Experimental Sensitivity for Majorana Neutrinos Produced via a Z Boson at Hadron Colliders arXiv:1001.1229v1 [hep-ph] 8 Jan 2010 A. Rajaraman 1 and D. Whiteson 1 $^{1}$University of California, Irvine, Irvine, California 92697 We present experimental sensitivity for the production of a fourth-generation Majorana neutrino ( N ) via an s -channel Z boson Z → NN → WℓWℓ at hadron colliders. This channel has not been studied for the Tevatron dataset with ∫ L = 5 fb − $^{1}$, where we find that a single experiment could significantly extend current 95% C.L. mass lower limits, to m N > 175 GeV /c $^{2}$, or report 3 σ evidence for the N if m N < 150 GeV /c $^{2}$. With 5 fb − $^{1}$, a single LHC experiment at √$_{s}$ = 10 TeV could expect to set a 95% C.L. mass lower limits of m N > 300 GeV /c $^{2}$, or report 3 σ evidence for the N if m N < 225 GeV /c $^{2}$. PACS numbers: 12.60.-i, 13.85.Rm, 14.80.-j or to muons, and about 80 GeV if it decayed to taus. Theoretical analyses of fourth generation neutrinos at hadron colliders [16–19] have focused on the process q ¯ q ′ → W ± → Nl ± where the fourth generation neu- trino is produced in association with a light charged lep- ton. This process has the significant advantage that only one heavy particle is produced, which increases the mass reach considerably. Furthermore, the neutrino will de- cay through N → l $^{±}$W ∓ which will produce the low- background like-sign dilepton signature in half the events. However, the production cross-section for this process depends on the mixing between the fourth generation with the first three generations due to the W → lN ver- tex. In many models, this mixing angle can be small; for example, if the mixing is generated by GUT or Planck- scale suppressed operators, the angle may be as small as 10 − 11 [12]. For mixing parameters smaller than about 10 − $^{6}$, the neutrino production rates in this channel are too small to be observable at colliders [16]. In models with small mixing angles, the dominant production mech- anism becomes pair production through an s -channel Z , for which the production rate is model-independent. These signals have been studied at various benchmark points for the LHC [20] and for future linear colliders [21]. However, the analysis of the s -channel Z process has not been performed for the Tevatron. In this Letter, we present a sensitivity study for the Tevatron and argue that the LEP limits on Majorana neutrinos can be sig- nificantly improved with an analysis of the data already taken. It would be of great interest to perform a full analysis of this data. We also perform a similar study for the LHC, which can probe the parameter space to much higher energies. The study of a fourth generation of quarks and lep- tons has undergone a renaissance. While earlier studies [1] claimed that the fourth generation was ruled out by experimental data, a recent analysis [2] has shown that the constraints can be satisfied by appropriate choices of mass splittings between the new fourth generation par- ticles. Furthermore, the existence of the fourth gener- ation can help to address certain discrepancies between the Standard Model and experimental results in the b- quark sector[3–8], leading to a revival of interest in this possibility (see [9] for a review). In view of the imminent arrival of LHC data, it is of particular interest to search for signals of these fourth- generation fermions at hadron colliders. Such analyses have been performed for the t ′ and b ′ quarks. CDF has searched for the t ′ quark [10] using the CKM suppressed decay t ′ → qW and for the b ′ quark [11] using the pro- cess b $^{′}$b′ → t ¯ tW $^{−}$W + which can yield like-sign dileptons. These searches have set limits of 311 GeV for the t ′ [10], and 338 GeV for the b ′ [11] (but see [12]). The LHC will discover or exclude fourth generation quarks up to about a TeV [13–15]. However, few detailed analyses have been performed in the lepton sector. By analogy with the first three gener- ations, one might expect the leptons of the fourth gener- ation to be lighter than the fourth generation quarks and in particular, the fourth generation neutrino might be expected to be the lightest new particle. It is of interest to see if this neutrino can be found at colliders. Past searches for fourth generation neutrinos have mainly been performed at lepton colliders. In particular, LEP II has looked for neutrinos produced in the process e $^{+}$e − → Z → NN , where the neutrinos subsequently decay via the process N → l $^{±}$W $^{∓}$. No excess of such events was found, which placed a limit of about 100 GeV for Dirac neutrinos decaying to electrons, muons or taus. For Majorana neutrinos the corresponding limits were about 90 GeV if the neutrino decayed either to electrons We consider an extension to the standard model by a fourth generation of fermions and a right-handed neu- trino. The mass term for the neutrinos can be written INTRODUCTION PRODUCTION AND DECAY