<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>ν</mml:mi><mml:mi>μ</mml:mi></mml:msub></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>ν</mml:mi><

We perform a detailed study of neutrino-electron elastic scattering using the monoenergetic <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Be</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>7</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> neutrinos in Borexino, with an emphasis on exploring the differences between the contributions of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>ν</c:mi><c:mi>e</c:mi></c:msub></c:math>, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>ν</e:mi><e:mi>μ</e:mi></e:msub></e:math>, and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>ν</g:mi><g:mi>τ</g:mi></g:msub></g:math>. We find that current data are capable of measuring these components such that the contributions from <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>ν</i:mi><i:mi>μ</i:mi></i:msub></i:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>ν</k:mi><k:mi>τ</k:mi></k:msub></k:math> cannot be zero, although distinguishing between them is challenging—the differences stemming from Standard Model radiative corrections are insufficient without significantly more precise measurements. In studying these components, we compare predicted neutrino-electron scattering event rates within the Standard Model (accounting for neutrino oscillations), as well as going beyond the Standard Model in two ways. We allow for nonunitary evolution to modify neutrino oscillations, and find that with a larger exposure (<m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:mo>∼</m:mo><m:mn>30</m:mn><m:mi mathvariant="normal">x</m:mi></m:mrow></m:math>), Borexino may provide relevant information for constraining nonunitarity, and that JUNO may be able to accomplish this with its data collection of <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mrow><p:mmultiscripts><p:mrow><p:mi>Be</p:mi></p:mrow><p:mprescripts/><p:none/><p:mrow><p:mn>7</p:mn></p:mrow></p:mmultiscripts></p:mrow></p:math> neutrinos. We also consider novel <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:msub><r:mi>ν</r:mi><r:mi>μ</r:mi></r:msub></r:math>- and <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:msub><t:mi>ν</t:mi><t:mi>τ</t:mi></t:msub></t:math>-electron scattering from a gauged <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mi>U</v:mi><v:mo stretchy="false">(</v:mo><v:mn>1</v:mn><v:msub><v:mo stretchy="false">)</v:mo><v:mrow><v:msub><v:mi>L</v:mi><v:mi>μ</v:mi></v:msub><v:mo>−</v:mo><v:msub><v:mi>L</v:mi><v:mi>τ</v:mi></v:msub></v:mrow></v:msub></v:math> model, showing consistency with previous analyses of Borexino and this scenario, but also demonstrating the impact of uncertainties on Standard Model mixing parameters on these results. Published by the American Physical Society 2024