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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 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.

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