missingmass

Missing mass is a concept in experimental physics used to infer the invariant mass of portions of a reaction that are not directly observed. It is determined from energy and momentum conservation by comparing the known initial four-momentum with the measured four-momenta of all detected final-state particles. If P_initial is the incoming system’s four-momentum and P_detected is the sum of the detected final-state four-momenta, the missing four-momentum is P_missing = P_initial − P_detected, and the missing mass is m_missing^2 = P_missing^2.

In collider and fixed-target experiments, the initial four-momentum is known from the beam conditions, so the missing mass can be reconstructed event-by-event. This technique is widely used to search for or study invisible or weakly interacting particles, such as neutrinos or hypothetical dark matter candidates, as well as to characterize unobserved reaction channels. Missing-mass analyses are also employed in nuclear and hadronic physics to determine binding energies and excitation spectra, for example in hypernuclear spectroscopy or meson production experiments.

Applications include identifying neutrinos in semi-leptonic decays, testing conservation laws in nuclear reactions, and constraining new physics where undetected particles carry away energy and momentum. In hadron colliders, a related concept is missing transverse energy (MET), which uses momentum conservation in the plane transverse to the beam to cope with incomplete information along the beam direction.

Limitations arise from detector resolution, calibration, and the presence of multiple undetected particles. Initial-state radiation and ambiguities in assigning detected products can blur the missing-mass distribution, requiring careful modeling and calibration.