The recent astronomical observation of GeV gamma-ray excess from the Galactic center was suggested due to a $b\overline{b}$ mode in the tens GeV of weakly interacting massive particle (WIMP) pair annihilations, and this mode was also explored by the new dwarf galaxy observation. Considering the case where the WIMP pair mass is below the top quark mass, a pseudoscalar $\varphi$ is studied in this article, which mediates the interactions between the standard model fermions and fermionic WIMPs and the neutral flavor-changing interactions in standard model fermion sectors. The $b\overline{b}$ mode is favored in WIMP pair annihilations, while the WIMP-nucleus scattering is highly suppressed in direct detection. Alternative schemes of $t\to c$ decay and single top production are employed to search the WIMPs. Assuming the mass of the WIMPs is around 5–60 GeV and with reasonable inputs by the constraints, the branching ratio ${\mathcal{B}}_{t\to c\overline{\chi }\chi }$ 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.

• Received 24 June 2015

DOI:https://doi.org/10.1103/PhysRevD.92.074006