Example

REDUCE

18.6 Example

We give here as an example of a simple calculation in high energy physics the computation of the Compton scattering cross-section as given in Bjorken and Drell Eqs. (7.72) through (7.74). We wish to compute the trace of

( ′ ′ )
γ-⋅ eγ-⋅ eγ-⋅ ki-+ γ-⋅ eγ-⋅ e-γ-⋅ kf
2k.pi 2k′ ⋅ pi

( γ ⋅ kγ ⋅ eγ ⋅ e′ γ ⋅ k γ ⋅ e′γ ⋅ e)
----i---------+ ----f-′-------
2k.pi 2k ⋅ pi

where k_i and k_f are the four-momenta of incoming and outgoing photons (with polarization vectors e and e^′ and laboratory energies k and k^′ respectively) and p_i, p_f are incident and ﬁnal electron four-momenta.

Omitting therefore an overall factor 2
α---
2m2 ( ′)
k-
k ² we need to ﬁnd one quarter of the trace of

( γ ⋅ e′γ ⋅ eγ ⋅ k γ ⋅ eγ ⋅ e′γ ⋅ k )
------------i-+ -------------f
2k.pi 2k ′.pi

( )
γ ⋅ kiγ ⋅ eγ ⋅ e′ γ ⋅ kf γ ⋅ e′γ ⋅ e
--------------+ -------′------
2k.pi 2k.pi

A straightforward REDUCE program for this, with appropriate substitutions (using P1 for p_i, PF for p_f, KI for k_i and KF for k_f) is

 on div; % this gives output in same form as Bjorken and Drell.
 mass ki= 0, kf= 0, p1= m, pf= m; vector e,ep;
 % if e is used as a vector, it loses its scalar identity
 %      as the base of natural logarithms.
 mshell ki,kf,p1,pf;
 let p1.e= 0, p1.ep= 0, p1.pf= m^2+ki.kf, p1.ki= m*k,p1.kf=
     m*kp, pf.e= -kf.e, pf.ep= ki.ep, pf.ki= m*kp, pf.kf=
     m*k, ki.e= 0, ki.kf= m*(k-kp), kf.ep= 0, e.e= -1,
     ep.ep=-1;
 operator gp;
 for all p let gp(p)= g(l,p)+m;
 comment this is just to save us a lot of writing;
 gp(pf)*(g(l,ep,e,ki)/(2*ki.p1) + g(l,e,ep,kf)/(2*kf.p1))
   * gp(p1)*(g(l,ki,e,ep)/(2*ki.p1) + g(l,kf,ep,e)/
     (2*kf.p1))$
 write "The Compton cxn is ",ws;

(We use P1 instead of PI in the above to avoid confusion with the reserved variable PI).

This program will print the following result

                         2    1      -1    1   -1
The Compton cxn is 2*E.EP  + ---*K*KP   + ---*K  *KP - 1
                              2            2

PrevTail

Front