Identifying top partners at LHC

TL;DR

The paper evaluates the LHC discovery potential for pair-produced vector-like quarks that mainly couple to the third generation, considering singlets T and B and doublets (T B), (X T), and (B Y). Using a full Monte Carlo framework with a fast detector simulation, it analyzes twelve multi-lepton final states (from one to four leptons) across different Z-candidate categories, optimizing event selection and employing likelihood-based reconstructions to identify decay modes and reconstruct heavy-quark masses. The study finds the single-lepton channel generally offers the best discovery reach for quark masses around 500 GeV, with $5\sigma$ luminosities as low as $\sim 0.16-1.9~\mathrm{fb}^{-1}$ depending on the model; multi-lepton channels play a crucial role in model discrimination by revealing specific decays like $T\to Zt$, $B\to Zb$, and $X\to Wt$, and enabling mass peak reconstruction when statistics permit. Overall, the results show a robust discovery and characterization framework for heavy top partners at the LHC, including Higgs-associated decays, with distinct signatures across the considered multiplets, and provide a roadmap for identifying the underlying new-physics structure from observed final-state patterns.

Abstract

We systematically study the possible signals at LHC of new vector-like quarks mainly coupled to the third generation. We consider heavy quarks T, B, X, Y of charges 2/3, -1/3, 5/3 and -4/3, respectively, in SU(2)_L isosinglets T_{L,R}, B_{L,R}, or isodoublets (T B)_{L,R}, (X T)_{L,R} or (B Y)_{L,R}. Analyses based on a fast detector simulation are presented for twelve different final states containing one, two, three or four charged leptons in several invariant mass regions, also considering various b quark multiplicities. It is shown that with the combination of the different channels the new quarks can be identified and their charged and neutral decays established. The comparison among final states also shows that the single lepton one offers the best discovery potential at LHC. For heavy quark masses of 500 GeV, the 5 sigma discovery luminosities range from 0.16 fb^-1 for a (X T)_{L,R} doublet to 1.9 fb^-1 for a B_{L,R} singlet.

Figures (32)

• Figure 1: Left: Heavy quark production cross sections at LHC. Right: branching ratios for $T$ and $B$ decays.
• Figure 2: $\ell_a^+ \ell_b^-$, $\ell_c^+ \ell_d^-$ invariant mass distributions for the six models in the $\ell^+ \ell^+ \ell^- \ell^-$ final state (see the text). The luminosity is 30 fb$^{-1}$.
• Figure 3: $\ell_a^+ \ell_b^- b$, $\ell_c^+ \ell_d^- b$ invariant mass distribution for the six models in the $\ell^+ \ell^+ \ell^- \ell^-$ ($ZZ$) final state, with two entries per event. The luminosity is 30 fb$^{-1}$.
• Figure 4: $\ell_a^+ \ell_b^-$ invariant mass distributions for the six models in the $\ell^\pm \ell^\pm \ell^\mp$ final state (see the text for the definition of $\ell_a^+$ and $\ell_b^-$). The luminosity is 30 fb$^{-1}$.
• Figure 5: Kinematical distributions of variables used in selection and recontruction criteria for the $\ell^\pm \ell^\pm \ell^\mp$ ($Z$) final state: light jet multiplicity $N_j$, $b$ jet multiplicity $N_b$, transverse momentum of the leading lepton and total transverse energy. The luminosity is 30 fb$^{-1}$.