real function M2_0_MINUS ( p1, p2, p3 )

c  Glen Cowan
c  12 December, 1999
c  Matrix element squared for 3-body decay of J^P = 0- particle.
c  P. Nyborg, Notes on Phase Space, p. 69, in M. Nikolic (ed.), 
c  Kinematics and Multiparticle Systems, Gordon and Breach, 1968.

       implicit             NONE

c  constants

       real              m_pi
       parameter         (m_pi = 0.139)      ! GeV

c  arguments

       real          p1 (*)
       real          p2 (*)
       real          p3 (*)

c  local variables

       real          E1, E2, E3

c begin

       E1 = SQRT (p1(1)**2 + p1(2)**2 + p1(3)**2 + m_pi**2)
       E2 = SQRT (p2(1)**2 + p2(2)**2 + p2(3)**2 + m_pi**2)
       E3 = SQRT (p3(1)**2 + p3(2)**2 + p3(3)**2 + m_pi**2)
       m2_0_minus = ( (E1 - E2) * (E2 - E3) * (E3 - E1) )**2

       return
       END