Light top partners and precision physics

Abstract

Abstract We analyze the corrections to the precision EW observables in minimal composite Higgs models by using a general effective parametrization that also includes the lightest fermionic resonances. A new, possibly large, logarithmically divergent contribution to $ \widehat{S} $ is identified, which comes purely from the strong dynamics. It can be interpreted as a running of $ \widehat{S} $ induced by the non-renormalizable Higgs interactions due to the non- linear σ-model structure. As expected, the corrections to the $ \widehat{T} $ parameter coming from fermion loops are finite and dominated by the contributions of the lightest composite states. The fit of the oblique parameters suggests a rather stringent lower bound on the σ-model scale f ≳ 750 GeV. The corrections to the Z $ \overline{b} $ L b L vertex coming from the lowest-order operators in the effective Lagrangian are finite and somewhat correlated to the corrections to $ \widehat{T} $ . Large additional contributions are generated by contact interactions with 4 composite fermions. In this case a logarithmic divergence can be generated and the correlation with $ \widehat{T} $ is removed. We also analyze the tree-level corrections to the top couplings, which are expected to be large due to the sizable degree of compositeness of the third generation quarks. We find that for a moderate amount of tuning the deviation in V tb can be of order 5% while the distortion of the Z $ \overline{t} $ L t L vertex can be 10%.