Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>ν</a:mi><a:mi>τ</a:mi></a:msub></a:math>. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msup><c:mi>e</c:mi><c:mo>+</c:mo></c:msup><c:msup><c:mi>e</c:mi><c:mo>−</c:mo></c:msup></c:math> collider. The data sample was collected at and just below the center-of-mass energies of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi mathvariant="normal">ϒ</e:mi><e:mo stretchy="false">(</e:mo><e:mn>4</e:mn><e:mi>S</e:mi><e:mo stretchy="false">)</e:mo></e:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi mathvariant="normal">ϒ</j:mi><j:mo stretchy="false">(</j:mo><j:mn>5</j:mn><j:mi>S</j:mi><j:mo stretchy="false">)</j:mo></j:math> resonances and has an integrated luminosity of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mn>915</o:mn><o:mtext> </o:mtext><o:mtext> </o:mtext><o:msup><o:mi>fb</o:mi><o:mrow><o:mo>−</o:mo><o:mn>1</o:mn></o:mrow></o:msup></o:math>, corresponding to <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mo stretchy="false">(</q:mo><q:mn>836</q:mn><q:mo>±</q:mo><q:mn>12</q:mn><q:mo stretchy="false">)</q:mo><q:mo>×</q:mo><q:msup><q:mn>10</q:mn><q:mn>6</q:mn></q:msup></q:math> <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:msup><u:mi>e</u:mi><u:mo>+</u:mo></u:msup><u:msup><u:mi>e</u:mi><u:mo>−</u:mo></u:msup><u:mo stretchy="false">→</u:mo><u:msup><u:mi>τ</u:mi><u:mo>+</u:mo></u:msup><u:msup><u:mi>τ</u:mi><u:mo>−</u:mo></u:msup></u:math> events. We search for production of the HNL (denoted <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:mi>N</x:mi></x:math>) in the decay <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:msup><z:mi>τ</z:mi><z:mo>−</z:mo></z:msup><z:mo stretchy="false">→</z:mo><z:msup><z:mi>π</z:mi><z:mo>−</z:mo></z:msup><z:mi>N</z:mi></z:math> followed by its decay via <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:mi>N</cb:mi><cb:mo stretchy="false">→</cb:mo><cb:msup><cb:mi>μ</cb:mi><cb:mo>+</cb:mo></cb:msup><cb:msup><cb:mi>μ</cb:mi><cb:mo>−</cb:mo></cb:msup><cb:msub><cb:mi>ν</cb:mi><cb:mi>τ</cb:mi></cb:msub></cb:math>. The search focuses on the parameter-space region in which the HNL is long-lived, so that the <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:msup><fb:mi>μ</fb:mi><fb:mo>+</fb:mo></fb:msup><fb:msup><fb:mi>μ</fb:mi><fb:mo>−</fb:mo></fb:msup></fb:math> originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:mi>τ</hb:mi></hb:math> neutrino as a function of the HNL mass. Published by the American Physical Society 2024