HERWIG/src/hwuaer.f

   1
   2 CDECK  ID>, HWUAER.
   3
   4 *CMZ :-        -23/08/94  13.22.29  by  Mike Seymour
   5
   6 *-- Author :    Ian Knowles
   7
   8 C-----------------------------------------------------------------------
   9
  10       FUNCTION HWUAER(R)
  11
  12 C-----------------------------------------------------------------------
  13
  14 C     Real part of photon self-energy: Pi_{gg}(R=M^2/Q^2)
  15
  16 C-----------------------------------------------------------------------
  17
  18       DOUBLE PRECISION HWUAER,R,ZERO,ONE,TWO,FOUR,FVTHR,THIRD,RMAX,BETA
  19
  20       PARAMETER (ZERO=0.D0, ONE=1.D0, TWO=2.D0, FOUR=4.D0,
  21
  22      &           FVTHR=1.666666666666667D0, THIRD=.3333333333333333D0)
  23
  24       PARAMETER (RMAX=1.D6)
  25
  26       IF (ABS(R).LT.1.D-3) THEN
  27
  28 C Use assymptotic formula
  29
  30          HWUAER=-FVTHR-LOG(ABS(R))
  31
  32       ELSEIF (ABS(R).GT.RMAX) THEN
  33
  34          HWUAER=ZERO
  35
  36       ELSEIF (FOUR*R.GT.ONE) THEN
  37
  38          BETA=SQRT(FOUR*R-ONE)
  39
  40          HWUAER=THIRD
  41
  42      &         -(ONE+TWO*R)*(TWO-BETA*ACOS(ONE-ONE/(TWO*R)))
  43
  44       ELSE
  45
  46          BETA=SQRT(ONE-FOUR*R)
  47
  48          HWUAER=THIRD
  49
  50      &         -(ONE+TWO*R)*(TWO+BETA*LOG(ABS((BETA-ONE)/(BETA+ONE))))
  51
  52       ENDIF
  53
  54       RETURN
  55
  56       END