SMEFT Constraints on Top Electroweak Couplings and a Composite Higgs Interpretation

Abstract

Recent LHC measurements of $tWZ$, $t\bar t Z$ and single top production provide complementary probes of top quark electroweak interactions. We incorporate these measurements into a global Standard Model effective field theory analysis of 46 independent dimension-six Wilson coefficients at linear order. The updated fit remains compatible with the Standard Model at $95\%$ CL, and favors negative values of $C_{Hq}^{(-)}\equiv C_{Hq}^{(1)}-C_{Hq}^{(3)}$ and $C_{Ht}$, with central values suggestive of TeV scale new physics. We also present an effective composite Higgs model based on a two-site description that realizes this pattern of Wilson coefficients through partial compositeness. The model includes custodial symmetry and protection of the $Z\bar b_L b_L$ coupling. A fermion bitriplet generates the required left-handed shift, and an additional embedding of the right-handed top allows negative contributions to $C_{Ht}$ from both fermion and spin-one exchange. Scanning four right-handed top embeddings, we find parameter regions within the $68\%$ CL contour that are compatible with electroweak precision data, Higgs coupling measurements and applicable direct searches. The $T$ parameter limits the negative fermionic contribution to $C_{Ht}$, favoring a spin-one contribution of comparable or greater magnitude. Larger $SU(2)_R$ representations extend the reach toward negative $C_{Ht}$, and also enhance four-quark interactions that increase the four-top cross section, in the direction favored by current measurements. The mechanism connects the preferred top electroweak shifts to complementary tests through single production of quarks with electric charge $-4/3$, modified Higgs couplings and four-top production.