More stringent constraints on the unitarised fermionic dark matter Higgs portal

Abstract: We revisit the simplest model of Higgs portal fermionic dark matter. The dark matter in this scenario is thermally produced in the early universe due to the interactions with the Higgs boson which is described by a non-renormalisable dimension-5 operator. The dark matter-Higgs scattering amplitude grows as $\propto \sqrt{s}$, signalling a breakdown of the effective description of the Higgs-dark matter interactions at large enough (compared to the mass scale $\Lambda$ of the dimention-5 operator) energies. Therefore, in order to reliably compute Higgs-dark matter scattering cross sections, we employ the K-matrix unitarisation procedure. To account for the desired dark matter abundance, the unitarised theory requires appreaciably smaller $\Lambda$ than the non-unitarised version, especially for dark matter masses around and below the Higgs resonance, $m_{\chi}\lesssim 65$ GeV, and $m_{\chi}\gtrsim $ few TeV. Consequently, we find that the pure scalar CP-conserving model is fully excluded by current direct dark matter detection experiments.