The heavy quark search at the LHC

TL;DR

This work investigates the discovery potential for heavy quarks in a sequential fourth family at the LHC, focusing on $t'$ and $b'$ linked to electroweak symmetry breaking. It introduces a jet mass technique to identify hadronic decays of $W$ and $t$, enabling reconstruction of $t'$ and $b'$ masses and permitting searches even without $b$-tagging, which is advantageous in early LHC running. The analysis employs improved matrix elements with MLM jet-parton matching (e.g., Alpgen-MadGraph with Pythia/Herwig) to model backgrounds and demonstrates that backgrounds, including QCD multijets, can be suppressed to manageable levels, with $t'\overline{t'}$ and $b'\overline{b'}$ signals remaining observable for $m_{t'}$ around $600$ GeV and $m_{b'}$ not far above. The Appendix discusses an axial-symmetry–based mechanism for heavy-quark mass generation, arguing that the mass hierarchy $m_{b'}>m_{t'}$ can arise while remaining compatible with electroweak precision constraints. Overall, the paper provides a promising strategy for early heavy-quark discovery at the LHC and contributes to the broader discussion of fourth-family scenarios and their EW implications.

Abstract

We explore further the discovery potential for heavy quarks at the LHC, with emphasis on the $t'$ and $b'$ of a sequential fourth family associated with electroweak symmetry breaking. We consider QCD multijets, $t\bar{t}+\rm{jets}$, $W+\rm{jets}$ and single $t$ backgrounds using event generation based on improved matrix elements and low sensitivity to the modeling of initial state radiation. We exploit a jet mass technique for the identification of hadronically decaying $W$'s and $t$'s, to be used in the reconstruction of the $t'$ or $b'$ mass. This along with other aspects of event selection can reduce backgrounds to very manageable levels. It even allows a search for both $t'$ and $b'$ in the absence of $b$-tagging, of interest for the early running of the LHC. A heavy quark mass of order 600 GeV is motivated by the connection to electroweak symmetry breaking, but our analysis is relevant for any new heavy quarks with weak decay modes.