A Strong Sector at the LHC: Top Partners in Same-Sign Dileptons

TL;DR

The paper analyzes top partners predicted by partial compositeness in strongly coupled electroweak-broken scenarios, focusing on the LHC signature in the clean same-sign dilepton channel. It demonstrates that single production, enhanced by large couplings to the top, can dominate over pair production and extend the discovery reach up to about $M \nobreaksim 1.5$ TeV, with discovery achievable under realistic assumptions. Through detailed collider-level simulations, it develops a comprehensive set of observables—HT, lepton kinematics, $M(LL)$, $m_T$, $m_{T2}$, hemisphere asymmetries, and forward-jet tagging—to identify the top partners, distinguish between $T_{5/3}$ and $B$, and measure their masses and couplings. The analysis provides strategies to reconstruct hadronic tops, exploit charge asymmetries, and extract $M_T$, $M_B$, $lu_T$, and $lu_B$, offering a practical starting point for experimental studies in both Higgsless and composite-Higgs frameworks. Overall, the work establishes a plausible and testable pathway to confirm the strongly coupled EWSB paradigm at the LHC.

Abstract

Heavy partners of the top quark are a common prediction of many models in which a new strongly-coupled sector is responsible for the breaking of the EW symmetry. In this paper we investigate their experimental signature at the LHC, focusing on the particularly clean channel of same-sign dileptons. We show that, thank to a strong interaction with the top quark which allows them to be singly produced at a sizable rate, the top partners will be discovered at the LHC if their mass is below 1.5 TeV, higher masses being possible in particularly favorable (but plausible) situations. Being the partners expected to be lighter in both the Higgsless and Composite-Higgs scenarios, the one of same-sign dileptons is found to be a very promising channel in which these models could be tested. We also discuss several experimental signatures which would allow, after the discovery of the excess, to uniquely attribute it to the top partners production and to measure the relevant physical parameters, i.e. the top partners masses and couplings. We believe that our results constitute a valid starting point for a more detailed experimental study.

Figures (14)

• Figure 1: Typical single and pair production diagrams for ${T_{5/3}}$ and ${B}$ for signals with two positively charged leptons. We notice that for ${T_{5/3}}$ the leptons always comes from its decay, while for ${B}$ they originate in two different legs.
• Figure 2: Cross sections, summed over charge, for pair (plain) and single (dashed) production of ${T_{5/3}}$ (or $B$) as a function of its mass. The dotted lines show the effect for the single production of varying $2<\lambda_{T,B}<4$.
• Figure 3: On the left, the partonic cross-section for two typical contributions to single ($ug\rightarrow {T_{5/3}} \bar{t}$; $\lambda=3$) and pair ($gg\rightarrow {T_{5/3}} \bar{T}_{5/3}$) production. To find the total cross-sections, these have to be convoluted with the corresponding partonic luminosities which are shown on the right for $ug$ and $gg$ as function of $\sqrt{\widehat{s}}$.
• Figure 4: Distribution of ${p_T}(L_1)$, ${p_T}(L_2)$, $H_T$ and ${\slashed{E}_T}$ for signal ($\times 100$) and background after the minimal set of cuts, ${p_T}(L_{1,2}>10)$ GeV, $M(LL)>120$ GeV. The case $\lambda_{T,B}=3$ is considered.
• Figure 5: Topology for which ${m_{T2}}$ is defined: pair production of particle with semi-invisible decay