HIJING/hipyset1_35/luxjet_hijing.F

   1 * $Id$
   2
   3 C*********************************************************************
   4
   5       SUBROUTINE LUXJET_HIJING(ECM,NJET,CUT)
   6
   7 C...Purpose: to select number of jets in matrix element approach.
   8 #include "ludat1_hijing.inc"
   9       DIMENSION ZHUT(5)
  10
  11 C...Relative three-jet rate in Zhu second order parametrization.
  12       DATA ZHUT/3.0922, 6.2291, 7.4782, 7.8440, 8.2560/
  13
  14 C...Trivial result for two-jets only, including parton shower.
  15       IF(MSTJ(101).EQ.0.OR.MSTJ(101).EQ.5) THEN
  16         CUT=0.
  17
  18 C...QCD and Abelian vector gluon theory: Q^2 for jet rate and R.
  19       ELSEIF(MSTJ(109).EQ.0.OR.MSTJ(109).EQ.2) THEN
  20         CF=4./3.
  21         IF(MSTJ(109).EQ.2) CF=1.
  22         IF(MSTJ(111).EQ.0) THEN
  23           Q2=ECM**2
  24           Q2R=ECM**2
  25         ELSEIF(MSTU(111).EQ.0) THEN
  26           PARJ(169)=MIN(1.,PARJ(129))
  27           Q2=PARJ(169)*ECM**2
  28           PARJ(168)=MIN(1.,MAX(PARJ(128),EXP(-12.*PARU(1)/
  29      &    ((33.-2.*MSTU(112))*PARU(111)))))
  30           Q2R=PARJ(168)*ECM**2
  31         ELSE
  32           PARJ(169)=MIN(1.,MAX(PARJ(129),(2.*PARU(112)/ECM)**2))
  33           Q2=PARJ(169)*ECM**2
  34           PARJ(168)=MIN(1.,MAX(PARJ(128),PARU(112)/ECM,
  35      &    (2.*PARU(112)/ECM)**2))
  36           Q2R=PARJ(168)*ECM**2
  37         ENDIF
  38
  39 C...alpha_strong for R and R itself.
  40         ALSPI=(3./4.)*CF*ULALPS_HIJING(Q2R)/PARU(1)
  41         IF(IABS(MSTJ(101)).EQ.1) THEN
  42           RQCD=1.+ALSPI
  43         ELSEIF(MSTJ(109).EQ.0) THEN
  44           RQCD=1.+ALSPI+(1.986-0.115*MSTU(118))*ALSPI**2
  45           IF(MSTJ(111).EQ.1) RQCD=MAX(1.,RQCD+(33.-2.*MSTU(112))/12.*
  46      &    LOG(PARJ(168))*ALSPI**2)
  47         ELSE
  48           RQCD=1.+ALSPI-(3./32.+0.519*MSTU(118))*(4.*ALSPI/3.)**2
  49         ENDIF
  50
  51 C...alpha_strong for jet rate. Initial value for y cut.
  52         ALSPI=(3./4.)*CF*ULALPS_HIJING(Q2)/PARU(1)
  53         CUT=MAX(0.001,PARJ(125),(PARJ(126)/ECM)**2)
  54         IF(IABS(MSTJ(101)).LE.1.OR.(MSTJ(109).EQ.0.AND.MSTJ(111).EQ.0))
  55      &  CUT=MAX(CUT,EXP(-SQRT(0.75/ALSPI))/2.)
  56         IF(MSTJ(110).EQ.2) CUT=MAX(0.01,MIN(0.05,CUT))
  57
  58 C...Parametrization of first order three-jet cross-section.
  59   100   IF(MSTJ(101).EQ.0.OR.CUT.GE.0.25) THEN
  60           PARJ(152)=0.
  61         ELSE
  62           PARJ(152)=(2.*ALSPI/3.)*((3.-6.*CUT+2.*LOG(CUT))*
  63      &    LOG(CUT/(1.-2.*CUT))+(2.5+1.5*CUT-6.571)*(1.-3.*CUT)+
  64      &    5.833*(1.-3.*CUT)**2-3.894*(1.-3.*CUT)**3+
  65      &    1.342*(1.-3.*CUT)**4)/RQCD
  66           IF(MSTJ(109).EQ.2.AND.(MSTJ(101).EQ.2.OR.MSTJ(101).LE.-2))
  67      &    PARJ(152)=0.
