Discovering the top partners at the LHC using same-sign dilepton final states

TL;DR

The paper investigates pair production of top partners $T_{5/3}$ and $B$ in a custodially protected, pseudo-Goldstone Higgs framework, focusing on the same-sign dilepton channel to suppress Standard Model backgrounds. Using a two-site composite-Higgs model, it derives the heavy fermion couplings and decay patterns $F\to Wt$, then performs LO Monte Carlo simulations to assess discovery reach at the LHC with no full detector effects. It shows that, for $M=500\,\mathrm{GeV}$, discovery can occur with as little as $\sim 50$--$150\,\mathrm{pb}^{-1}$ depending on the partner content, and for $M=1\,\mathrm{TeV}$ with $\sim 10$--$50\,\mathrm{fb}^{-1}$, even though backgrounds and jet structures become more challenging. The work also provides mass-reconstruction strategies using hadronic $W$ and top decays, demonstrating resonant structures at $2M$ in the total invariant mass and at $M$ in the $Wt$ system when $T_{5/3}$ is present, and discusses the potential impact of single-production channels and detector effects on the ultimate discovery potential.

Abstract

A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500 GeV. We study the pair production and detection at the LHC of the top partners with electric charge Q=5/3 (T_{5/3}) and Q=-1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T_{5/3} fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to Wt, leading to a t\bar{t}WW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100 pb^{-1} (less than 20 fb^{-1}) of integrated luminosity for masses M=500 GeV (M=1TeV). In the case of the T_{5/3}, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large t\bar{t} background. Our analysis also directly applies to the search of 4th generation b' quarks.

Figures (10)

• Figure 1: Pair production of $T_{5/3}$ and $B$ to same-sign dilepton final states.
• Figure 2: Associated single production of $B$ and $T_{5/3}$ at the LHC.
• Figure 3: Production cross sections at the LHC for $T_{5/3}$ as functions of its mass. The dashed line refers to pair-production; the solid and the two dotted curves refer to single production for the three values of the coupling (from highest to lowest) $\lambda_{T_{5/3}}=Y_* \sin \varphi_{R}=4, 3, 2$. Cross sections for $B$ are given by the same curves for the same values of $\lambda_B=Y_* \cos\varphi_{L}\sin \varphi_{R}$.
• Figure 4: Fractions of signal and background events with a given number of jets for $E^{min}_T=30\,\text{GeV}$ and two different jet cone sizes: $\Delta R= 0.4$ (left plot), and $\Delta R= 0.7$ (right plot).
• Figure 5: Invariant mass of the first (left) and second (right) hardest jet for $\Delta R=0.4$ (top) and $\Delta R=0.7$ (bottom). All distributions are normalized to unit area.