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Abstract
A search for the pair-production of heavy leptons ($N^0,L^{\pm}$) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels $N^0\rightarrow W^{\pm}l^{\mp}$ ($\ell = e, \mu, \tau$) and $L^{\pm}\rightarrow W^\pm \nu$ ($\nu=\nu_e, \nu_\mu, \nu_\tau$) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying $W$ boson, and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of $20.3\,fb^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair-production is observed. Heavy leptons with masses below 325--540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.
Highlights
Experiments show that neutrinos have much smaller masses than charged leptons
Events selected in the opposite sign (OS) (SS) final state are required to have a EmT iss of at least 110 (100) GpeVffiffiffiffiaffiffinffiffiffidffiffi,ffiffiffiffifffioffiffiffirffiffiffiffiOffiffiffiffiSffiffi events, an angular separation ΔRjj 1⁄4 ðΔηÞ2 þ ðΔφÞ2 < 2 between the two jets with highest pT, where Δη and Δφ are defined as the differences in pseudorapidity and azimuthal angle between the jets
The results of limit calculations combining the observations in the OS and SS final states are shown in Figs. 3 and 4
Summary
Experiments show that neutrinos have much smaller masses than charged leptons (see Ref. [1] and references therein). Among different models for the seesaw mechanism, the type-III model [2,5] introduces at least two extra triplets of fermionic fields with zero hypercharge in the adjoint representation of SUð2ÞL that generate neutrino masses and couple to gauge bosons. This model predicts new charged and neutral heavy leptons that could be produced in proton-proton collisions at the LHC. The search is performed for the process pp → N0LÆ → WÆl∓WÆν, where one W boson decays leptonically and the other W boson decays hadronically, resulting in a lepton pair in the final state with either the same charge [same sign (SS)] or with the opposite charge [opposite sign (OS)]
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