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Missing Transverse Energy 101
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Weakly interacting neutral particles produced in proton-proton (pp) collisions at the LHC traverse the CMS detector unobserved.
Their presence is inferred from the measurable momentum imbalance in the plane perpendicular to the beam direction when produced alongside electromagnetically charged or neutral particles. This measurable transverse momentum imbalance is referred to as missing transverse momentum (MET).
Precise determination of MET is critical for Standard Model measurements involving final states with neutrinos and searches for physics beyond the SM targeting new weakly interacting particles.
MET reconstruction is sensitive to experimental resolutions and mis-measurements of reconstructed particles, detector artifacts, and the effects of additional pp interactions within the same or nearby bunch crossings (pileup).
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MET Calibrations
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Inaccurate MET estimation can result from sources such as non-linearity in the calorimeter’s response to hadrons, minimum energy thresholds in the calorimeters, and pT thresholds or inefficiencies in track reconstruction. These issues are mitigated through calibration procedures discussed in this exercise.
Type-1 MET is the default MET calibration recommended by CMS.
Type-1 smear MET enhances data-MC agreement, and JME POG advises analysts to assess its impact in their studies.
MET is influenced by uncertainties from all contributing objects, including jets, leptons, photons, and unclustered energy. Systematic variations in the scale and resolution of each component must be propagated to the MET calculation to evaluate their impact on the analysis.
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MET performance
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The performance of MET is studied in events with a well-measured Z boson (decaying to electrons or muons) or an isolated photon, which should have little to no genuine MET.
Transverse momentum conservation is used to study MET response and resolution along z-axis.
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Handling Anomalous MET Events
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Large MET in an event may be caused by detector noise, cosmic rays, and beam-halo particles. Such MET with uninteresting origins is called false MET, anomalous MET, or fake MET and can be an indication of problematic event reconstruction.
Events with anomalos mets can be rejected using the Noisy event filters.
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