C********************************************************************* 
 
      SUBROUTINE LUXDIF(NC,NJET,KFL,ECM,CHI,THE,PHI) 
 
C...Purpose: to give the angular orientation of events. 
      COMMON/LUJETS/N,K(4000,5),P(4000,5),V(4000,5) 
      COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) 
      COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4) 
      SAVE /LUJETS/,/LUDAT1/,/LUDAT2/ 
 
C...Charge. Factors depending on polarization for QED case. 
      QF=KCHG(KFL,1)/3. 
      POLL=1.-PARJ(131)*PARJ(132) 
      POLD=PARJ(132)-PARJ(131) 
      IF(MSTJ(102).LE.1.OR.MSTJ(109).EQ.1) THEN 
        HF1=POLL 
        HF2=0. 
        HF3=PARJ(133)**2 
        HF4=0. 
 
C...Factors depending on flavour, energy and polarization for QFD case. 
      ELSE 
        SFF=1./(16.*PARU(102)*(1.-PARU(102))) 
        SFW=ECM**4/((ECM**2-PARJ(123)**2)**2+(PARJ(123)*PARJ(124))**2) 
        SFI=SFW*(1.-(PARJ(123)/ECM)**2) 
        AE=-1. 
        VE=4.*PARU(102)-1. 
        AF=SIGN(1.,QF) 
        VF=AF-4.*QF*PARU(102) 
        HF1=QF**2*POLL-2.*QF*VF*SFI*SFF*(VE*POLL-AE*POLD)+ 
     &  (VF**2+AF**2)*SFW*SFF**2*((VE**2+AE**2)*POLL-2.*VE*AE*POLD) 
        HF2=-2.*QF*AF*SFI*SFF*(AE*POLL-VE*POLD)+2.*VF*AF*SFW*SFF**2* 
     &  (2.*VE*AE*POLL-(VE**2+AE**2)*POLD) 
        HF3=PARJ(133)**2*(QF**2-2.*QF*VF*SFI*SFF*VE+(VF**2+AF**2)* 
     &  SFW*SFF**2*(VE**2-AE**2)) 
        HF4=-PARJ(133)**2*2.*QF*VF*SFW*(PARJ(123)*PARJ(124)/ECM**2)* 
     &  SFF*AE 
      ENDIF 
 
C...Mass factor. Differential cross-sections for two-jet events. 
      SQ2=SQRT(2.) 
      QME=0. 
      IF(MSTJ(103).GE.4.AND.IABS(MSTJ(101)).LE.1.AND.MSTJ(102).LE.1.AND. 
     &MSTJ(109).NE.1) QME=(2.*ULMASS(KFL)/ECM)**2 
      IF(NJET.EQ.2) THEN 
        SIGU=4.*SQRT(1.-QME) 
        SIGL=2.*QME*SQRT(1.-QME) 
        SIGT=0. 
        SIGI=0. 
        SIGA=0. 
        SIGP=4. 
 
C...Kinematical variables. Reduce four-jet event to three-jet one. 
      ELSE 
        IF(NJET.EQ.3) THEN 
          X1=2.*P(NC+1,4)/ECM 
          X2=2.*P(NC+3,4)/ECM 
        ELSE 
          ECMR=P(NC+1,4)+P(NC+4,4)+SQRT((P(NC+2,1)+P(NC+3,1))**2+ 
     &    (P(NC+2,2)+P(NC+3,2))**2+(P(NC+2,3)+P(NC+3,3))**2) 
          X1=2.*P(NC+1,4)/ECMR 
          X2=2.*P(NC+4,4)/ECMR 
        ENDIF 
 
C...Differential cross-sections for three-jet (or reduced four-jet). 
        XQ=(1.-X1)/(1.-X2) 
        CT12=(X1*X2-2.*X1-2.*X2+2.+QME)/SQRT((X1**2-QME)*(X2**2-QME)) 
        ST12=SQRT(1.-CT12**2) 
        IF(MSTJ(109).NE.1) THEN 
          SIGU=2.*X1**2+X2**2*(1.+CT12**2)-QME*(3.+CT12**2-X1-X2)- 
     &    QME*X1/XQ+0.5*QME*((X2**2-QME)*ST12**2-2.*X2)*XQ 
          SIGL=(X2*ST12)**2-QME*(3.-CT12**2-2.5*(X1+X2)+X1*X2+QME)+ 
     &    0.5*QME*(X1**2-X1-QME)/XQ+0.5*QME*((X2**2-QME)*CT12**2-X2)*XQ 
          SIGT=0.5*(X2**2-QME-0.5*QME*(X2**2-QME)/XQ)*ST12**2 
          SIGI=((1.-0.5*QME*XQ)*(X2**2-QME)*ST12*CT12+QME*(1.-X1-X2+ 
     &    0.5*X1*X2+0.5*QME)*ST12/CT12)/SQ2 
          SIGA=X2**2*ST12/SQ2 
          SIGP=2.*(X1**2-X2**2*CT12) 
 
