PYTHIA/pythia/pystfl.F

   1
   2 C*********************************************************************
   3
   4       SUBROUTINE PYSTFL(KF,X,Q2,XPQ)
   5
   6 C...Give proton structure function at small x and/or Q^2 according to
   7 C...correct limiting behaviour.
   8       COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
   9       COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4)
  10       COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
  11       COMMON/PYINT1/MINT(400),VINT(400)
  12       SAVE /LUDAT1/,/LUDAT2/
  13       SAVE /PYPARS/,/PYINT1/
  14       DIMENSION XPQ(-25:25),XPA(-25:25),XPB(-25:25),WTSB(-3:3)
  15       DATA RMR/0.92/,RMP/0.38/,WTSB/0.5,1.,1.,5.,1.,1.,0.5/
  16
  17 C...Send everything but protons/neutrons/VMD pions directly to PYSTFU.
  18       MINT(92)=0
  19       KFA=IABS(KF)
  20       IACC=0
  21       IF((KFA.EQ.2212.OR.KFA.EQ.2112).AND.MSTP(57).GE.2) IACC=1
  22       IF(KFA.EQ.211.AND.MSTP(57).GE.3) IACC=1
  23       IF(KFA.EQ.22.AND.MINT(109).EQ.2.AND.MSTP(57).GE.3) IACC=1
  24       IF(IACC.EQ.0) THEN
  25         CALL PYSTFU(KF,X,Q2,XPQ)
  26         RETURN
  27       ENDIF
  28
  29 C...Reset. Check x.
  30       DO 100 KFL=-25,25
  31       XPQ(KFL)=0.
  32   100 CONTINUE
  33       IF(X.LE.0..OR.X.GE.1.) THEN
  34         WRITE(MSTU(11),5000) X
  35         RETURN
  36       ENDIF
  37
  38 C...Define valence content.
  39       KFC=KF
  40       NV1=2
  41       NV2=1
  42       IF(KF.EQ.2212) THEN
  43         KFV1=2
  44         KFV2=1
  45       ELSEIF(KF.EQ.-2212) THEN
  46         KFV1=-2
  47         KFV2=-1
  48       ELSEIF(KF.EQ.2112) THEN
  49         KFV1=1
  50         KFV2=2
  51       ELSEIF(KF.EQ.-2112) THEN
  52         KFV1=-1
  53         KFV2=-2
  54       ELSEIF(KF.EQ.211) THEN
  55         NV1=1
  56         KFV1=2
  57         KFV2=-1
  58       ELSEIF(KF.EQ.-211) THEN
  59         NV1=1
  60         KFV1=-2
  61         KFV2=1
  62       ELSEIF(MINT(105).LE.223) THEN
  63         KFV1=1
  64         WTV1=0.2
  65         KFV2=2
  66         WTV2=0.8
  67       ELSEIF(MINT(105).EQ.333) THEN
  68         KFV1=3
  69         WTV1=1.0
  70         KFV2=1
  71         WTV2=0.0
  72       ELSEIF(MINT(105).EQ.443) THEN
  73         KFV1=4
  74         WTV1=1.0
  75         KFV2=1
  76         WTV2=0.0
  77       ENDIF
  78
  79 C...Do naive evaluation and find min Q^2, boundary Q^2 and x_0.
  80       CALL PYSTFU(KFC,X,Q2,XPA)
  81       Q2MN=MAX(3.,VINT(231))
  82       Q2B=2.+0.052**2*EXP(3.56*SQRT(MAX(0.,-LOG(3.*X))))
  83       XMN=EXP(-(LOG((Q2MN-2.)/0.052**2)/3.56)**2)/3.
  84
  85 C...Large Q2 and large x: naive call is enough.
  86       IF(Q2.GT.Q2MN.AND.Q2.GT.Q2B) THEN
  87         DO 110 KFL=-25,25
