Strong Dark Matter Self-Interaction from a Stable Scalar Mediator
  In face of the small-scale structure problems of the collisionless cold dark
matter (DM) paradigm, a popular remedy is to introduce a strong DM
self-interaction which can be generated nonperturbatively by a MeV-scale light
mediator. However, if such the mediator is unstable and decays into SM
particles, the model is severely constrained by the DM direct and indirect
detection experiments. In the present paper, we study a model of a
self-interacting fermionic DM, endowed with a light stable scalar mediator. In
this model, the DM relic abundance is dominated by the fermionic DM particle
which is generated mainly via the freeze-out of its annihilations to the stable
mediator. Since this channel is invisible, the DM indirect detection
constraints should be greatly relaxed. Furthermore, the direct detection
signals are suppressed to an unobservable level since fermionic DM scatterings
with a nucleon appear at one-loop level. By further studying the bounds from
the CMB and BBN on the visible channels involving the dark sector, we show that
there is a large parameter space which can generate appropriate DM
self-interactions at dwarf galaxy scales, while remaining compatible with other
experimental constraints.
