Abstract
A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of $pp$ collisions at $\sqrt{s}=8$ TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to $e\tau$ or $\mu\tau$, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale $\Lambda$, with the strongest mass constraints arising in regions where the mass equals $\Lambda$. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.
Highlights
Background estimationStandard Model processes that produce events with three or more lepton candidates fall into three classes
For events classified as on-Z, the transverse mass is constructed using the ETmiss and the highest-pT lepton not associated with a Z boson candidate
The production cross section of pair-produced excited leptons via the GM process is independent of Λ, which leads to improved sensitivity at low excited-lepton masses
Summary
The ATLAS detector [34] at the LHC covers nearly the entire solid angle around the collision point. It consists of an inner tracking detector surrounded by a thin superconducting. The ATLAS detector [34] at the LHC covers nearly the entire solid angle around the collision point.1 It consists of an inner tracking detector surrounded by a thin superconducting. A high-granularity silicon pixel detector covers the vertex region and typically provides three measurements per track, with one hit being usually registered in the innermost layer. It is followed by a silicon microstrip tracker, which usually provides four two-dimensional measurement points per track. The muon spectrometer comprises separate trigger and high-precision tracking chambers measuring the deflection of muons in a magnetic field generated by superconducting air-core toroids. This is followed by two software-based trigger levels which together reduce the event rate to about 400 Hz
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