<<FeynCalc`

<<feyncalc.m

SpinorUBar[p2,m].DiracMatrix[nu].(DiracSlash[l] + DiracSlash[q]+m).DiracMatrix[mu].(DiracSlash[l]+m).DiracMatrix[nu].SpinorU[p1,m]

% /. DiracSlash[l] -> DiracSlash[k] - DiracSlash[q]*y + z*DiracSlash[p1]

% /. DiracSlash[q] -> DiracSlash[p2]- DiracSlash[p1]

DiracSimplify[%]

%  /. {ScalarProduct[p1, p1] -> m^2, ScalarProduct[p2,p2]->m^2, ScalarProduct[p1,p2] -> m^2 - ScalarProduct[q,q]/2, ScalarProduct[k,p1]-> 0, ScalarProduct[k,p2]-> 0}

Expand[%]

res = %;

res;

% /. {FourVector[p1, mu] -> (FourVector[p,mu]-FourVector[q,mu])/2, FourVector[p2,mu]->(FourVector[p,mu]+FourVector[q,mu])/2}

Expand[%]

+2 ((OverBar[k])^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")-4 ((OverBar[k])^mu) ("φ""("(OverBar[p2])","m")").((OverBar[γ])·(OverBar[k])).("φ""("(OverBar[p1])","m")")-2 (m^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")+2 (m^2) (z^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")+4 (m^2) z ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")-2 m (z^2) ((OverBar[p])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")+2 m z ((OverBar[p])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")+4 m ((OverBar[q])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")+2 (y^2) ((OverBar[q])^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")-2 y ((OverBar[q])^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")-8 m y ((OverBar[q])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")+2 y z ((OverBar[q])^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")+4 m y z ((OverBar[q])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")+2 m (z^2) ((OverBar[q])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")-2 z ((OverBar[q])^2) ("φ""("(OverBar[p2])","m")").((OverBar[γ])^mu).("φ""("(OverBar[p1])","m")")-6 m z ((OverBar[q])^mu) ("φ""("(OverBar[p2])","m")").("φ""("(OverBar[p1])","m")")