Subhojit Roy
Dark matter (DM) and the baryon asymmetry of the Universe (BAU) are among the most compelling indications of physics beyond the Standard Model. We revisit the inelastic Higgs-portal complex singlet, a minimal framework in which a complex scalar splits into two nearly degenerate real states, with an off-diagonal Higgs-portal interaction that drives coannihilation to set the relic density, while the elastic DM-Higgs coupling can be tuned small enough to evade direct-detection limits. This setup naturally supports a strong first-order electroweak phase transition (SFOEWPT) and can account for the longstanding Galactic Center gamma-ray excess (GCE) via present-day DM annihilation into Higgs pairs. In this work, we show that the same framework, extended by a Z 2 -symmetric dimension-6 C P -violating top Yukawa operator, can also generate the BAU via the electroweak baryogenesis (EWBG) mechanism. The cosmological history involves a two-step electroweak phase transition: first, the singlet fields acquire nonzero vacuum expectation values ( v e v s ); then a strongly first-order transition occurs in which the Higgs develops its nonzero v e v while the singlet v e v s vanish. During this second step, both fields remain nonzero only within the advancing bubble wall, generating wall-localized C P violation that biases sphaleron transitions and enables EWBG. After the phase transition, C P and Z 2 symmetries are restored: the lightest singlet state becomes a stable DM candidate, while the vanishing singlet v e v s allow the model to naturally satisfy the stringent constraints on C P violation. We delineate the SFOEWPT-favored parameter space, identifying the criteria for the two-step phase transition region that simultaneously yields the observed BAU and relic density, explains the GCE, and predicts gravitational wave spectra accessible to next-generation space-based detectors.