C...Example how to set and have documented the main parameters that C...determine e+e- -> W+W- event generation (for LEP 2). C...Part of a call for standardization during the LEP 2 workshop. COMMON/LUJETS/N,K(4000,5),P(4000,5),V(4000,5) DIMENSION PGAM(4) C...Main parameters of run: c.m. energy and number of events. ECM=175. NEV=5000 C...Specify desired parameter values. WMASS=80.22 ALPHAH=1./128. ALPHAS=0.123 IRAD=0 CALL PYWSET(WMASS,ALPHAH,ALPHAS,IRAD) C...Initialize. CALL PYINIT('CMS','e+','e-',ECM) EGAM=0. PZGAM=0. PTGAM=0. C...Print values of parameters set (PYWGET must be called after PYINIT). CALL PYWGET C...Generate a few events. DO 200 IEV=1,NEV CALL PYEVNT c IF(IEV.LE.5) CALL LULIST(1) C...Do statistics on photon emission. DO 100 J=1,4 100 PGAM(J)=0. DO 120 I=1,N IF(K(I,2).EQ.22.AND.K(I,3).LE.4) THEN DO 110 J=1,4 110 PGAM(J)=PGAM(J)+P(I,J) ENDIF 120 CONTINUE EGAM=EGAM+PGAM(4) PZGAM=PZGAM+ABS(PGAM(3)) PTGAM=PTGAM+SQRT(PGAM(1)**2+PGAM(2)**2) 200 CONTINUE C...Print final cross section and photon statistics. CALL PYSTAT(1) WRITE(6,'(A,F10.3)') 'Average total E of emitted photons =', &EGAM/NEV WRITE(6,'(A,F10.3)') 'Ditto abs(sum p_longitudinal) =', &PZGAM/NEV WRITE(6,'(A,F10.3)') 'Ditto abs(sum p_T(vectorial)) =', &PTGAM/NEV END C********************************************************************* SUBROUTINE PYWSET(WMASS,ALPHAH,ALPHAS,IRAD) C...A subroutine to set parameters for WW pair generation, to be used C...for comparison purposes. It also indicates some of the additional C...degrees of freedom available for the PYTHIA generation. C...WMASS = the desired W mass value. C...ALPHAH = alpha_em evaluated at the scale of the hard process. C...ALPHAS = alpha_s evaluated at the scale of W decay. C...IRAD = 0 or 1 for without/with initial state QED radiation. COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4) COMMON/LUDAT3/MDCY(500,3),MDME(2000,2),BRAT(2000),KFDP(2000,5) COMMON/PYSUBS/MSEL,MSUB(200),KFIN(2,-40:40),CKIN(200) COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200) SAVE /LUDAT1/,/LUDAT2/,/LUDAT3/ SAVE /PYSUBS/,/PYPARS/ C...Set W and Z masses. PMAS(24,1)=WMASS PMAS(23,1)=91.187 C...Fixed alpha_strong value given by user input. MSTP(2)=0 PARU(111)=ALPHAS C...Fixed alpha_em value value given by user input. MSTU(101)=2 PARU(103)=ALPHAH PARU(104)=1. C...Decide on internal set of electroweak parameters in PYTHIA: C...MSTP(8)=0 : use alpha_em(Q2) C... =1 : translate alpha_em -> sqrt(2)*G_F*m_W^2*sin2(t_W)/pi. C... =2 : also translate sin2(t_W) -> 1.-(m_W/m_Z)^2 except in C... v = a - 4 sin2(t_W) e. MSTP(8)=1 C...Calculate sin2(theta_W) from G_F and m_W. C...Done automatically for MSTP(8)>=1, so not needed any longer. c GF=PARU(105) c PI=3.141593 c SIN2TW=PI*ALPHAH/(SQRT(2.)