where ε ε 1 = − − p 1 ⋅ ⋅ P − − P 2 = − − P 2 + p 1 2 − − p 2 2 2 − − P 2 ε ε 2 = − − p 2 ⋅ ⋅ P − − P 2 = − − P 2 + p 2 2 − − p 1 2 2 − − P 2 are the projections of the momenta p 1 and p 2 along the direction of the total momentum P . Subtracting the two constraints H 1 Ψ Ψ = 0 and H 2 Ψ Ψ = 0 , gives