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ulalps.f
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1 
2 C*********************************************************************
3 
4  FUNCTION ulalps(Q2)
5 
6 C...Purpose: to give the value of alpha_strong.
7  common/ludat1/mstu(200),paru(200),mstj(200),parj(200)
8  SAVE /ludat1/
9  common/ludat2/kchg(500,3),pmas(500,4),parf(2000),vckm(4,4)
10  SAVE /ludat2/
11 
12 C...Constant alpha_strong trivial.
13  IF(mstu(111).LE.0) THEN
14  ulalps=paru(111)
15  mstu(118)=mstu(112)
16  paru(117)=0.
17  paru(118)=paru(111)
18  RETURN
19  ENDIF
20 
21 C...Find effective Q2, number of flavours and Lambda.
22  q2eff=q2
23  IF(mstu(115).GE.2) q2eff=max(q2,paru(114))
24  nf=mstu(112)
25  alam2=paru(112)**2
26  100 IF(nf.GT.max(2,mstu(113))) THEN
27  q2thr=paru(113)*pmas(nf,1)**2
28  IF(q2eff.LT.q2thr) THEN
29  nf=nf-1
30  alam2=alam2*(q2thr/alam2)**(2./(33.-2.*nf))
31  goto 100
32  ENDIF
33  ENDIF
34  110 IF(nf.LT.min(8,mstu(114))) THEN
35  q2thr=paru(113)*pmas(nf+1,1)**2
36  IF(q2eff.GT.q2thr) THEN
37  nf=nf+1
38  alam2=alam2*(alam2/q2thr)**(2./(33.-2.*nf))
39  goto 110
40  ENDIF
41  ENDIF
42  IF(mstu(115).EQ.1) q2eff=q2eff+alam2
43  paru(117)=sqrt(alam2)
44 
45 C...Evaluate first or second order alpha_strong.
46  b0=(33.-2.*nf)/6.
47  algq=log(q2eff/alam2)
48  IF(mstu(111).EQ.1) THEN
49  ulalps=paru(2)/(b0*algq)
50  ELSE
51  b1=(153.-19.*nf)/6.
52  ulalps=paru(2)/(b0*algq)*(1.-b1*log(algq)/(b0**2*algq))
53  ENDIF
54  mstu(118)=nf
55  paru(118)=ulalps
56 
57  RETURN
58  END