A naturally light Higgs without light top partners

Abstract

We demonstrate that the inclusion of a realistic lepton sector can relax significantly the upper bound on top partner masses in minimal composite Higgs models, induced by the lightness of the Higgs boson. To that extend, we present a comprehensive survey of the impact of different realizations of the fermion sectors on the Higgs potential, with a special emphasis on the role of the leptons. The non-negligible compositeness of the τ R in a general class of models that address the flavor structure of the lepton sector and the smallness of the corresponding FCNCs, can have a significant effect on the potential. We find that, with the τ R in the symmetric representation of SO(5), an increase in the maximally allowed mass of the lightest top partner of 1 TeV is possible for minimal quark setups like the MCHM 5,10 , without increasing the tuning. A light Higgs boson m H ∼ (100−200) GeV is a natural prediction of such models, which thus provide a new setup that can evade ultralight top partners without ad-hoc tuning in the Higgs mass. Moreover, we advocate a more minimal realization of the lepton sector than generally used in the literature, which still can avoid light partners due to its contributions to the Higgs mass in a different and very natural way, triggered by the seesaw mechanism. This allows to construct the most economical SO(5)/SO(4) composite Higgs models possible. Using both a transparent 4D approach, as well as presenting numerical results in the 5D holographic description, we demonstrate that, including leptons, minimality and naturalness do not imply light partners. Leptonic effects, not considered before, could hence be crucial for the viability of composite models.

Constraining fine tuning in composite Higgs models with partially composite leptons

“…of previous studies [21,22], it must be noted that our new measure will give larger absolute numbers for the fine tuning. To see why, consider three factors: arbitrary increase of parameters, arbitrary increase of observables, and genuinely more sensitive expressions (i.e.…”

“…Such a large number of observables and parameters requires a sophisticated treatment, both in the definition of the fine tuning measure, and the method used to find the small regions of the considerable parameter space volume that give rise to the correct SM behaviour. We approach the first problem by generalising the fine-tuning measure of [22] to cope with N observables, and in doing so we count the total fine tuning in a more accurate way than in [21] which leads to interesting conclusions. We solve the second problem by using a combination of the MultiNest implementation of the nested sampling algorithm, plus a second stage of MCMC sampling, to efficiently find the desired regions in our candidate MCHM parameter spaces.…”

“…These observables are the Higgs VEV and mass, and the masses of the top quark, bottom quark and tau lepton. The tau neutrino will be treated as massless, however certain representations of the lepton composite partners can realise a see-saw model [21]. In practise, the Higgs VEV only ever appears in the ratio v 2 /f 2 and hence we can simply rescale f to give the correct Higgs VEV instead of treating it as an extra input parameter.…”

“…Rather than make simplifying assumptions to reduce the complexity of the parameter space (as in [21], where a random sampling approach was used), we scan the full dimensionality of each model using the Multinest implementation of the nested sampling technique [38][39][40]. This has proven very successful in exploring complicated, multidimensional functions encountered in a range of cosmology and particle physics examples.…”

Neutrino masses from neutral top partners

“…See also Ref. [48] for pNGB composite Higgs models exploring the interplay between top partners, the lepton sector, and naturalness.…”