  88         XPQ(KFL)=XPA(KFL)
  89   110   CONTINUE
  90         MINT(92)=1
  91
  92 C...Small Q2 and large x: dampen boundary value.
  93       ELSEIF(X.GT.XMN) THEN
  94
  95 C...Evaluate at boundary and define dampening factors.
  96         CALL PYSTFU(KFC,X,Q2MN,XPA)
  97         FV=(Q2*(Q2MN+RMR)/(Q2MN*(Q2+RMR)))**(0.55*(1.-X)/(1.-XMN))
  98         FS=(Q2*(Q2MN+RMP)/(Q2MN*(Q2+RMP)))**1.08
  99
 100 C...Separate valence and sea parts of structure function.
 101         IF(KFA.NE.22) THEN
 102           XFV1=XPA(KFV1)-XPA(-KFV1)
 103           XPA(KFV1)=XPA(-KFV1)
 104           XFV2=XPA(KFV2)-XPA(-KFV2)
 105           XPA(KFV2)=XPA(-KFV2)
 106         ELSE
 107           XPA(KFV1)=XPA(KFV1)-WTV1*VINT(232)
 108           XPA(-KFV1)=XPA(-KFV1)-WTV1*VINT(232)
 109           XPA(KFV2)=XPA(KFV2)-WTV2*VINT(232)
 110           XPA(-KFV2)=XPA(-KFV2)-WTV2*VINT(232)
 111         ENDIF
 112
 113 C...Dampen valence and sea separately. Put back together.
 114         DO 120 KFL=-25,25
 115         XPQ(KFL)=FS*XPA(KFL)
 116   120   CONTINUE
 117         IF(KFA.NE.22) THEN
 118           XPQ(KFV1)=XPQ(KFV1)+FV*XFV1
 119           XPQ(KFV2)=XPQ(KFV2)+FV*XFV2
 120         ELSE
 121           XPQ(KFV1)=XPQ(KFV1)+FV*WTV1*VINT(232)
 122           XPQ(-KFV1)=XPQ(-KFV1)+FV*WTV1*VINT(232)
 123           XPQ(KFV2)=XPQ(KFV2)+FV*WTV2*VINT(232)
 124           XPQ(-KFV2)=XPQ(-KFV2)+FV*WTV2*VINT(232)
 125         ENDIF
 126         MINT(92)=2
 127
 128 C...Large Q2 and small x: interpolate behaviour.
 129       ELSEIF(Q2.GT.Q2MN) THEN
 130
 131 C...Evaluate at extremes and define coefficients for interpolation.
 132         CALL PYSTFU(KFC,XMN,Q2MN,XPA)
 133         VI232A=VINT(232)
 134         CALL PYSTFU(KFC,X,Q2B,XPB)
 135         VI232B=VINT(232)
 136         FLA=LOG(Q2B/Q2)/LOG(Q2B/Q2MN)
 137         FVA=(X/XMN)**0.45*FLA
 138         FSA=(X/XMN)**(-0.08)*FLA
 139         FB=1.-FLA
 140
 141 C...Separate valence and sea parts of structure function.
 142         IF(KFA.NE.22) THEN
 143           XFVA1=XPA(KFV1)-XPA(-KFV1)
 144           XPA(KFV1)=XPA(-KFV1)
 145           XFVA2=XPA(KFV2)-XPA(-KFV2)
 146           XPA(KFV2)=XPA(-KFV2)
 147           XFVB1=XPB(KFV1)-XPB(-KFV1)
 148           XPB(KFV1)=XPB(-KFV1)
 149           XFVB2=XPB(KFV2)-XPB(-KFV2)
 150           XPB(KFV2)=XPB(-KFV2)
 151         ELSE
 152           XPA(KFV1)=XPA(KFV1)-WTV1*VI232A
 153           XPA(-KFV1)=XPA(-KFV1)-WTV1*VI232A
 154           XPA(KFV2)=XPA(KFV2)-WTV2*VI232A
 155           XPA(-KFV2)=XPA(-KFV2)-WTV2*VI232A
 156           XPB(KFV1)=XPB(KFV1)-WTV1*VI232B
 157           XPB(-KFV1)=XPB(-KFV1)-WTV1*VI232B
 158           XPB(KFV2)=XPB(KFV2)-WTV2*VI232B
 159           XPB(-KFV2)=XPB(-KFV2)-WTV2*VI232B
 160         ENDIF
 161
