See if i have done anything wrong or not~
First do it in the frame where the target proton (marked 2 below) is stationary.
P_1 = (E_1/c,p_1,0,0) \mbox{ ; } P_2 = (mc,0,0,0)
By conservation of four-momentum,
P_1 + P_2 = P_3 + P_4 +P_5 +P_6
(P_1 + P_2)^2 = (P_3 + P_4 +P_5 +P_6)^2
(P_1...