Search for pseudoscalar-mediated WIMPs in t rightarrow c transitions with missing energy

The recent astronomical observation of GeV gamma-ray excess from the Galactic Center was suggested due to a $b \bar{b}$ mode in tens GeV WIMP (weakly interacting massive particle) pair annihilations, and this mode was also explored by the new dwarf galaxy observation. Considering the case the WIMP pair mass below top quark mass, a pseudoscalar $\phi$ is studied in this article, which mediates the interactions between the standard model fermions and fermionic WIMPs, and neutral flavor-changing interactions in standard model fermion sectors. The $b \bar{b}$ mode is favored in WIMP pair annihilations, while the WIMP-nucleus scattering is highly suppressed in direct detection. Alternative schemes of $t \rightarrow c$ decay and single top production are employed to search WIMPs. Assuming the mass of WIMP around 5-60 GeV and with the reasonable inputs by the constraints, the branching ratio $\mathcal {B}_{t\rightarrow c \bar{\chi} \chi}$ of a top quark decaying into a charm quark and a WIMP pair is derived of order $10^{-8} - 10^{-5}$, thus careful studies in the future on top-physics may help to gain a better understanding of WIMPs.