Abstract
A search for physics beyond the standard model in events with at least three charged leptons (electrons or muons) is presented. The data sample corresponds to an integrated luminosity of 137 fb−1 of proton-proton collisions at sqrt{s} = 13 TeV, collected with the CMS detector at the LHC in 2016–2018. The two targeted signal processes are pair production of type-III seesaw heavy fermions and production of a light scalar or pseudoscalar boson in association with a pair of top quarks. The heavy fermions may be manifested as an excess of events with large values of leptonic transverse momenta or missing transverse momentum. The light scalars or pseudoscalars may create a localized excess in the dilepton mass spectra. The results exclude heavy fermions of the type-III seesaw model for masses below 880 GeV at 95% confidence level in the scenario of equal branching fractions to each lepton flavor. This is the most restrictive limit on the flavor-democratic scenario of the type-III seesaw model to date. Assuming a Yukawa coupling of unit strength to top quarks, branching fractions of new scalar (pseudoscalar) bosons to dielectrons or dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level for masses in the range 15–75 and 108–340 GeV, respectively. These are the first limits in these channels on an extension of the standard model with scalar or pseudoscalar particles.
Highlights
Background estimation and systematic uncertaintiesThe irreducible backgrounds are estimated using simulated event samples and are dominated by the WZ, ZZ, ttZ, and Zγ processes
The event yields of these processes are obtained from theoretical predictions, with normalization corrections derived in dedicated control regions as described below
All of the LT+pmTiss and MT bins are used for the seesaw signal masses under consideration, whereas the appropriate subset of the lepton flavor and dilepton mass bins is used for a given φ mass and branching fraction scenario in the ttφ signal model, such that the low dielectron and dimuon mass spectra are considered for a light ttφ signal with the φ → ee and φ → μμ decays, respectively
Summary
Background estimation and systematic uncertaintiesThe irreducible backgrounds are estimated using simulated event samples and are dominated by the WZ, ZZ, ttZ, and Zγ processes. Unlike the typeIII seesaw heavy fermions, relatively light scalar or pseudoscalar decays do not necessarily produce energetic charged leptons, but can yield striking resonant dilepton signatures in events with high hadronic activity and b tagged jets.
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