Odd Top Partners at the LHC

TL;DR

This paper explores a novel class of top partner models at the LHC where top partners are odd under a conserved parity, leading to different decay channels and weaker experimental bounds, thus impacting naturalness considerations.

Contribution

It introduces a simplified model with parity-odd top partners decaying into new scalars, and provides bounds on their masses based on LHC searches, including an explicit realization in the Oddest Little Higgs model.

Findings

Top partners can be lighter than previously constrained in conventional models.

Bounds on top partner mass are weaker when decays involve parity-odd scalars.

A 500 GeV top partner is allowed if the parity-odd scalar mass is between 325 and 500 GeV.

Abstract

LHC searches for fermionic top partners $T$ focus on three decay topologies: $T\to bW$, $T\to tZ$, and $T\to th$. However, top partners may carry new conserved quantum numbers that forbid these decays. The simplest possibility is a conserved parity, under which the top partner is odd and all SM states are even. In this case, decays of top partners may involve new particle-odd scalars, leading to signal topologies more commonly associated with supersymmetry, either with or without R-parity conservation. We study a simplified model in which this possibility is realized, and estimate the bounds on the top partner mass in this model implied by LHC searches for supersymmetry. We find that the bounds can be significantly weaker than in the conventional top partner decay scenario. For example, if the new parity is exact, a 500 GeV top partner is allowed as long as the lightest parity-odd scalar mass is between 325 and 500 GeV. The lower allowed top partner mass reduces the need for fine-tuning in the Higgs mass parameter, compared to the conventional decay scenario. We also present an explicit model, the Oddest Little Higgs, which exhibits this phenomenology.