Looking for sterile neutrinos in the CMS muon system

The CMS collaboration has recently presented new results in searches for long-lived heavy neutral leptons (HNLs). These particles, also known as “sterile neutrinos,” are intriguing hypothetical entities that hold the potential to solve three significant puzzles in particle physics. Firstly, they could explain why neutrinos have such small masses through a mechanism known as the “see-saw.” Secondly, they may offer an explanation for the matter-antimatter asymmetry in the universe. Lastly, they could provide a candidate for dark matter, which constitutes a substantial portion of our universe’s mass.

However, detecting HNLs is a challenging task since they interact very weakly with known particles. This analysis showcases the researchers’ increasingly creative methods of detecting particles that the detectors were not originally designed to measure.

Most particles studied in large LHC experiments share a common trait – they are unstable and quickly decay after production. The resulting decay products typically consist of electrons, muons, photons, and hadrons – well-known particles that particle detectors are specifically designed to observe and measure.

The analysis of short-lived particles involves a careful examination of the observed decay products. Many significant LHC results have been obtained through this approach, such as the discovery of the Higgs boson decaying into photon pairs and four leptons, as well as studies on the top quark and exotic hadrons.

However, studying HNLs requires a different approach. These neutral particles have relatively long lifetimes, allowing them to travel several meters undetected before decaying within the detector. The analysis presented in this study focuses on cases where an HNL appears after the decay of a W boson in a proton-proton collision and subsequently decays within the CMS detector’s muon system.

The muon system, situated on the outermost portion of CMS, is primarily designed to detect muons. Muons produced in LHC collisions traverse the entire detector, leaving traces in both the inner tracking system and the muon system. By combining these traces, physicists can identify muons and measure their properties. In the search for HNLs, a muon is replaced by a weakly interacting heavy particle that leaves no trace until it decays.

If the HNL decays within the muon system, it can generate a visible shower of particles in the muon detectors. However, unlike a muon, an HNL leaves no trace in the inner tracking detector and shows no other activity within the muon system. This analysis revolves around identifying “out-of-nowhere” clusters of tracks in the muon detectors.

The analysis begins by selecting collision events containing a reconstructed electron or muon from the decay of the W boson and an isolated cluster of traces within the muon system. The subsequent analysis involves eliminating cases where standard processes could imitate the HNL signal. After completing the full analysis, no excess of signal above the expected background has been observed. As a result, a range of potential HNL parameters has been excluded, setting the most stringent limits to date for HNLs with masses of 2-3 GeV.

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