Search for a heavy composite Majorana neutrino in events with dilepton signatures from proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract: Results are presented of a search for a heavy Majorana neutrino N$\ell$ decaying into two same-flavor leptons $\ell$ (electrons or muons) and a quark-pair jet. A model is considered in which the N$\ell$ is an excited neutrino in a compositeness scenario. The analysis is performed using a sample of proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. The data are found to be in agreement with the standard model prediction. For the process in which the N$\ell$ is produced in association with a lepton, followed by the decay of the N$\ell$ to a same-flavor lepton and a quark pair, an upper limit at 95% confidence level on the product of the cross section and branching fraction is obtained as a function of the N$\ell$ mass $m{\text{N}\ell}$ and the compositeness scale $\Lambda$. For this model the data exclude the existence of N$\text{e}$ (N$\mu$) for $m{\text{N}\ell}$ below 6.0 (6.1) TeV, at the limit where $m{\text{N}\ell}$ is equal to $\Lambda$. For $m{\text{N}_\ell}$ $\approx$ 1 TeV, values of $\Lambda$ less than 20 (23) TeV are excluded. These results represent a considerable improvement in sensitivity, covering a larger parameter space than previous searches in pp collisions at 13 TeV.