Relaxing vector-like top quark mass limits through exotic decays in the type-II two-Higgs-doublet model

Abstract We investigate the phenomenological implications of extending the Two-Higgs-Doublet Model (2HDM) with vector-like top quarks (VLTs), emphasizing exotic decay channels into non-standard Higgs bosons, $$T \rightarrow Ht$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>→</mml:mo> <mml:mi>H</mml:mi> <mml:mi>t</mml:mi> </mml:mrow> </mml:math> , $$T \rightarrow At$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>→</mml:mo> <mml:mi>A</mml:mi> <mml:mi>t</mml:mi> </mml:mrow> </mml:math> , and $$T \rightarrow H^+b$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>H</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>b</mml:mi> </mml:mrow> </mml:math> , in addition to the conventional Standard Model (SM) modes. These beyond-the-SM (BSM) decays, currently absent from Large Hadron Collider (LHC) searches, can substantially alter the exclusion limits on VLT masses. In the Type-II 2HDM with a singlet VLT, the lower bound decreases from approximately 1.43 TeV to 1.28 TeV, while for a ( T , B ) doublet it drops from 1.54 TeV to 0.99 TeV, driven by the dominance of the $$T \rightarrow H^+b$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>H</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>b</mml:mi> </mml:mrow> </mml:math> mode with a branching ratio approaching 98%.