Light thermal dark matter models in the light of DAMIC-M 2025 constraints

We study the viability of light thermal dark matter (DM) in the sub-GeV mass range in view of the stringent new DAMIC-M limits on DM-electron scattering. Considering a Dirac fermion singlet DM charged under a new Abelian gauge symmetry <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>U</a:mi> <a:mo stretchy="false">(</a:mo> <a:mn>1</a:mn> <a:mo stretchy="false">)</a:mo> </a:math> , we outline two possibilities: (i) family nonuniversal <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>U</e:mi> <e:mo stretchy="false">(</e:mo> <e:mn>1</e:mn> <e:mo stretchy="false">)</e:mo> </e:math> gauge coupling with resonantly enhanced DM annihilation into Standard Model (SM) fermions, and (ii) family universal dark <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>U</i:mi> <i:mo stretchy="false">(</i:mo> <i:mn>1</i:mn> <i:mo stretchy="false">)</i:mo> </i:math> gauge symmetry where relic density is set by DM annihilation into light gauge bosons. As an illustrative example of the first class of models, we consider a gauged <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:msub> <m:mi>L</m:mi> <m:mi>μ</m:mi> </m:msub> <m:mo>−</m:mo> <m:msub> <m:mi>L</m:mi> <m:mi>τ</m:mi> </m:msub> </m:math> extension of the SM having interesting detection prospects at several experiments. While both of these classes of models lead to the observed DM relic and consistency with DAMIC-M together with other experimental limits, the second class of models also lead to strong DM self-interactions, potentially solving the small-scale structure issues of cold dark matter. While a vast part of the parameter space in both the models is already ruled out, the current allowed region of parameter space can be further probed at ongoing or future experiments, keeping the models testable.