Muon-induced baryon number violation

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>μ</a:mi><a:mo stretchy="false">→</a:mo><a:mi>e</a:mi></a:math> conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mi>μ</d:mi><d:mo stretchy="false">→</d:mo><d:mi>e</d:mi></d:math> signal peak. A concrete example of such light new physics is <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msup><g:mi>μ</g:mi><g:mo>−</g:mo></g:msup></g:math>-nucleon annihilation into a light dark sector, which can produce an energetic <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msup><i:mi>e</i:mi><i:mo>−</i:mo></i:msup></i:math> as well as <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msup><k:mi>e</k:mi><k:mo>+</k:mo></k:msup><k:msup><k:mi>e</k:mi><k:mo>−</k:mo></k:msup></k:math> by-products. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars. Published by the American Physical Society 2024