<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math> rays from in-flight positron annihilation as a probe of new physics

The <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>γ</a:mi></a:math>-ray emission originating from in-flight annihilation (IA) of positrons is a powerful observable for constraining high-energy positron production from exotic sources. By comparing diffuse <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>γ</c:mi></c:math>-ray observations of INTEGRAL, COMPTEL, and EGRET to theoretical predictions, we set the most stringent constraints on electrophilic feebly interacting particles, thereby proving IA as a valuable probe of new physics. In particular, we extensively discuss the case of MeV-scale sterile neutrinos, where IA sets the most stringent constraints, excluding <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mo stretchy="false">|</e:mo><e:msub><e:mrow><e:mi>U</e:mi></e:mrow><e:mrow><e:mi>μ</e:mi><e:mn>4</e:mn></e:mrow></e:msub><e:msup><e:mrow><e:mo stretchy="false">|</e:mo></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msup><e:mo>≳</e:mo><e:msup><e:mrow><e:mn>10</e:mn></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>13</e:mn></e:mrow></e:msup></e:mrow></e:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mo stretchy="false">|</i:mo><i:msub><i:mrow><i:mi>U</i:mi></i:mrow><i:mrow><i:mi>τ</i:mi><i:mn>4</i:mn></i:mrow></i:msub><i:msup><i:mrow><i:mo stretchy="false">|</i:mo></i:mrow><i:mrow><i:mn>2</i:mn></i:mrow></i:msup><i:mo>≳</i:mo><i:mn>2</i:mn><i:mo>×</i:mo><i:msup><i:mrow><i:mn>10</i:mn></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>13</i:mn></i:mrow></i:msup></i:mrow></i:math> for sterile neutrinos mixed with <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>μ</m:mi></m:math> and <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>τ</o:mi></o:math> neutrinos, respectively. These constraints improve existing limits by more than an order of magnitude. We briefly discuss the application of these results to a host of exotic positron sources such as dark photons, axionlike particles, primordial black holes, and sub-GeV dark matter.