Missing energy

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Missing energy, often denoted $E^{\mathrm{miss}}$, refers to energy that is not detected in a detector but is expected because of conservation of momentum. It is also called missing momentum. Both terms are used to refer both to the vectorial form $\vec{E}^{\mathrm{miss}}$ and its norm $E^{\mathrm{miss}} = |\vec{E}^{\mathrm{miss}}|$.
$\vec{E}^{\mathrm{miss}} = - \sum_{i}^{\mathrm{visible}} \vec{E}_i.$
In hadron colliders, the longitudinal momentum component of the initial state is unknown, because hadrons are composite particles whose energy is shared by their constituent partons and constantly transferred between them. Therefore, conservation of momentum can only be applied to infer missing energy in the transverse plane, this is called missing transverse energy or missing transverse momentum, $\vec{E}_{\mathrm{T}}^{\mathrm{miss}}$.