Abstract
Many standard model extensions that address the hierarchy problem contain Dirac-fermion partners of the top quark, which are typically expected around the TeV scale. Searches for these vector-like quarks mostly focus on their decay into electroweak gauge bosons and Higgs plus a standard model quark. In this article, backed by models of composite Higgs, we propose a set of simplified scenarios, with effective Lagrangians and benchmarks, that include more exotic decay channels, which modify the search strategies and affect the bounds. Analysing several classes of underlying models, we show that exotic decays are the norm and commonly appear with significant rates. All these models contain light new scalars that couple to top partners with charge 5/3, 2/3, and −1/3.
Highlights
In this article, we use the framework of partial compositeness and a composite pNGB Higgs as a guide for characterising the phenomenology of VLQs
As a second example for exotic decays of a charge 5/3 top partner, we consider a model with a colour-neutral, electrically charged scalar φ+. The latter arises for example as part of the pNGB spectrum in composite Higgs models with SU(5)/SO(5) breaking [2], where it is accompanied by a doubly-charged scalar
We further focus on model M9, which has a smaller value for fχ, in order to maximise the branching ratios (BRs) X5/3 → bπ6 with respect to the standard one X5/3 → tW +
Summary
As a first simplified scenario, we consider a model with a charge 2/3 top partner T and a lighter pseudo-scalar a. As a second simplified scenario, we consider a model with a top partner T with charge 2/3 that does not mix with the SM top, and a lighter pseudo-scalar η This situation is realised, for example, in composite Higgs models based on SU(4)/Sp(4) breaking, where η is the additional singlet and the top partner couplings respect a parity associated with η. As a second example for exotic decays of a charge 5/3 top partner, we consider a model with a colour-neutral, electrically charged scalar φ+ The latter arises for example as part of the pNGB spectrum in composite Higgs models with SU(5)/SO(5) breaking [2] (see section 3.4), where it is accompanied by a doubly-charged scalar. One interesting final state that we want to mention is due to decays of a charge 2/3 partner in the charged scalar leading to T → b φ+ → bW +γ, which is similar to a top final state with the addition of a hard photon
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