Constraining a lighter exotic scalar via same-sign top

It was shown recently that, in two Higgs doublet models without Z2 symmetry, extra Yukawa couplings such as ρtc, ρtt can fuel enough CP violation for electroweak baryogenesis (EWBG). We revisit an old proposal where a pseudoscalar A0 has mass between tc¯ and tt¯ thresholds. With ρtt small, it evades gg→A0→h0(125)Z constraints, where approximate alignment also helps. We find this scenario with relatively light A0 is not yet ruled out, and cg→tA0→ttc¯ can probe sizable ρtc at the LHC, giving access to the second mechanism of EWBG provided by such models.