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Fresh findings in the search for long-lived heavy neutral leptons (HNLs) have been recently revealed by the CMS collaboration. These, also referred to as “sterile neutrinos,” are fascinating theoretical particles that might answer three significant conundrums in particle physics: the possibility to account for the minute neutrino masses through the “see-saw” mechanism, the capacity to elucidate the matter-antimatter imbalance in the universe, and simultaneously offering a prospective candidate for dark matter.
Nonetheless, these particles are exceptionally challenging to identify, as they engage feebly with recognized particles. The ongoing analysis illustrates a situation where scientists are employing progressively inventive techniques to perceive particles that the detectors were not particularly intended to evaluate.
The majority of the particles scrutinized in the vast LHC experiments share a common characteristic: they are unsteady and deteriorate almost instantly after generation. The decay outcomes are typically electrons, photons, muons, and hadrons — familiar particles that large particle detectors were configured to watch and gauge.
Inquiries into the initial fleeting particles are conducted through meticulous examination of the discernible decay byproducts. Numerous prominent LHC findings were achieved this way, ranging from the Higgs boson transforming into pairs of photons and four leptons to investigations of the top quark and detection of new unusual hadrons.
The approach to the HNLs examined in this analysis must vary. These are neutral entities with relatively extended lifespans, allowing them to journey for meters unnoticed before breaking down somewhere within the detector. The analysis presented here stresses situations in which an HNL could appear after a W boson decays during a proton-proton collision, then decay within the CMS detector’s muon system.
The muon system, which is the outermost segment of CMS and designed explicitly to identify muons — as the name implies — allows muons created in the LHC proton-proton collisions to pass through the entire detector, marking a path in the inner tracking system and another within the muon system. Merging these two paths into a complete muon trail enables physicists to pinpoint muons and evaluate their characteristics. In the HNL search, a muon is substituted with a feebly interacting weighty particle that remains invisible until it deteriorates.
Should it disintegrate within the muon system, it could create a particle spray that is plainly perceptible in the muon detectors. However, unlike a muon, it leaves no marks in the inner tracking detector or any other indications within the muon system. This examination hinges on the search for “out-of-nowhere” track clusters in the muon detectors.
The selection of collision events with an individual collection of traces within the muon system and a recreated electron or muon from the decay of the W boson served as the starting point for the analysis. Subsequently, the analysis called for the elimination of instances where conventional procedures might mimic the HNL indication. Following the comprehensive evaluation, no surplus signal beyond what was anticipated has been found. Consequently, a spectrum of conceivable HNL attributes was ruled out, establishing the strictest restrictions so far for HNLs weighing between 2-3 GeV.