= Missing transverse momentum
{title2=$\mathbf p_T^{\rm miss}=-\sum_{\rm visible}\mathbf p_T$}
<Missing transverse momentum> is the negative <vector sum> of reconstructed visible <transverse momenta>, with detector corrections specified. Under the ideal hypotheses of zero total incoming <transverse momentum>, complete visible reconstruction and exactly one invisible <neutrino>, <momentum conservation> gives
$$
\mathbf p_T^{\rm miss}=\mathbf p_{T\nu}.
$$
The <neutrino> is \b[not directly seen]. Even under these ideal hypotheses, its longitudinal <momentum> $p_{z\nu}$ is not measured. Beam remnants can carry substantial unobserved longitudinal <momentum>; the colliding <quarks>' longitudinal <momenta> are also unknown event by event. Imposing an assumed parent <mass> can constrain $p_{z\nu}$, but that imports a hypothesis rather than measuring it. Finite coverage, poorly reconstructed <particle jets> and other invisible <particles> can also generate or modify <missing transverse momentum>.
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