Anomalous Wtb coupling effects in the weak radiative B-meson decay

TL;DR

The paper addresses how gauge-invariant dimension-six operators modifying the Wtb vertex influence the rare decay $\bar B \to X_s \gamma$. It constructs an EFT with operators $Q_{RR}$, $Q_{LL}$, $Q_{LRt}$, $Q_{LRb}$ and computes their loop-induced contributions to the dipole Wilson coefficients through a matching at a low scale $x = m_t^2/M_W^2$, yielding shifts in $C_7$ and $C_8$ that feed the branching ratio. The authors find that $v_R$ and $g_L$ are especially constrained due to an $m_t/m_b$ enhancement, while $\delta v_L$ and $g_R$ enter with smaller coefficients but remain relevant for LHC comparisons. They derive 95% CL bounds on the six parameters $(\delta v_L, v_R, g_L, g_R, C_7^{(p)}, C_8^{(p)})$ and discuss how future collider measurements could improve sensitivity, noting the importance of the assumed flavor alignment and the potential need to extend the analysis to additional FCNC observables. The results demonstrate that B-physics constraints provide complementary and often stronger bounds on certain anomalous couplings than direct top-quark measurements at the LHC, while highlighting areas for further study in related processes.

Abstract

We study the effect of anomalous Wtb couplings on the B -> Xs gamma branching ratio. The considered couplings are introduced as parts of gauge-invariant dimension-six operators that are built out of the Standard Model fields only. One-loop contributions from the charged-current vertices are assumed to be of the same order as the tree-level flavour-changing neutral current ones. Bounds on the corresponding Wilson coefficients are derived.

Figures (2)

• Figure 1: Diagrams with non-SM $b\to t$ vertices that contribute to $f_7^{g_{L,R}}(x)$. The pseudogoldstone boson is denoted by $\pi$.
• Figure 2: Diagrams with non-SM $\bar{t}t\gamma$ vertices that contribute to $f_7^{g_R}(x)$.