  68         ENDIF
  69
  70 C...Parametrization of second order three-jet cross-section.
  71         IF(IABS(MSTJ(101)).LE.1.OR.MSTJ(101).EQ.3.OR.MSTJ(109).EQ.2.OR.
  72      &  CUT.GE.0.25) THEN
  73           PARJ(153)=0.
  74         ELSEIF(MSTJ(110).LE.1) THEN
  75           CT=LOG(1./CUT-2.)
  76           PARJ(153)=ALSPI**2*CT**2*(2.419+0.5989*CT+0.6782*CT**2-
  77      &    0.2661*CT**3+0.01159*CT**4)/RQCD
  78
  79 C...Interpolation in second/first order ratio for Zhu parametrization.
  80         ELSEIF(MSTJ(110).EQ.2) THEN
  81           IZA=0
  82           DO 110 IY=1,5
  83   110     IF(ABS(CUT-0.01*IY).LT.0.0001) IZA=IY
  84           IF(IZA.NE.0) THEN
  85             ZHURAT=ZHUT(IZA)
  86           ELSE
  87             IZ=100.*CUT
  88             ZHURAT=ZHUT(IZ)+(100.*CUT-IZ)*(ZHUT(IZ+1)-ZHUT(IZ))
  89           ENDIF
  90           PARJ(153)=ALSPI*PARJ(152)*ZHURAT
  91         ENDIF
  92
  93 C...Shift in second order three-jet cross-section with optimized Q^2.
  94         IF(MSTJ(111).EQ.1.AND.IABS(MSTJ(101)).GE.2.AND.MSTJ(101).NE.3.
  95      &  AND.CUT.LT.0.25) PARJ(153)=PARJ(153)+(33.-2.*MSTU(112))/12.*
  96      &  LOG(PARJ(169))*ALSPI*PARJ(152)
  97
  98 C...Parametrization of second order four-jet cross-section.
  99         IF(IABS(MSTJ(101)).LE.1.OR.CUT.GE.0.125) THEN
 100           PARJ(154)=0.
 101         ELSE
 102           CT=LOG(1./CUT-5.)
 103           IF(CUT.LE.0.018) THEN
 104             XQQGG=6.349-4.330*CT+0.8304*CT**2
 105             IF(MSTJ(109).EQ.2) XQQGG=(4./3.)**2*(3.035-2.091*CT+
 106      &      0.4059*CT**2)
 107             XQQQQ=1.25*(-0.1080+0.01486*CT+0.009364*CT**2)
 108             IF(MSTJ(109).EQ.2) XQQQQ=8.*XQQQQ
 109           ELSE
 110             XQQGG=-0.09773+0.2959*CT-0.2764*CT**2+0.08832*CT**3
 111             IF(MSTJ(109).EQ.2) XQQGG=(4./3.)**2*(-0.04079+0.1340*CT-
 112      &      0.1326*CT**2+0.04365*CT**3)
 113             XQQQQ=1.25*(0.003661-0.004888*CT-0.001081*CT**2+0.002093*
 114      &      CT**3)
 115             IF(MSTJ(109).EQ.2) XQQQQ=8.*XQQQQ
 116           ENDIF
 117           PARJ(154)=ALSPI**2*CT**2*(XQQGG+XQQQQ)/RQCD
 118           PARJ(155)=XQQQQ/(XQQGG+XQQQQ)
 119         ENDIF
 120
 121 C...If negative three-jet rate, change y' optimization parameter.
 122         IF(MSTJ(111).EQ.1.AND.PARJ(152)+PARJ(153).LT.0..AND.
 123      &  PARJ(169).LT.0.99) THEN
 124           PARJ(169)=MIN(1.,1.2*PARJ(169))
 125           Q2=PARJ(169)*ECM**2
 126           ALSPI=(3./4.)*CF*ULALPS_HIJING(Q2)/PARU(1)
 127           GOTO 100
 128         ENDIF
 129
 130 C...If too high cross-section, use harder cuts, or fail.
 131         IF(PARJ(152)+PARJ(153)+PARJ(154).GE.1) THEN
 132           IF(MSTJ(110).EQ.2.AND.CUT.GT.0.0499.AND.MSTJ(111).EQ.1.AND.
 133      &    PARJ(169).LT.0.99) THEN
 134             PARJ(169)=MIN(1.,1.2*PARJ(169))
 135             Q2=PARJ(169)*ECM**2
 136             ALSPI=(3./4.)*CF*ULALPS_HIJING(Q2)/PARU(1)
 137             GOTO 100
 138           ELSEIF(MSTJ(110).EQ.2.AND.CUT.GT.0.0499) THEN
 139             CALL LUERRM_HIJING(26,
 140      &            '(LUXJET_HIJING:) no allowed y cut value for '/
 141      $            /'Zhu parametrization')
 142           ENDIF
 143           CUT=0.26*(4.*CUT)**(PARJ(152)+PARJ(153)+PARJ(154))**(-1./3.)
 144           IF(MSTJ(110).EQ.2) CUT=MAX(0.01,MIN(0.05,CUT))
 145           GOTO 100
 146         ENDIF
 147
 148 C...Scalar gluon (first order only).
 149       ELSE
 150         ALSPI=ULALPS_HIJING(ECM**2)/PARU(1)
 151         CUT=MAX(0.001,PARJ(125),(PARJ(126)/ECM)**2,EXP(-3./ALSPI))
 152         PARJ(152)=0.
 153         IF(CUT.LT.0.25) PARJ(152)=(ALSPI/3.)*((1.-2.*CUT)*
 154      &  LOG((1.-2.*CUT)/CUT)+0.5*(9.*CUT**2-1.))
 155         PARJ(153)=0.
 156         PARJ(154)=0.
 157       ENDIF
 158
 159 C...Select number of jets.
 160       PARJ(150)=CUT
 161       IF(MSTJ(101).EQ.0.OR.MSTJ(101).EQ.5) THEN
 162         NJET=2
 163       ELSEIF(MSTJ(101).LE.0) THEN
 164         NJET=MIN(4,2-MSTJ(101))
 165       ELSE
 166         RNJ=RLU_HIJING(0)
 167         NJET=2
 168         IF(PARJ(152)+PARJ(153)+PARJ(154).GT.RNJ) NJET=3
 169         IF(PARJ(154).GT.RNJ) NJET=4
 170       ENDIF
 171
 172       RETURN
 173       END