Abstract
A search is performed for heavy Majorana neutrinos (N) decaying into a W boson and a lepton using the CMS detector at the Large Hadron Collider. A signature of two jets and either two same sign electrons or a same sign electron-muon pair is searched for using 19.7 inverse femtobarns of data collected during 2012 in proton-proton collisions at a centre-of-mass energy of 8 TeV. The data are found to be consistent with the expected standard model (SM) background and, in the context of a Type-1 seesaw mechanism, upper limits are set on the cross section times branching fraction for production of heavy Majorana neutrinos in the mass range between 40 and 500 GeV. The results are additionally interpreted as limits on the mixing between the heavy Majorana neutrinos and the SM neutrinos. In the mass range considered, the upper limits range between 0.00015 - 0.72 for |V[eN]|^2 and 6.6E-5 - 0.47 for |V[eN] V*[muN]|^2 / ( |V[eN]|^2 + |V[muN]|^2 ), where V[lN] is the mixing element describing the mixing of the heavy neutrino with the SM neutrino of flavour l. These limits are the most restrictive direct limits for heavy Majorana neutrino masses above 200 GeV.
Highlights
Background estimation4.1 Background from prompt same-sign leptons
The data are found to be consistent with the expected standard model (SM) background and, in the context of a Type-1 seesaw mechanism, upper limits are set on the cross section times branching fraction for production of heavy Majorana neutrinos in the mass range between 40 and 500 GeV
In the mass range considered, the upper limits range between 0.00015–0.72 for |VeN|2 and 6.6 × 10−5–0.47 for |VeNVμ∗N|2/(|VeN|2 + |VμN|2), where V N is the mixing element describing the mixing of the heavy neutrino with the SM neutrino of flavour
Summary
The central feature of the CMS detector is a superconducting solenoid with an internal radius of 3 m. Several samples of simulated SM processes, which include a full treatment of the pp collisions, were used These samples include the full simulation of the CMS detector based on Geant4 [35] and are reconstructed using the same CMS software as used for data. A number of Monte Carlo (MC) event generators are used to simulate signal and background events: alpgen v2.14 [36], MadGraph v5.1.3.30 [37] and pythia v6.4.22 [38]. In order to simulate heavy Majorana neutrino events, a leading-order (LO) event generator described in ref. The pythia v6.4.22 generator is used to model the production of WZ and ZZ with fully leptonic final states. Events from double W-strahlung and double parton scatterings as well as triboson and tt plus boson (ttW, ttZ, ttWW) are generated with MadGraph v5.1.3.30. For s-channel W production, which has the same production kinematics as the signal
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