ClearAll["Global'*"]

<<Feyncalc`

(*Pair production/Setting momentum values*)
SP[p1,p1]=SP[p4,p4]=0
SP[p2,p2]=SP[p3,p3]=m^2
SP[p1,p2]=SP[p3,p4]=s/2-m^2/2
SP[p1,p3]=SP[p2,p4]=m^2/2-t/2
SP[p1,p4]=SP[p2,p3]=-u/2

paira=(e^4/(t-m^2)^2)Tr[GA[ν].(GS[p1]-GS[p3]+m).GA[μ].(GS[p3]+m).GA[μ].(GS[p1]-GS[p3]+m).GA[ν].(GS[p4]-m)]//Contract//Simplify

pairb=(e^4/(u-m^2)^2)Tr[GA[μ].(GS[p1]-GS[p4]+m).GA[ν].(GS[p3]+m).GA[ν].(GS[p1]-GS[p4]+m).GA[μ].(GS[p4]-m)]//Contract//Simplify

pairab=2*e^4/((t-m^2)(u-m^2))Tr[GA[ν].(GS[p1]-GS[p3]+m).GA[μ].(GS[p3]+m).GA[ν].(GS[p1]-GS[p4]+m).GA[μ].(GS[p4]-m)]//Contract//Simplify

pair=paira+pairb+pairab//Simplify

pair/.u->2m^2-s-t//Simplify

pair/.m->0