It is often necessary to put a particle “on the mass shell” in a calculation. This can, of course, be accomplished with a LET command such as
but an alternative method is provided by two commands MASS and MSHELL. MASS takes a list of equations of the form:
⟨vector variable⟩=⟨scalar variable⟩
The only effect of this command is to associate the relevant scalar variable as a mass with the corresponding vector. If we now say
mshell ⟨vector variable⟩,…,⟨vector variable⟩⟨terminator⟩
and a mass has been associated with these arguments, a substitution of the form
⟨vector variable⟩.⟨vector variable⟩ = ⟨mass⟩^2
is set up. An error results if the variable has no preassigned mass.