Search for events with one displaced vertex from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in p p collisions at s = 13 TeV…

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>140</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:msup> <a:mi>fb</a:mi> <a:mrow> <a:mo>−</a:mo> <a:mn>1</a:mn> </a:mrow> </a:msup> </a:math> of proton-proton collision data at <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msqrt> <c:mi>s</c:mi> </c:msqrt> <c:mo>=</c:mo> <c:mn>13</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>TeV</c:mi> </c:math> recorded in 2015–2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi mathvariant="script">O</e:mi> <e:mo stretchy="false">(</e:mo> <e:mn>10</e:mn> <e:mo stretchy="false">)</e:mo> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:mi>cm</e:mi> </e:math> .