18.5 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:

vector variable=scalar variable

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

mshell vector variable,,vector variableterminator

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.