C...Differential cross-sect for scalar gluons (no mass effects). 
        ELSE 
          X3=2.-X1-X2 
          XT=X2*ST12 
          CT13=SQRT(MAX(0.,1.-(XT/X3)**2)) 
          SIGU=(1.-PARJ(171))*(X3**2-0.5*XT**2)+ 
     &    PARJ(171)*(X3**2-0.5*XT**2-4.*(1.-X1)*(1.-X2)**2/X1) 
          SIGL=(1.-PARJ(171))*0.5*XT**2+ 
     &    PARJ(171)*0.5*(1.-X1)**2*XT**2 
          SIGT=(1.-PARJ(171))*0.25*XT**2+ 
     &    PARJ(171)*0.25*XT**2*(1.-2.*X1) 
          SIGI=-(0.5/SQ2)*((1.-PARJ(171))*XT*X3*CT13+ 
     &    PARJ(171)*XT*((1.-2.*X1)*X3*CT13-X1*(X1-X2))) 
          SIGA=(0.25/SQ2)*XT*(2.*(1.-X1)-X1*X3) 
          SIGP=X3**2-2.*(1.-X1)*(1.-X2)/X1 
        ENDIF 
      ENDIF 
 
C...Upper bounds for differential cross-section. 
      HF1A=ABS(HF1) 
      HF2A=ABS(HF2) 
      HF3A=ABS(HF3) 
      HF4A=ABS(HF4) 
      SIGMAX=(2.*HF1A+HF3A+HF4A)*ABS(SIGU)+2.*(HF1A+HF3A+HF4A)* 
     &ABS(SIGL)+2.*(HF1A+2.*HF3A+2.*HF4A)*ABS(SIGT)+2.*SQ2* 
     &(HF1A+2.*HF3A+2.*HF4A)*ABS(SIGI)+4.*SQ2*HF2A*ABS(SIGA)+ 
     &2.*HF2A*ABS(SIGP) 
 
C...Generate angular orientation according to differential cross-sect. 
  100 CHI=PARU(2)*RLU(0) 
      CTHE=2.*RLU(0)-1. 
      PHI=PARU(2)*RLU(0) 
      CCHI=COS(CHI) 
      SCHI=SIN(CHI) 
      C2CHI=COS(2.*CHI) 
      S2CHI=SIN(2.*CHI) 
      THE=ACOS(CTHE) 
      STHE=SIN(THE) 
      C2PHI=COS(2.*(PHI-PARJ(134))) 
      S2PHI=SIN(2.*(PHI-PARJ(134))) 
      SIG=((1.+CTHE**2)*HF1+STHE**2*(C2PHI*HF3-S2PHI*HF4))*SIGU+ 
     &2.*(STHE**2*HF1-STHE**2*(C2PHI*HF3-S2PHI*HF4))*SIGL+ 
     &2.*(STHE**2*C2CHI*HF1+((1.+CTHE**2)*C2CHI*C2PHI-2.*CTHE*S2CHI* 
     &S2PHI)*HF3-((1.+CTHE**2)*C2CHI*S2PHI+2.*CTHE*S2CHI*C2PHI)*HF4)* 
     &SIGT-2.*SQ2*(2.*STHE*CTHE*CCHI*HF1-2.*STHE*(CTHE*CCHI*C2PHI- 
     &SCHI*S2PHI)*HF3+2.*STHE*(CTHE*CCHI*S2PHI+SCHI*C2PHI)*HF4)*SIGI+ 
     &4.*SQ2*STHE*CCHI*HF2*SIGA+2.*CTHE*HF2*SIGP 
      IF(SIG.LT.SIGMAX*RLU(0)) GOTO 100 
 
      RETURN 
      END