Looking for sterile neutrinos in the CMS muon system

The CMS collaboration has just lately introduced new ends in searches for long-lived heavy impartial leptons (HNLs). Also called “sterile neutrinos”, HNLs are fascinating hypothetical particles that might resolve three main puzzles in particle physics: they may clarify the smallness of neutrino plenty by way of the so-called “see-saw” mechanism, they may clarify the matter-antimatter asymmetry of the universe, and on the identical time they may present a candidate for darkish matter.

They're nevertheless very tough to detect since they work together very weakly with recognized particles. The present evaluation is an instance of researchers having to make use of more and more artistic strategies to detect particles that the detectors weren't particularly designed to measure.

Many of the particles studied within the massive LHC experiments have one factor in widespread: they're unstable and decay nearly instantly after being produced. The merchandise of those decays are normally electrons, muons, photons and hadrons—well-known particles that the large particle detectors had been designed to look at and measure.

Research of the unique short-lived particles are carried out primarily based on cautious evaluation of the noticed decay merchandise. Lots of the flagship LHC outcomes had been obtained this manner, from the Higgs boson decaying into photon pairs and 4 leptons to research of the highest quark and discoveries of latest unique hadrons.

The HNLs studied on this evaluation require a distinct strategy. They're impartial particles with comparatively lengthy lifetimes that enable them to fly for meters undetected, earlier than decaying someplace within the detector. The evaluation introduced right here focuses on instances the place an HNL would seem after the decay of a W boson in a proton-proton collision, and would then itself decay someplace within the muon system of the CMS detector.

The muon system constitutes the outermost a part of CMS and was designed—as its title suggests—to detect muons. Muons produced within the LHC proton-proton collisions traverse the entire detector, leaving a hint within the interior monitoring system after which one other one within the muon system. Combining these two traces into the complete muon observe lets physicists establish muons and measure their properties. Within the HNL search, a muon is changed by a weakly interacting heavy particle that leaves no hint—till it decays.

If it decays within the muon system it may well produce a bathe of particles clearly seen within the muon detectors. However—not like a muon—it leaves no hint within the interior monitoring detector, and no different exercise within the muon system. This evaluation is predicated on on the lookout for “out-of-nowhere” clusters of tracks within the muon detectors.

The evaluation began by choosing collision occasions with a reconstructed electron or muon from the decay of the W boson and an remoted cluster of traces within the muon system. Then, the evaluation required the elimination of instances the place commonplace processes may imitate the HNL sign. After the complete evaluation, no extra of sign above expectation has been noticed. Consequently, a spread of doable HNL parameters was excluded, setting essentially the most stringent limits up to now for HNLs with plenty of 2-3 GeV.