Limits on dark matter annihilation in prompt cusps from the isotropic gamma-ray background

Recent studies indicate that thermally produced dark matter will form highly concentrated, low-mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>γ</a:mi></a:math>-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi-LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>γ</c:mi></c:math>-ray background, excluding with 95% confidence dark matter annihilation to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>b</e:mi><e:mover accent="true"><e:mi>b</e:mi><e:mo stretchy="false">¯</e:mo></e:mover></e:math> final states for dark matter masses below 120 GeV.