The invariant mass of a system made of two massless particles whose momenta form an angle θ θ has a convenient expression: M 2 = ( E 1 + E 2 ) 2 − − ‖ p 1 + p 2 ‖ 2 = [ ( p 1 , 0 , 0 , p 1 ) + ( p 2 , 0 , p 2 sin ⁡ ⁡ θ θ , p 2 cos ⁡ ⁡ θ θ ) ] 2 = ( p 1 + p 2 ) 2 − − p 2 2 sin 2 ⁡ ⁡ θ θ − − ( p 1 + p 2 cos ⁡ ⁡ θ θ ) 2 = 2 p 1 p 2 ( 1 − − cos ⁡ ⁡ θ θ ) .