CMS hunts for heavy neutral leptons

The quest to search for new physics inspires searches in CMS for very rare processes, which, if discovered, could open the door to a new understanding of particle physics.

One such process is the production and decay of heavy sterile Majorana neutrinos, a type of heavy neutral lepton (HNL) introduced to describe the very small neutrino masses via the so-called seesaw mechanism. Two further fundamental puzzles of particle physics can be solved by adding three HNLs to the Standard Model (SM) particle spectrum: the lightest (with a mass of a few keV) can serve as a dark-matter candidate; the two heavier ones (heavier than about a GeV) could, when mass-degenerate, be responsible for a sizable amount of CP violation and thus help explain the cosmological matter–antimatter asymmetry.

Through their mixing with the SM neutrinos (see figure, left), the heavier HNLs could be produced at the LHC in leptonic W-boson decays. Subsequently, the HNL can decay to another W boson and a lepton, leading to a signal containing three isolated leptons. Depending on how weakly the new particles couple to the SM neutrinos, characterised by the parameters |V_eN|², |V_μN|² and |V_τN|², they can either decay shortly after production, or after flying some distance in the detector.

A new search performed with data collected in 2016 by CMS focuses on prompt trilepton (electrons or muons) signatures of HNL production. It explores a mass range from 1 GeV to 1.2 TeV, more than doubling the scope of LHC results so far. It also probes a mass regime that was unexplored since the days of the Large Electron-Positron collider (LEP), indicating that eventually the LHC will supersede these results with more data.

The trilepton final state does not lead to a sharp peak in an invariant mass spectrum, and therefore the search has to employ various kinematic properties of the events to be able to detect a possible presence of HNLs. To be sensitive to very low HNL masses, the search uses soft muons (with p_T > 5 GeV) and electrons (p_T > 10 GeV). While no signs of HNL have been found so far (see figure, right), the constraints on |V_μN|² (|V_eN|² is similar) in the high-mass region are the strongest to date. In the low mass region, the analysis has comparable sensitivity to previous searches.

Using dedicated analysis techniques, it is foreseen to extend this search to explore the parameter space where HNLs have longer lifetimes and so travel large distances in the detector before they decay. Together with more data this will enable CMS to significantly improve the sensitivity at low masses and eventually probe unexplored territory in this important region of HNL parameter space.