Abstract
Abstract A search for heavy Majorana neutrinos in events containing a pair of high-p T leptons of the same charge and high-p T jets is presented. The search uses 20.3 fb−1 of pp collision data collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of $$ \sqrt{s}=8 $$ s = 8 TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons W R and Z′.
Highlights
Background and signal simulationThere are several SM interactions that can produce pairs of isolated charged leptons from vector boson decays, from Drell-Yan and diboson processes and from the decay products of top quarks
Processes that contribute to the background with pairs of same-sign leptons are indicated by the label ‘SS’ and processes which contribute with pairs of opposite-sign leptons, which are included in the charge-misidentification background estimate, are indicated by the label ‘OS’
The numbers of events measured in data are compared to the expected numbers of background events in the signal regions, with the intention of interpreting an excess of events in data in terms of a heavy Majorana neutrino in the mTISM or left-right symmetric model (LRSM) models
Summary
The ATLAS detector [19] surrounds the interaction point and covers nearly the entire solid angle. The detector consists of an inner detector (ID) tracking system, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS) that surrounds the other detector systems. The ID tracking system consists of a silicon pixel detector, a silicon microstrip tracker, both covering |η| < 2.5, and a transition radiation tracker covering |η| < 2.0. The electromagnetic accordion calorimeter is composed of lead and liquidargon (LAr) and provides coverage for |η| < 3.2. Hadronic calorimetry is provided by steel and scintillator tile calorimeters for |η| < 1.7 and copper and LAr calorimeters for 1.5 < |η| < 3.2. Additional LAr calorimeters with copper and tungsten absorbers cover the forward region. The MS consists of dedicated trigger chambers covering |η| < 2.4 and precision tracking detectors covering |η| < 2.7. This is followed by two software-based trigger levels (L2 and EF), which together further reduce the event rate to less than 1 kHz
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