Extending the limits in the hunt for long-lived heavy neutral leptons with the ATLAS experiment at the Large Hadron Collider at CERN

A search for a long-lived, heavy neutral lepton ($\mathcal{N}$) in 139~fb$^{-1}$ of $\sqrt{s}=13$~TeV $pp$ collision data collected between 2015 and 2018 by the ATLAS detector at the Large Hadron Collider is presented. The $\mathcal{N}$ is assumed to be produced via $W \rightarrow \mathcal{N} \mu$ or $W \rightarrow \mathcal{N} e$ and decays into two charged leptons and a neutrino, forming a unique displaced vertex signature. A novel technique is used to reconstruct the $\mathcal{N}$ mass that discriminates between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the $\mathcal{N}$ with the left-handed neutrino states for the $\mathcal{N}$ mass range $3$~GeV~$< m_{\mathcal{N}} < 15$~GeV. For the first time, limits are given for both single-flavour and multi-flavour mixing scenarios motivated by neutrino-flavour oscillation results for both the normal and inverted neutrino-mass hierarchies. In addition to the concluded search for $\mathcal{N}$, ongoing efforts are outlined that include 66~fb$^{-1}$ of $\sqrt{s}=13.6$~TeV $pp$ collision data collected between 2022 and 2023 by the ATLAS detector and semileptonic $\mathcal{N}$ decays into a charged lepton and a charged pion.