Time to Go Beyond Triple-Gauge-Boson-Coupling Interpretation of $W$ Pair Production

$W$ boson pair production processes at $e^{+}{e}^{-}$ and $pp$ colliders have been conventionally interpreted as measurements of $WWZ$ and $WW\gamma$ triple gauge couplings (TGCs). Such an interpretation is based on the assumption that new physics effects other than anomalous TGCs are negligible. While this “TGC dominance assumption” was well motivated and useful at LEP2 thanks to precision electroweak constraints, it is already challenged by recent LHC data. In fact, contributions from anomalous $Z$ boson couplings that are allowed by electroweak precision data but neglected in LHC analyses, which are enhanced at high energy, can even dominate over those from the anomalous TGCs considered. This limits the generality of the anomalous TGC constraints derived in current analyses and necessitates extension of the analysis framework and a change of physics interpretation. The issue will persist as we continue to explore the high-energy frontier. We clarify and analyze the situation in the effective field theory framework, which provides a useful organizing principle for understanding standard model deviations in the high-energy regime.

Figure 1

Fractional shift in LEP2 $e^{+}{e}^{-}\to W^{+}{W}^{-}\to qq\ell \nu$ differential cross section induced by each of the anomalous couplings in Eq. (8), compared with experimental uncertainties (gray dotted line) reported in Ref. [2]. Assuming lepton flavor universality, effects of the anomalous TGCs being constrained (solid lines) [35] are seen to dominate over those of $Zff$ vertex and $W$ mass corrections (dashed lines), even when the latter are set to maximum values allowed by EWPD [3, 34], providing justification for the conventional TGC analysis procedure.

Figure 2

Leading lepton $p_T$ distribution of 8 TeV LHC $W^{+}{W}^{-}$ events in the $e\mu$ channel when each anomalous coupling is turned on individually, compared with experimental data (dots with error bars) and SM predictions (gray dotted line). The latter, together with non-$WW$ backgrounds (gray shaded area), are taken from Ref. [8]. Effects of anomalous TGCs being considered in recent TGC fits (solid lines) are clearly not dominant over those of $\delta g_R^{Zu}$, $\delta g_R^{Zd}$ (dashed lines) consistent with EWPD, calling for extension of the conventional TGC analysis procedure.