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In experimental particle physics, missing energy refers to energy which is not detected in a particle detector, but is expected due to the laws of conservation of energy and conservation of momentum. For example, if an electron and a positron collide head-on at equal speeds in the lab frame, any net momentum of outgoing particles indicates a missing energy. Missing energy is generally attributed to particles which escape the detector without being detected. Although, apparent missing energy may also be caused by mismeasurement within a field that is also attributed to executive functions of physics on the energy/momentum of detected particles.
In hadron colliders, the initial momentum of the colliding partons along the beam axis is not known (because the energy of each hadron is split, and constantly exchanged, between its constituents), so the amount of total missing energy cannot be determined. However, the initial energy in particles travelling transverse to the beam axis is zero, so any net momentum in the transverse direction indicates missing transverse energy.
Missing energy is commonly used to infer the presence of non-detectable particles such as the standard model neutrino and is expected to be a signature of many predicted physics events that contain particles that do not interact with the detector, for example the Lightest Supersymmetric Particle.
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