22.5 Mass Declarations

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

        let p.p= m^2;

but an alternative method is provided by two commands mass and mshell. mass takes a list of equations of the form:

for example,

        mass p1=m, q1=mu;

The only effect of this command is to associate the relevant scalar variable as a mass with the corresponding vector. If we now say

and a mass has been associated with these arguments, a substitution of the form

is set up. An error results if the variable has no preassigned mass.

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