2 4 jets. If one decays leptonically, we expect approxi- mately 2 jets and a third lepton. If both decay leptoni- cally, we expect approximately zero jets but four leptons. All but the fully leptonic mode, the smallest fraction, contribute to the ℓ $^{±}$ℓ$^{±}$jj signature and allow for direct reconstruction of the N . where ψ c = − iγ $^{2}$ψ $^{∗}$. This theory has two mass eigen- states of masses m 1 = − ( M/ 2) + √ m 2 D + M $^{2}$/ 4 , m 2 = ( M/ 2) + √ m 2 D + M $^{2}$/ 4. In addition, the mass of the fourth generation lepton is constrained to be close in mass to the neutrinos by precision electroweak constraints [2]. The lighter neutrino mass eigenstate is the Majorana fermion N = N c $_{L}$+ N$_{L}$ . The coupling for pair production via the Z is through the coupling L$_{Z}$ = Z$_{µ}$J µ where We consider processes where only the lightest neu- trino is produced, providing the most model-independent bound on this theory. In future work, we will study the effect of the second neutrino and fourth generation lep- ton. For this analysis, we treat the lepton and second neutrino as infinitely massive, corresponding to a limit where M, m$_{D}$ go to infinity with m 2 D M fixed. L$_{m}$ = − 1 2 ( Q c $_{R}$N c $_{R}$) ( 0 m$_{D}$ m$_{d}$ M ) ( Q$_{R}$ N$_{R}$ ) + h.c. (1) as We select events with the ℓ $^{±}$ℓ$^{±}$jj signature: • two like-signed reconstructed leptons ( e or µ ), each with p$_{T}$ > 20 GeV and | η | < 2 . 0 • at least two reconstructed jets, each with p$_{T}$ > 15 GeV and | η | < 2 . 5 EXPERIMENTAL SENSITIVITY In the case that two jets are reconstructed, the mass of the N can be reconstructed from the two jets and either of the leptons. In the case that three jets are recon- structed, we form the mass of the N from the invariant mass of the two jets which are closest to the W mass, and either of the leptons. In the case that four jets are reconstructed, the mass of the two N s come from the ljj assignments that give the best W masses and the small- est difference between the two reconstructed m$_{N}$ s. See Figure 2. The heavy neutrino will decay through N → W $^{±}$l∓ (the neutral current process is suppressed.) Note that N can decay to either sign of lepton, giving like-sign leptons in half of the events, see Fig 1. We assume that the heavy neutrino decays promptly; this will be the case unless the mixing between the fourth generation and the first three is extremely small [12]. We consider the possible decay modes N → W ( e, µ, τ ). In a hadron collider, backgrounds to τ leptons are much larger and efficiencies are much lower than for e and µ , giving the τ decay mode little power. We consider two cases, (a) µµ , in which the non- τ decays appear solely as muons: BR( N → W µ ) = 1 - BR( N → W τ ); and (b) ℓℓ , with ℓ = e, µ in which the non- τ decays appear as both electrons and muons: BR( N → W µ ) + BR( N → W e ) = 1 - BR( N → W τ ). The µ $^{±}$µ± mode has significantly smaller background rate than ℓ $^{±}$ℓ$^{±}$. J µ = e 2 sin θ$_{W}$ cos θ$_{W}$ ( ¯ N 1 γ $^{µ}$γ $^{5}$N 1 ) ] 2 Neutrino Mass [GeV/c 80 100 120 140 160 180 200 Events 50 100 150 200 250 300 350 400 450 2j 3j 4j FIG. 2: Reconstructed Majorana neutrino ( N ) mass in events with 2, 3 or at least 4 jets for m N = 150 GeV/ c $^{2}$. Backgrounds At the Tevatron, the largest backgrounds to the ℓ $^{±}$ℓ$^{±}$jj signature come from W γ or W Z production or misidentified leptons [22] either from semi-leptonic t ¯ t de- cays or direct W + jets production. For the Tevatron, we extrapolate the number of ex- pected backgrounds events from Ref. [22] to a dataset If the N decays to ℓW , then the decay of NN → ℓW ℓW can be categorized by the decays of the W bosons. If both W s decay hadronically, we expect approximately FIG. 1: Pair production of the heavy Majorana neutrino N via a Z boson, and subsequent decay W $^{±}$l$^{∓}$.