missing transverse energy
/ MET = em-ee-tee, or 'met' /
Suppose you weigh a sealed box of marbles, open it, weigh the marbles you can see, and find some weight unaccounted for. You would conclude an invisible marble must have escaped, and you could even infer how heavy. Particle physicists play the same accounting game to catch particles a detector cannot see directly. By tallying up what came out of a collision and noticing what is missing from the books, they infer the presence of invisible particles, above all neutrinos.
The bookkeeping uses conservation of momentum, but with a clever twist. In a head-on collision, the two beams travel along one axis, and physicists focus on momentum in the plane perpendicular to that axis — the transverse plane. Before the collision, the total transverse momentum is essentially zero, because the beams have none sideways. By conservation, the transverse momenta of everything produced must also sum to zero. So the experiment adds up the transverse momenta of all the particles it actually detected; if they do not cancel, the leftover imbalance must have been carried away by something unseen. That imbalance is the missing transverse energy: it points in the direction, and gives the size, of the momentum that escaped detection. (The trick is restricted to the transverse plane because particles also vanish unmeasured down the beam pipe, spoiling the balance along the beam axis.)
Missing transverse energy is the standard way to infer neutrinos at colliders, and it is one of the most powerful handles in searches for new physics: many proposed particles, including candidates for dark matter, would leave a detector invisibly, betrayed only by an imbalance in the visible momentum. The crucial honesty here is that missing energy is an inference, not a sighting. A large imbalance can also come from a mismeasured jet or instrumental glitch, so experiments treat it carefully and never claim to have 'seen' the invisible particle — only to have detected its missing footprint.
When a W boson decays to an electron and a neutrino, the detector records the electron but not the neutrino. The neutrino reveals itself only as missing transverse energy — an imbalance pointing opposite the electron, whose size matches the momentum the neutrino must have carried off.
What does not balance must have escaped unseen.
Missing energy is inferred, not seen: it is restricted to the transverse plane (particles also escape down the beam) and can be faked by mismeasurement, so it is never proof of a particle on its own.