Analysis of tbW and ttZ couplings from CLEO and LEP/SLC data

TL;DR

The paper investigates possible deviations in the top-quark couplings to $W$ and $Z$ bosons using a non-linear electroweak chiral Lagrangian with four parameters $\kappa_{L}^{\rm NC}$, $\kappa_{R}^{\rm NC}$, $\kappa_{L}^{\rm CC}$, and $\kappa_{R}^{\rm CC}$. By combining the NLO prediction for $B\to X_s\gamma$ with CLEO and LEP/SLC data, it derives very tight bounds on these couplings and reveals strong correlations among them, especially between neutral- and charged-current sectors. The results show that the observed $Z$-pole asymmetries are unlikely to be reconciled with the SM by these anomalous couplings alone, and any sizeable effect would require $\kappa_{L}^{\rm CC}$ of order $10^{-1}$ with a specific $m_t$ near $172$ GeV. The study also highlights potential avenues for future experimental tests, including a $\sim5\%$ sensitivity to $\kappa_{L}^{\rm CC}$ via single-top production at Tevatron Run-2.

Abstract

We update the constraints on anomalous dimension four $\tbw$ and $\ttz$ couplings by using CLEO $b \ra s γ$ and LEP/SLC precision $Z$-pole data. It is found that the data imposes very stringent bounds on them. Moreover, the $2 σ$ pull from SM predictions of $A_{LR}$(hadrons), $A_b$ and $A_{FB}$(b) have little chance of being explained by the strongly constrained anomalous couplings.

Figures (3)

• Figure 1: Allowed region for $\kappa_{R}^{CC}$ from the measurement of the branching ratio of $b \to s \gamma$, between the upper and lower solid line curves. Region between dashed lines defined for the asymmetry $A_{\rm CP}^{b\to s\gamma} \leq 0.25$.
• Figure 2: Allowed regions for $\kappa_{R}^{NC}$ and $\kappa_{L}^{NC}$ at different values of $\kappa_{L}^{CC}$.
• Figure 3: Asymmetries of $Z$ decays as a function of $\kappa_{L}^{CC}$. The narrow regions between solid lines come from the small possible variation of the correlated $\kappa_{R}^{NC}$ and $\kappa_{L}^{NC}$ couplings.