from particle energy deposits in the calorimeters and from noise events [@METJINST]. Muons produced from proton collisions upstream of the detector (beam halo) can mimic proton-proton collisions with large $\ETm$ and are identified using information obtained from the CSCs. The geometry of the CSCs allows efficient identification of beam halo muons, since halo muons that traverse the calorimetry will mostly also traverse one or both CSC endcaps. Events are rejected if a significant amount of energy is lost in the masked crystals that constitute approximately 1% of the ECAL, using information either from the separate readout of the L1 hardware trigger or by measuring the energy deposited around the masked crystals. We select events with a well-reconstructed primary vertex and with the scalar $\sum \pt$ of tracks associated to it greater than 10% of the scalar $\sum \pt$ of all jet transverse momenta. These requirements reject 0.003% of an otherwise good inclusive sample of proton-proton interactions (minimum bias events).

Event selection and classification\[sec:BOX2011\]
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Electrons enter the megajet definition as ordinary jets. Reconstructed muons are not included in the megajet grouping because, unlike electrons, they are distinguished from jets in the HLT. This choice also allows the use of $\PW(\Pgm\cPgn)$+jets 