Missing energy is carried by particles that do not interact with the electromagnetic or strong forces and thus are not easily detectable, most notably neutrinos.
[1] In general, missing energy is used to infer the presence of non-detectable particles and is expected to be a signature of many theories of physics beyond the Standard Model.
[2][3][4] The concept of missing energy is commonly applied in hadron colliders.
[5] The initial momentum of the colliding partons along the beam axis is not known — the energy of each hadron is split, and constantly exchanged, between its constituents — so the amount of total missing energy cannot be determined.
Accurate measurements of missing energy are difficult, as they require full, accurate, energy reconstruction of all particles produced in an interaction.