for mass generation, can mix with SM quarks and induce flavor-changing neutral currents (FCNCs). We investigate the impact of down-type iso-singlet VLQs on the rare decay $B^{+}\to K^{+}\nu \bar{\nu }$, a theoretically clean channel sensitive to new physics. The Belle-II collaboration recently reported a branching ratio of $[2.3\pm 0.5]\times 10^{-5}$, significantly higher than the SM prediction of $[0.45\pm 0.07]\times 10^{-5}$, suggesting possible new contributions. To constrain the relevant mixing parameter $U_{sb}$, we also analyze related decays such as $B_s\to {\mu }^{+}{\mu }^{-}$ and $B\to X_s{\mu }^{+}{\mu }^{-}$. Incorporating VLQ effects into the effective field theory framework, we compute modified Wilson coefficients ($C_7$, $C_9$, $C_{10}$, $C_L$) and predict enhanced branching ratios for $B\to X_s\nu \bar{\nu }$ and $B\to K\nu \bar{\nu }$. ${\chi }^2$ contour analyses indicate that VLQs can account for observed anomalies, providing a viable and testable extension to the SM.

Comments: 171 pages, 23 figures, PhD Thesis