 162 C...Interpolate for valence and sea. Put back together.
 163         DO 130 KFL=-25,25
 164         XPQ(KFL)=FSA*XPA(KFL)+FB*XPB(KFL)
 165   130   CONTINUE
 166         IF(KFA.NE.22) THEN
 167           XPQ(KFV1)=XPQ(KFV1)+(FVA*XFVA1+FB*XFVB1)
 168           XPQ(KFV2)=XPQ(KFV2)+(FVA*XFVA2+FB*XFVB2)
 169         ELSE
 170           XPQ(KFV1)=XPQ(KFV1)+WTV1*(FVA*VI232A+FB*VI232B)
 171           XPQ(-KFV1)=XPQ(-KFV1)+WTV1*(FVA*VI232A+FB*VI232B)
 172           XPQ(KFV2)=XPQ(KFV2)+WTV2*(FVA*VI232A+FB*VI232B)
 173           XPQ(-KFV2)=XPQ(-KFV2)+WTV2*(FVA*VI232A+FB*VI232B)
 174         ENDIF
 175         MINT(92)=3
 176
 177 C...Small Q2 and small x: dampen boundary value and add term.
 178       ELSE
 179
 180 C...Evaluate at boundary and define dampening factors.
 181         CALL PYSTFU(KFC,XMN,Q2MN,XPA)
 182         FB=(XMN-X)*(Q2MN-Q2)/(XMN*Q2MN)
 183         FA=1.-FB
 184         FVC=(X/XMN)**0.45*(Q2/(Q2+RMR))**0.55
 185         FVA=FVC*FA*((Q2MN+RMR)/Q2MN)**0.55
 186         FVB=FVC*FB*1.10*XMN**0.45*0.11
 187         FSC=(X/XMN)**(-0.08)*(Q2/(Q2+RMP))**1.08
 188         FSA=FSC*FA*((Q2MN+RMP)/Q2MN)**1.08
 189         FSB=FSC*FB*0.21*XMN**(-0.08)*0.21
 190
 191 C...Separate valence and sea parts of structure function.
 192         IF(KFA.NE.22) THEN
 193           XFV1=XPA(KFV1)-XPA(-KFV1)
 194           XPA(KFV1)=XPA(-KFV1)
 195           XFV2=XPA(KFV2)-XPA(-KFV2)
 196           XPA(KFV2)=XPA(-KFV2)
 197         ELSE
 198           XPA(KFV1)=XPA(KFV1)-WTV1*VINT(232)
 199           XPA(-KFV1)=XPA(-KFV1)-WTV1*VINT(232)
 200           XPA(KFV2)=XPA(KFV2)-WTV2*VINT(232)
 201           XPA(-KFV2)=XPA(-KFV2)-WTV2*VINT(232)
 202         ENDIF
 203
 204 C...Dampen valence and sea separately. Add constant terms.
 205 C...Put back together.
 206         DO 140 KFL=-25,25
 207         XPQ(KFL)=FSA*XPA(KFL)
 208   140   CONTINUE
 209         IF(KFA.NE.22) THEN
 210           DO 150 KFL=-3,3
 211           XPQ(KFL)=XPQ(KFL)+FSB*WTSB(KFL)
 212   150     CONTINUE
 213           XPQ(KFV1)=XPQ(KFV1)+(FVA*XFV1+FVB*NV1)
 214           XPQ(KFV2)=XPQ(KFV2)+(FVA*XFV2+FVB*NV2)
 215         ELSE
 216           DO 160 KFL=-3,3
 217           XPQ(KFL)=XPQ(KFL)+VINT(281)*FSB*WTSB(KFL)
 218   160     CONTINUE
 219           XPQ(KFV1)=XPQ(KFV1)+WTV1*(FVA*VINT(232)+FVB*VINT(281))
 220           XPQ(-KFV1)=XPQ(-KFV1)+WTV1*(FVA*VINT(232)+FVB*VINT(281))
 221           XPQ(KFV2)=XPQ(KFV2)+WTV2*(FVA*VINT(232)+FVB*VINT(281))
 222           XPQ(-KFV2)=XPQ(-KFV2)+WTV2*(FVA*VINT(232)+FVB*VINT(281))
 223         ENDIF
 224         XPQ(21)=XPQ(0)
 225         MINT(92)=4
 226       ENDIF
 227
 228 C...Format for error printout.
 229  5000 FORMAT(' Error: x value outside physical range; x =',1P,E12.3)
 230
 231       RETURN
 232       END