The Need for New Search Strategies for Fourth Generation Quarks at the LHC

TL;DR

This paper argues that current search strategies for fourth-generation quarks at the LHC are insufficient if their decay patterns differ from the minimal SM4 model, proposing alternative methods to improve detection sensitivity.

Contribution

It introduces a new search strategy for fourth-generation quarks based on a Two Higgs Doublet Model, highlighting the need to go beyond SM4 assumptions for accurate detection.

Findings

Standard CMS bounds do not apply if t' decays via t' -> ht.

Alternative decay channels can relax the lower mass bounds.

Proposes a more sensitive search strategy for beyond SM4 scenarios.

Abstract

Most limits on the fourth generation heavy top quark (the t') are based on the assumed dominance of t' -> Wb, which is expected to be case in the minimal fourth generation framework with a single Higgs (the so called SM4). Here we show, within a variant of a Two Higgs Doublet Model with four generations of fermions, that, in general, a different t' detection strategy is required if the physics that underlies the new heavy fermionic degrees of freedom goes beyond the "naive" SM4. We find that the recent CMS lower bounds: m_{t'}< 450 GeV in the semi-leptonic channel pp -> t't' -> l\nu qqbb and m_{t'}< 557 GeV in the dilepton channel pp -> t't' ->ll\nu \nu bb, that were obtained using the customary (SM4-driven) detection strategies, do not apply. In particular, we demonstrate that if the decay t' -> ht dominates, then applying the "standard" CMS search tools leads to a considerably relaxed lower bound: m_{t'} >~350 GeV. We, therefore, suggest an alternative search strategy that is more sensitive to beyond SM4 dynamics of the fourth generation fermions.