Reopening the Z portal with semi-annihilations

In one-component dark matter (DM) scenarios is commonly assumed that a scalar weakly interacting massive particle must either be part of an <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>S</a:mi><a:mi>U</a:mi><a:mo stretchy="false">(</a:mo><a:mn>2</a:mn><a:msub><a:mo stretchy="false">)</a:mo><a:mi>L</a:mi></a:msub></a:math> multiplet with zero hypercharge or have suppressed vector interactions with the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>Z</e:mi></e:math>-gauge boson to circumvent stringent direct detection (DD) bounds. In this work, we demonstrate that multicomponent scenarios with a dark scalar doublet exhibiting vectorlike interactions with the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>Z</g:mi></g:math> boson are also compatible with bounds arising from DD searches. Specifically, we consider a simple extension of the Standard Model wherein the dark sector comprises a doublet and a complex singlet <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>ϕ</i:mi></i:math>, both charged under a <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>Z</k:mi><k:mn>6</k:mn></k:msub></k:math> symmetry. We find that semi-annihilation processes drastically reduce the relic abundance of the neutral component of the doublet, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msup><m:mi>H</m:mi><m:mn>0</m:mn></m:msup></m:math>, sufficiently attenuating the effects of its large <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>Z</o:mi></o:math>-mediated elastic scattering cross-section with nucleons to satisfy the DD constraints. Although the contribution of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msup><q:mi>H</q:mi><q:mn>0</q:mn></q:msup></q:math> to the total relic abundance is nearly negligible, with <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>ϕ</s:mi></s:math> dominating, both dark matter components are expected to be detectable in ongoing and future DD experiments. The viability of the model is tested against several theoretical and experimental constraints, resulting in a parameter space featuring a nondegenerate mass spectrum at the electroweak scale. Published by the American Physical Society 2024