1/30/2024 0 Comments Neutrinos plus![]() ![]() A high-energy muon can sometimes radiate energy while passing through the steel placed between the muon detectors, leaving more hits than usual, like an MDS. A very energetic jet can “punch-through” the hadron calorimeter and deposit some of its energy in the muon system, creating an MDS. Not many other processes can mimic the same configuration (a lepton and an MDS). ![]() The energetic electrons or muons lead to very clean signatures in the detectors, allowing us to trigger (select) these events and collect them for offline analysis, where they are further selected by imposing that the MDS is seen in the hemisphere opposite to that of the electron or muon. The CMS physicists searched for these HNL showers in events with a W-boson decay, because the W boson usually decays into an energetic electron or muon plus a SM neutrino, which can turn into an HNL (as illustrated in Fig. This signature is commonly referred to as Muon Detector Shower (MDS).įigure 1: Illustration of the production and decay of a heavy neutral lepton (N). Similarly to what happens for jets, the larger is the energy of the LLP, the larger will be the particle shower, activating a larger number of channels in the detectors. ![]() When the long-lived HNL decays, it produces a cascade of particles (a shower) that is measured by the gas detectors as a concentrated outburst of signals, unlike anything that happens for muons or other SM particles produced in the LHC collisions. Nevertheless, CMS physicists recently established that LLPs could be seen in the muon detectors. Such “long-lived” particles (LLPs) are particularly difficult to detect in the CMS experiment, which was not designed for that purpose. In particular, if their mass is small enough and/or their mixing with SM leptons is weak enough, HNLs could travel macroscopic distances (up to several meters) before decaying. How they could be seen in an experiment strongly depends on these parameters. Besides solving the neutrino mass problem, models with HNLs can also explain other SM mysteries, such as the matter-antimatter asymmetry of the universe through CP violation, the nature of dark matter, and the anomalous magnetic moment of the muon.Įxperiments have been searching for HNLs for decades, but it is challenging: we do not know their mass nor the strength of their interaction with the SM leptons. These heavier cousins of the SM neutrinos are not supposed to directly interact with the SM particles through the electroweak or strong interactions, but can be produced through mixing, together with the SM electron, muon, and tau neutrinos. With a “right-handed” partner, the SM neutrinos can have masses, just like the other SM particles. The puzzle of the neutrino mass can be solved by introducing a “right-handed” partner to the neutrino, often called “heavy neutral lepton” (HNL). Furthermore, being at least a million times lighter than the electron (the lightest massive fermion in the SM) may suggest that the neutrinos get their mass generated by a different mechanism. The observation of neutrino oscillations posed a big challenge to the completeness of the SM, by implying that neutrinos are massive. their spin is always opposite to their momentum. Two key properties of neutrinos in the standard model of particle physics (SM) are that they are massless and “left-handed”, i.e. ![]()
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