In the center of momentum frame where the total momentum equals zero, p 1 = − − p 2 p 1 2 = p 2 2 E = m 1 2 c 4 + p 1 2 c 2 + m 2 2 c 4 + p 2 2 c 2 = E p 1 = ± ± E 4 − − 2 E 2 m 1 2 c 4 − − 2 E 2 m 2 2 c 4 + m 1 4 c 8 − − 2 m 1 2 m 2 2 c 8 + m 2 4 c 8 2 c E u 1 = − − v 1 .