An ordinary annihilation event in the continuum, at a c.m. energy of 91 GeV, may be generated with
CALL PYEEVT(0,91D0)In this case a event is generated, including weak effects, followed by parton-shower evolution and fragmentation/decay treatment. Before a call to PYEEVT, however, a number of default values may be changed, e.g. MSTJ(101) = 2 to use second-order QCD matrix elements, giving a mixture of , , , and events, MSTJ(102) = 1 to have QED only, MSTJ(104) = 6 to allow production as well, MSTJ(107) = 1 to include initial-state photon radiation (including a treatment of the pole), PARJ(123) = 92.0 to change the mass, PARJ(81) = 0.3 to change the parton-shower value, or PARJ(82) = 1.5 to change the parton-shower cut-off. If initial-state photon radiation is used, some restrictions apply to how one can alternate the generation of events at different energies or with different mass, etc. These restrictions are not there for efficiency reasons (the extra time for recalculating the extra constants every time is small), but because it ties in with the cross-section calculations (see PARJ(144)).
Most parameters can be changed independently of each other. However, if just one or a few parameters/switches are changed, one should not be surprised to find a rather bad agreement with the data, like e.g. a too low or high average hadron multiplicity. It is therefore usually necessary to retune one parameter related to the perturbative QCD description, like or , one of the two parameters and of the Lund symmetric fragmentation function (since they are so strongly correlated, it is often not necessary to retune both of them), and the average fragmentation transverse momentum -- see Note 2 of the MSTJ(101) description for an example. For very detailed studies it may be necessary to retune even more parameters.
The three-gluon and gluon-gluon-photon decays of may be simulated by a call
A typical program for analysis of annihilation events at 200 GeV might look something like
IMPLICIT DOUBLE PRECISION(A-H, O-Z) IMPLICIT INTEGER(I-N) INTEGER PYK,PYCHGE,PYCOMP COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5) COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) COMMON/PYDAT2/KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4) COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5) MDCY(PYCOMP(111),1)=0 ! put pi0 stable MSTJ(107)=1 ! include initial-state radiation PARU(41)=1D0 ! use linear sphericity ..... ! other desired changes CALL PYTABU(10) ! initialize analysis statistics DO 100 IEV=1,1000 ! loop over events CALL PYEEVT(0,200D0) ! generate new event IF(IEV.EQ.1) CALL PYLIST(2) ! list first event CALL PYTABU(11) ! save particle composition ! statistics CALL PYEDIT(2) ! remove decayed particles CALL PYSPHE(SPH,APL) ! linear sphericity analysis IF(SPH.LT.0D0) GOTO 100 ! too few particles in event for ! PYSPHE to work on it (unusual) CALL PYEDIT(31) ! orient event along axes above IF(IEV.EQ.1) CALL PYLIST(2) ! list first treated event ..... ! fill analysis statistics CALL PYTHRU(THR,OBL) ! now do thrust analysis ..... ! more analysis statistics 100 CONTINUE ! CALL PYTABU(12) ! print particle composition ! statistics ..... ! print analysis statistics END