The need for new search strategies for fourth generation quarks at the LHC

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 the 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: mt'>450 GeV in the semileptonic channel pp→t't'→ℓνbqqb and mt'>557 GeV in the dilepton channel pp→t't'→ℓ+ℓ-ννbb, 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: mt'350 GeV. We, therefore, suggest an alternative search strategy that is more sensitive to beyond SM4 dynamics of the fourth generation fermions.