Abstract
Many standard model extensions, including composite Goldstone Higgs models, predict vector-like fermionic top-partners at the TeV scale. The intensive search programmes by ATLAS and CMS focus on decays into a 3rd generation quark and an electroweak boson (W,Z,h). However, underlying models of partial compositeness contain additional states that give rise to exotic top partner decays. We consider a well-motivated scenario in which a charge-2/3 top-partner decays into a pseudo-scalar, T→ta, with a→gg or bb¯ dominating below the tt¯ threshold. We show that the constraints on the top partner mass from QCD pair production are substantially weakened, still allowing a top partner mass as light as 400 GeV.
Highlights
There has been raising interest in exploring “exotic” decays into non-Standard Model (SM) bosons: they could arise as new pseudo-Nambu-Goldstones in non-minimal cosets [16,17,18,19], as additional pseudo-scalars carrying QCD colour charge [17], as Dark Matter candidates [20, 21], or they could be a gluon or a photon [22, 23] or new scalar states [19, 24,25,26]
While in this work we are interested in composite Higgs models, top partners arise in other classes of models: extra dimensions, Little-Higgs models [31], and models where vector-like quarks are added “by hand” to the theory, like in supersymmetry [24] and two-Higgs-doublet models [19, 25, 26]
While the precise numerical values vary among different models, the branching ratios share the following features: above the ttthreshold, ma > 2mt, the dominant decay is into tops, while below the dominant decay is in a pair of gluons, a → gg, with decays into heavy fermions following, a → bb, τ +τ −
Summary
1.3 ÷ 1.4 TeV, depending on the pattern of decays to the standard channels t Z, t h, or b W , exclusively. All models with an underlying confining gauge group and underlying fermions predict the existence of a light pseudo-Nambu-Goldstone boson, SM singlet, a that, if lighter than 2mt, decays dominantly to gg or bb This provides a theoretically well-justified motivation to focus on the exotic decays T → t a → t gg and T → t a → t bb. A → jj decays resemble the hadronic decays of the W /Z and the Higgs in the SM, the sensitivity could be improved by hadronic taggers, either cut-based or via BDT/machine learning techniques, which could identify di-jet resonances (boosted or resolved) with different masses This problem is most severe for light a, with ma shown by mW , while the reach of heavier masses the T → t h are covered, as → t bb broadband search we recast.
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