4 *CMZ :- -23/08/94 13.22.29 by Mike Seymour
6 *-- Author : Ian Knowles
8 C-----------------------------------------------------------------------
12 C-----------------------------------------------------------------------
14 C Real part of photon self-energy: Pi_{gg}(R=M^2/Q^2)
16 C-----------------------------------------------------------------------
18 DOUBLE PRECISION HWUAER,R,ZERO,ONE,TWO,FOUR,FVTHR,THIRD,RMAX,BETA
20 PARAMETER (ZERO=0.D0, ONE=1.D0, TWO=2.D0, FOUR=4.D0,
22 & FVTHR=1.666666666666667D0, THIRD=.3333333333333333D0)
26 IF (ABS(R).LT.1.D-3) THEN
28 C Use assymptotic formula
30 HWUAER=-FVTHR-LOG(ABS(R))
32 ELSEIF (ABS(R).GT.RMAX) THEN
36 ELSEIF (FOUR*R.GT.ONE) THEN
42 & -(ONE+TWO*R)*(TWO-BETA*ACOS(ONE-ONE/(TWO*R)))
50 & -(ONE+TWO*R)*(TWO+BETA*LOG(ABS((BETA-ONE)/(BETA+ONE))))