TEvtGen/PHOTOS/phint.F

   1
   2       FUNCTION PHINT(IDUM)
   3 C.----------------------------------------------------------------------
   4 C.
   5 C.    PHINT:   PHotos INTerference
   6 C.
   7 C.    Purpose:  Calculates interference between emission of photons from
   8 C.              different possible chaged daughters stored in
   9 C.              the  HEP common /PHOEVT/.
  10 C.
  11 C.    Input Parameter:    commons /PHOEVT/ /PHOMOM/ /PHOPHS/
  12 C.
  13 C.
  14 C.    Output Parameters:
  15 C.
  16 C.
  17 C.    Author(s):  Z. Was,                         Created at:  10/08/93
  18 C.                                                Last Update:
  19 C.
  20 C.----------------------------------------------------------------------
  21
  22       IMPLICIT NONE
  23       REAL*8 PHINT,phint1
  24       REAL*8 PHOCHA
  25       INTEGER IDUM
  26       INTEGER NMXPHO
  27       PARAMETER (NMXPHO=10000)
  28       INTEGER IDPHO,ISTPHO,JDAPHO,JMOPHO,NEVPHO,NPHO
  29       REAL*8 PPHO,VPHO
  30       COMMON/PHOEVT/NEVPHO,NPHO,ISTPHO(NMXPHO),IDPHO(NMXPHO),
  31      &JMOPHO(2,NMXPHO),JDAPHO(2,NMXPHO),PPHO(5,NMXPHO),VPHO(4,NMXPHO)
  32       DOUBLE PRECISION MCHSQR,MNESQR
  33       REAL*8 PNEUTR
  34       COMMON/PHOMOM/MCHSQR,MNESQR,PNEUTR(5)
  35       DOUBLE PRECISION COSTHG,SINTHG
  36       REAL*8 XPHMAX,XPHOTO
  37       COMMON/PHOPHS/XPHMAX,XPHOTO,COSTHG,SINTHG
  38       REAL*8 MPASQR,XX,BETA,PQ1(4),PQ2(4),PPHOT(4)
  39       REAL*8 SS,PP2,PP,E1,E2,Q1,Q2,COSTHE
  40       LOGICAL IFINT
  41       INTEGER K,IDENT
  42 C
  43       DO  K=JDAPHO(2,1),JDAPHO(1,1),-1
  44          IF(IDPHO(K).NE.22) THEN
  45            IDENT=K
  46            GOTO 20
  47          ENDIF
  48       ENDDO
  49  20   CONTINUE
  50 C check if there is a photon
  51       IFINT= NPHO.GT.IDENT
  52 C check if it is two body + gammas reaction
  53       IFINT= IFINT.AND.(IDENT-JDAPHO(1,1)).EQ.1
  54 C check if two body was particle antiparticle (we improve on it !
  55 C      IFINT= IFINT.AND.IDPHO(JDAPHO(1,1)).EQ.-IDPHO(IDENT)
  56 C check if particles were charged
  57       IFINT= IFINT.AND.abs(PHOCHA(IDPHO(IDENT))).GT.0.01D0
  58 C check if they have both charge
  59       IFINT= IFINT.AND.
  60      $       abs(PHOCHA(IDPHO(JDAPHO(1,1)))).gt.0.01D0
  61 C calculates interference weight contribution
  62       IF(IFINT) THEN
  63         MPASQR = PPHO(5,1)**2
  64         XX=4.D0*MCHSQR/MPASQR*(1.-XPHOTO)/(1.-XPHOTO+(MCHSQR-MNESQR)/
  65      &     MPASQR)**2
  66          BETA=SQRT(1.D0-XX)
  67          PHINT  = 2D0/(1D0+COSTHG**2*BETA**2)
  68          SS =MPASQR*(1.D0-XPHOTO)
  69          PP2=((SS-MCHSQR-MNESQR)**2-4*MCHSQR*MNESQR)/SS/4
  70          PP =SQRT(PP2)
  71          E1 =SQRT(PP2+MCHSQR)
  72          E2 =SQRT(PP2+MNESQR)
  73          PHINT= (E1+E2)**2/((E2+COSTHG*PP)**2+(E1-COSTHG*PP)**2)
  74 C
  75       q1=PHOCHA(IDPHO(JDAPHO(1,1)))
  76       q2=PHOCHA(IDPHO(IDENT))
  77       do k=1,4
  78        pq1(k)=ppho(k,JDAPHO(1,1))
  79        pq2(k)=ppho(k,JDAPHO(1,1)+1)
  80        pphot(k)=ppho(k,npho)
  81       enddo
  82        costhe=(pphot(1)*pq1(1)+pphot(2)*pq1(2)+pphot(3)*pq1(3))
  83        costhe=costhe/sqrt(pq1(1)**2+pq1(2)**2+pq1(3)**2)
  84        costhe=costhe/sqrt(pphot(1)**2+pphot(2)**2+pphot(3)**2)
  85 C
  86 ! --- this IF checks whether JDAPHO(1,1) was MCH or MNE.
  87 ! --- COSTHG angle (and in-generation variables) may be better choice
  88 ! --- than costhe. note that in the formulae below amplitudes were
  89 ! --- multiplied by (E2+COSTHG*PP)*(E1-COSTHG*PP).
  90         IF (costhg*costhe.GT.0) then
  91
  92          PHINT= (q1*(E2+COSTHG*PP)-q2*(E1-COSTHG*PP))**2
  93      &         /(q1**2*(E2+COSTHG*PP)**2+q2**2*(E1-COSTHG*PP)**2)
  94         ELSE
  95
  96          PHINT= (q1*(E1-COSTHG*PP)-q2*(E2+COSTHG*PP))**2
  97      &         /(q1**2*(E1-COSTHG*PP)**2+q2**2*(E2+COSTHG*PP)**2)
  98         ENDIF
  99       ELSE
 100          PHINT  = 1D0
 101       ENDIF
 102          phint1=1
 103       END