In particle collider experiments, one often defines the angular position of a particle in terms of an azimuthal angle ϕ ϕ and pseudorapidity η η . Additionally the transverse momentum, p T , is usually measured. In this case if the particles are massless, or highly relativistic ( E ≫ ≫ m ) then the invariant mass becomes: M 2 = 2 p T 1 p T 2 ( cosh ⁡ ⁡ ( η η 1 − − η η 2 ) − − cos ⁡ ⁡ ( ϕ ϕ 1 − − ϕ ϕ 2 ) ) .