Top-philic Scalar Dark Matter with a Vector-like Fermionic Top Partner

We consider a simple extension of the Standard Model with a scalar top-philic Dark Matter (DM) $S$ coupling, apart from the Higgs portal, exclusively to the right-handed top quark $t_R$ and a colored vector-like top partner $T$ with a Yukawa coupling $y_{ST}$ which we call the topVL portal. When the Higgs portal is closed and $y_{ST}$ is perturbative $(\lesssim 1)$, $TS\to (W^+b, gt)$, $SS\to t\bar{t}$ and $T\bar{T}\to (q\bar{q},gg)$ provide the dominant (co)annihilation contributions to obtain $Ω_{\rm DM} h^2\simeq 0.12$ in light, medium and heavy DM mass range, respectively. However, large $y_{ST}\sim\mathcal{O}(10)$ can make $SS\to gg$ dominate via the loop-induced coupling $C_{SSgg}$ in the $m_S<m_t$ region. In this model it is the $C_{SSgg}$ coupling that generates DM-nucleon scattering in the direct detection, which can be large and simply determined by $Ω_{\rm DM} h^2\simeq 0.12$ when $SS\to gg$ dominates the DM annihilation. The current LUX results can exclude the $SS\to gg$ dominating scenario and XENON-1T experiment may further test $y_{ST}\gtrsim 1$, and $0.5\lesssim y_{ST}\lesssim 1$ may be covered in the future LUX-ZP experiment. The current indirect detection results from Fermi gamma-ray observations can also exclude the $SS\to gg$ dominating scenario and are sensitive to the heavy DM mass region, of which the improved sensitivity by one order will push DM mass to be above 400, 600, 1000 GeV for $y_{ST}=0.3, 0.5, 1.0$, respectively. $T\bar{T}$ pair produced at the hadron collider will decay $100\%$ into $t\bar{t}+E^{miss}_T$ signal when kinematically open. The latest ATLAS 13 TeV 13.2 $\mathrm{fb^{-1}}$ data can excluded $m_T$ between 300 (650) and 1150 (1100) GeV for $m_S$ =40 (400) GeV and the exclusion region can reach up to $m_S\sim 500$ GeV.

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