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What Happens When Energy Goes Missing?

11 Oct 2016, 14:30 UTC
What Happens When Energy Goes Missing?
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Article: Performance of algorithms that reconstruct missing transverse momentum in √s = 8 TeV proton-proton collisions in the ATLAS detector
Authors: The ATLAS Collaboration
Reference: arXiv:1609.09324
The ATLAS experiment recently released a note detailing the nature and performance of algorithms designed to calculate what is perhaps the most difficult quantity in any LHC event: missing transverse energy. Missing transverse energy (MET) is so difficult because by its very nature, it is missing, thus making it unobservable in the detector. So where does this missing energy come from, and why do we even need to reconstruct it?
Figure 1: LHC momentum conservation.
The LHC accelerate protons towards one another on the same axis, so that they collide head on. Therefore, the incoming partons have net momentum along the direction of the beamline, but no net momentum in the transverse direction (see Figure 1). MET is then defined as the negative vectorial sum (in the transverse plane) of all recorded particles. Any nonzero MET indicates a particle that escaped the detector. This escaping particle could be a regular Standard Model neutrino, or something much more exotic, such as the lightest supersymmetric particle or a dark matter candidate.
Figure 2: ATLAS event display ...

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