GEANT321/gphys/gdeca2.F

   1 *
   2 * $Id$
   3 *
   4 * $Log$
   5 * Revision 1.1.1.1  1995/10/24 10:21:23  cernlib
   6 * Geant
   7 *
   8 *
   9 #include "geant321/pilot.h"
  10 *CMZ :  3.21/02 29/03/94  15.41.21  by  S.Giani
  11 *-- Author :
  12       SUBROUTINE GDECA2(XM0,XM1,XM2,PCM)
  13 C.
  14 C.    ******************************************************************
  15 C.    *                                                                *
  16 C.    *  Simulates two body decay process with isotropic angular       *
  17 C.    *  distribution in CMS.                                          *
  18 C.    *                                                                *
  19 C.    *    ==>Called by : GDECAY                                       *
  20 C.    *       Author    G.Patrick  *********                           *
  21 C.    *                                                                *
  22 C.    ******************************************************************
  23 C.
  24 #include "geant321/gconsp.inc"
  25       DIMENSION PCM(4,2)
  26       DIMENSION RNDM(2)
  27 C.
  28 C.    ------------------------------------------------------------------
  29 C.
  30 C             Generate first decay product in CMS.
  31 C
  32       E1=(XM0*XM0+XM1*XM1-XM2*XM2)/(2.*XM0)
  33       P1=SQRT(ABS((E1-XM1)*(E1+XM1)))
  34 C
  35 C             Isotropic decay angular distribution.
  36 C
  37       CALL GRNDM(RNDM,2)
  38       COSTH=2.*RNDM(1)-1.
  39       IF(ABS(COSTH).GE.1.0) THEN
  40          COSTH=SIGN(1.,COSTH)
  41          SINTH=0.
  42       ELSE
  43          SINTH=SQRT((1.-COSTH)*(1.+COSTH))
  44       ENDIF
  45       PHI=TWOPI*RNDM(2)
  46 C
  47 C             Polar co-ordinates to momentum components.
  48 C
  49       PCM(1,1)=P1*SINTH*COS(PHI)
  50       PCM(2,1)=P1*SINTH*SIN(PHI)
  51       PCM(3,1)=P1*COSTH
  52       PCM(4,1)=E1
  53 C
  54 C             Generate second decay product.
  55 C
  56       PCM(1,2)=-PCM(1,1)
  57       PCM(2,2)=-PCM(2,1)
  58       PCM(3,2)=-PCM(3,1)
  59       PCM(4,2)=SQRT(PCM(1,2)**2+PCM(2,2)**2+PCM(3,2)**2+XM2*XM2)
  60 C
  61       END