Speaker
Description
The absence of confirmed signal in dark matter (DM) direct detection (DD) may suggest a weak coupling between DM and the first generation quarks. In this work we consider a real scalar dark matter $S$ which has new Yukawa interactions with charm $c$ and top quark $t$ via a vector-like fermion mediator $\psi$. By setting the Higgs portal to be negligible, we focus on the new Yukawa interactions. Since there is no valence $c,t$ quark in nucleons, DM-gluon scattering at loop level becomes important. We found that renormalization group equation (RGE) effects are crucial in calculating the DM-nucleon scattering rate at $\mu_{had}\sim 1\, {\rm GeV}$ if one constructs the effective theory at $\mu_{EFT}\sim m_Z$. For the perturbative benchmark couplings we choose, combined results from relic abundance requirement $\Omega h^2=0.12$, direct/indirect detection constraints, 13 TeV LHC data have excluded a thermal relic DM with ${m_S}<{m_t/2}$ in this model. FCNC processes of top quark can be generated at both tree level $t\to \psi^{(*)}S \to cSS$ and loop level $t\to c+\gamma/g/Z$, of which the branching fractions are usually below $10^{-9}$ after passing the other constraints, which are still safe from the current top quark width measurements.
