S-channel dark matter simplified models in the resonance region

The particle-physics nature of dark matter (DM) remains one of the central open questions in modern physics. A widely used framework to investigate DM properties is provided by <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>s</a:mi> </a:math> -channel simplified models (), which extend the Standard Model with a DM particle and a mediator that connects the visible and dark sectors. Much of the parameter space is already constrained by direct and indirect detection, collider searches, and the measured DM relic abundance. We show, however, that the regime ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>m</c:mi> <c:mrow> <c:mi>DM</c:mi> </c:mrow> </c:msub> <c:mo>≃</c:mo> <c:msub> <c:mi>m</c:mi> <c:mrow> <c:mi>med</c:mi> </c:mrow> </c:msub> <c:mo>/</c:mo> <c:mn>2</c:mn> </c:math> ) remains viable under current bounds and will be stringently tested by forthcoming experiments. Using a full Boltzmann treatment that allows for departures from kinetic equilibrium near resonance, we demonstrate that this regime can reproduce the observed relic density with couplings compatible with direct-detection limits. We also show that models with <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>s</e:mi> </e:math> -wave dominated annihilation can explain the -LAT Galactic Center Excess with couplings consistent with relic-density and direct-detection constraints. Finally, we propose two minimal constructions that naturally realize <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>m</g:mi> <g:mrow> <g:mi>med</g:mi> </g:mrow> </g:msub> <g:mo>≈</g:mo> <g:mn>2</g:mn> <g:msub> <g:mi>m</g:mi> <g:mrow> <g:mi>DM</g:mi> </g:mrow> </g:msub> </g:math> , making the resonant scenario generic rather than fine-tuned.