Revealing the origin of neutrino masses through displaced shower searches in the CMS muon system

We study the potential to probe the origin of neutrino masses, by searching for long-lived right-handed neutrinos (RHNs) <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>N</a:mi></a:math> in the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>B</c:mi><c:mo>−</c:mo><c:mi>L</c:mi></c:math> model and in the RHN-extended Standard Model (SM) effective field theory (EFT). Despite the small active-sterile mixing <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mo stretchy="false">|</e:mo><e:msub><e:mi>V</e:mi><e:mrow><e:mo>ℓ</e:mo><e:mi>N</e:mi></e:mrow></e:msub><e:msup><e:mo stretchy="false">|</e:mo><e:mn>2</e:mn></e:msup></e:math>, RHNs are produced abundantly via SM and exotic Higgs production, as long as the Higgs mixing or EFT operator coefficient is sufficiently large. We reinterpret a search for displaced showers in the CMS muon system and we find that it is sensitive to parameter space at and below the seesaw floor, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mo stretchy="false">|</i:mo><i:msub><i:mi>V</i:mi><i:mrow><i:mo>ℓ</i:mo><i:mi>N</i:mi></i:mrow></i:msub><i:msup><i:mo stretchy="false">|</i:mo><i:mn>2</i:mn></i:msup><i:mo>≈</i:mo><i:msup><i:mn>10</i:mn><i:mrow><i:mo>−</i:mo><i:mn>12</i:mn></i:mrow></i:msup></i:math> (<m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mo>ℓ</m:mo><m:mo>=</m:mo><m:mi>e</m:mi></m:math>, <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>τ</o:mi></o:math>) for <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msub><q:mi>m</q:mi><q:mi>N</q:mi></q:msub><q:mo>≈</q:mo><q:mn>40</q:mn><q:mtext> </q:mtext><q:mtext> </q:mtext><q:mi>GeV</q:mi></q:math>. With existing data constraining such well-motivated scenarios of neutrino mass generation, we determine the projected sensitivity at the HL-LHC, motivating dedicated searches for long-lived RHNs with decay lengths <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mo>≈</s:mo><s:mn>10</s:mn><s:mtext> </s:mtext><s:mtext> </s:mtext><s:mi mathvariant="normal">m</s:mi></s:math>.