Physics:Missing mass

The missing mass is the effective mass of all particles missing in a partly measured interaction. Separating into particles of initial and final state, the missing mass is defined by

In exclusive measurements, File:Hepb img469.gif can serve as a test for the completeness of measurement: no missing particle results in File:Hepb img470.gif = 0.

The observed value of File:Hepb img471.gif may well be negative due to measurement errors, whereas an effective mass computed from observed particles will always be real, even in the presence of measurement errors. Note that the error propagation for the case File:Hepb img472.gif gives in first order a zero error for File:Hepb img471.gif , and higher order terms result in a strongly asymmetric probability distribution for File:Hepb img471.gif peaking at negative values.