Light top partners and precision physics

TL;DR

This paper investigates how minimal composite Higgs models affect precision electroweak measurements, identifying new divergences and setting bounds on model parameters through detailed analysis of fermionic and gauge sector corrections.

Contribution

It introduces a comprehensive effective parametrization including light fermionic resonances, revealing a new divergence in the S parameter and analyzing their impact on electroweak observables.

Findings

A new logarithmic divergence in the S parameter from strong dynamics.

Lower bound on the sigma-model scale f > 750 GeV from oblique parameter fits.

Potential 5% deviation in Vtb and 10% in Z tL tL vertex due to compositeness.

Abstract

We analyze the corrections to the precision EW observables in minimal composite Higgs models by using a general effective parametrization which also includes the lightest fermionic resonances. A new, possibly large, logarithmically divergent contribution to S is identified, which comes purely from the strong dynamics. It can be interpreted as a running of S induced by the non-renormalizable Higgs interactions due to the non-linear sigma-model structure. As expected, the corrections to the 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 sigma-model scale f > 750GeV. The corrections to the Z bL bL vertex coming from the lowest-order operators in the effective Lagrangian are finite and somewhat correlated to the corrections to 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 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 Vtb can be of order 5% while the distortion of the Z tL tL vertex can be 10%.