Hello everyone. How to contract lorentz tensors and to specify contraction rules for them? I'm doing High Energy Physics and have to calculate the square of S-matrix (actually, M-matrix) amplitude.

For simplicity, let's say my amplitude has the following form: M = A*(K1,J)+B(K2,J) where A,B - are constants and K1,K2, J - are 4-vectors.

Also let's say that i know contraction rule for JmJn that gives me some known tensor (JJ)mn. (m and n are lorentz indices)

So the task is to specify explicitly the (JJ)mn tensor and learn mathematica how to contract it with K1 and K2 when calculating M^2.

Any ideas? I tried FeynCalc, but didn't manage how to specify (JJ)mn.