Search for Scalar Dark Matter via Pseudoscalar Portal Interactions: In Light of the Galactic Center Gamma-Ray Excess

In light of the observed Galactic center gamma-ray excess, we investigate a simplified model, for which the scalar dark matter interacts with quarks through a pseudoscalar mediator. The viable regions of the parameter space, that can also account for the relic density and evade the current searches, are identified, if the low-velocity dark matter annihilates through an $s$-channel off-shell mediator mostly into $\bar{b} b$, and/or annihilates directly into two ${\it hidden}$ on-shell mediators, which subsequently decay into the quark pairs. These two kinds of annihilations are $s$ wave. The projected monojet limit set by the high luminosity LHC sensitivity could constrain the favored parameter space, where the mediator's mass is larger than the dark matter mass by a factor of 2. We show that the projected sensitivity of 15-year Fermi-LAT observations of dwarf spheroidal galaxies can provide a stringent constraint on the most parameter space allowed in this model. If the on-shell mediator channel contributes to the dark matter annihilation cross sections over 50$\%$, this model with a lighter mediator can be probed in the projected PICO-500L experiment.