Constraining a Lighter Exotic Scalar via Same-sign Top.

It was shown recently that, in two Higgs doublet models without $Z_2$ symmetry, extra Yukawa couplings such as $\rho_{tc}$, $\rho_{tt}$ can fuel enough $CP$ violation for electroweak baryogenesis (EWBG). We revisit an old proposal where a pseudoscalar $A^0$ has mass between $t\bar c$ and $t\bar t$ thresholds. With $\rho_{tt}$ small, it evades $gg \to A^0 \to h^0(125)Z$ constraints, where approximate alignment also helps. We find this scenario with relatively light $A^0$ is not yet ruled out, and $cg \to tA^0 \to tt\bar c$ can probe sizable $\rho_{tc}$ at the LHC, giving access to the second mechanism of EWBG provided by such models.