*WMASS**2*GF) c PARU(102)=SIN2TW C...Select W pair production only. MSEL=0 MSUB(25)=1 C...Switch off or on initial state QED radiation. IF(IRAD.LE.0) THEN MSTP(11)=0 MSTP(61)=0 ELSE MSTP(11)=1 MSTP(61)=1 ENDIF C...Set scale of hard process to be s-hat for consistency with C...initial state radiation of Excalibur. (Not so important in C...practice!) MSTP(32)=4 C...Switch off QCD showers and fragmentation - irrelevent for C...comparison with electroweak generators. MSTP(71)=0 MSTP(111)=0 RETURN END C********************************************************************* SUBROUTINE PYWGET C...A subroutine to write some of the parameters used for WW generation C...in PYTHIA. COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4) COMMON/LUDAT3/MDCY(500,3),MDME(2000,2),BRAT(2000),KFDP(2000,5) COMMON/PYSUBS/MSEL,MSUB(200),KFIN(2,-40:40),CKIN(200) COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200) SAVE /LUDAT1/,/LUDAT2/,/LUDAT3/ SAVE /PYSUBS/,/PYPARS/ C...Write alpha_strong, alpha_em and sin2(theta_W). WRITE(6,'(//A)') ' Parameter survey by PYWGET:' PI=3.141593 GF=PARU(105) ALPS=ULALPS(PMAS(23,1)**2) WRITE(6,'(/A,F10.6)') ' alpha_strong; fixed value used here:', &ALPS ALEM=ULALEM(0.) WRITE(6,'(A,F10.6,F10.2)') &' alpha_em and 1/alpha_em at Q2=0: ',ALEM,1./ALEM ALEM=ULALEM(PMAS(24,1)**2) WRITE(6,'(A,F10.6,F10.2)') &' alpha_em and 1/alpha_em at large Q2:',ALEM,1./ALEM SIN2TW=PARU(102) WRITE(6,'(A,F10.6)') ' sin2(theta_W); derived from G_F etc:', &SIN2TW C...Write W and Z mass and width. WRITE(6,'(/A,F10.3)') ' W mass, set by user in PYWSET: ', &PMAS(24,1) WRITE(6,'(A,F10.3)') ' W width, calculated, s-dependent: ', &PMAS(24,2) WRITE(6,'(A,F10.3)') ' Z mass, set to PDG value in PYWSET: ', &PMAS(23,1) WRITE(6,'(A,F10.3)') ' Z width, calculated, s-dependent: ', &PMAS(23,2) C...W partial widths: how they come about. COMFC0=ALEM*PMAS(24,1)/(12.*SIN2TW) COMFC1=GF*PMAS(24,1)**3/(6.*SQRT(2.)*PI) WRITE(6,'(/A)') ' Common prefactor in all W partial widths:' WRITE(6,'(A,F10.4)') ' alpha_em*m_W/(12*sin2(theta_W): ', &COMFC0 WRITE(6,'(A,F10.4)') ' G_F*m_W**3/(6*sqrt(2)*pi) ', &COMFC1 COMFAC=COMFC0 IF(MSTP(8).EQ.1) COMFAC=COMFC1 COLFAC=3.*(1.+ALPS/PI) WRITE(6,'(A,F10.4)') ' Quark colour factor 3*(1+alpha_s/pi) ', &COLFAC WRITE(6,'(A)') ' Fermion pair Cabbibo factor and partial width =' WRITE(6,'(A)') ' prefactor * colour factor * Cabbibo, ' WRITE(6,'(A)') ' final column also with phase space suppression:' WRITE(6,'(A,3F10.5)') ' u + dbar',VCKM(1,1), &COMFAC*COLFAC*VCKM(1,1),PMAS(24,2)*BRAT(172) WRITE(6,'(A,3F10.5)') ' c + dbar',VCKM(2,1), &COMFAC*COLFAC*VCKM(2,1),PMAS(24,2)*BRAT(173) WRITE(6,'(A,3F10.5)') ' u + sbar',VCKM(1,2), &COMFAC*COLFAC*VCKM(1,2),PMAS(24,2)*BRAT(176) WRITE(6,'(A,3F10.5)') ' c + sbar',VCKM(2,2), &COMFAC*COLFAC*VCKM(2,2),PMAS(24,2)*BRAT(177) WRITE(6,'(A,3F10.5)') ' u + bbar',VCKM(1,3), &COMFAC*COLFAC*VCKM(1,3),PMAS(24,2)*BRAT(180) WRITE(6,'(A,3F10.5)') ' c + bbar',VCKM(2,3), &COMFAC*COLFAC*VCKM(2,3),PMAS(24,2)*BRAT(181) WRITE(6,'(A,3F10.5)') ' e + nu ',1.,COMFAC,PMAS(24,2)*BRAT(188) WRITE(6,'(A,3F10.5)') ' mu + nu ',1.,COMFAC,PMAS(24,2)*BRAT(189) WRITE(6,'(A,3F10.5)') ' tau + nu',1.,COMFAC,PMAS(24,2)*BRAT(190) C...Z couplings. WRITE(6,'(/A)') &' electron couplings to Z calculated from sin2(theta_W):' WRITE(6,'(A,3F10.5)') ' e_e, a_e, v_e: ',-1.,-1.,-1.+4.*SIN2TW RETURN END