Very often, the output of the program is to be fed into a subsequent detector simulation program. It therefore becomes necessary to set up an interface between the PYJETS common block and the detector model. Preferably this should be done via the HEPEVT standard common block, see section , but sometimes this may not be convenient. If a PYEDIT(2) call is made, the remaining entries exactly correspond to those an ideal detector could see: all non-decayed particles, with the exception of neutrinos. The translation of momenta should be trivial (if need be, a PYROBO call can be made to rotate the `preferred' direction to whatever is the longitudinal direction of the detector), and so should the translation of particle codes. In particular, if the detector simulation program also uses the standard Particle Data Group codes, no conversion at all is needed. The problem then is to select which particles are allowed to decay, and how decay vertex information should be used.
Several switches regulate which particles are allowed to decay. First, the master switch MSTJ(21) can be used to switch on/off all decays (and it also contains a choice of how fragmentation should be interfaced). Second, a particle must have decay modes defined for it, i.e. the corresponding MDCY(KC,2) and MDCY(KC,3) entries must be non-zero for compressed code KC = PYCOMP(KF). This is true for all colour neutral particles except the neutrinos, the photon, the proton and the neutron. (This statement is actually not fully correct, since irrelevant `decay modes' with MDME(IDC,2) = 102 exist in some cases.) Third, the individual switch in MDCY(KC,1) must be on. Of all the particles with decay modes defined, only , , and are by default considered stable.
Finally, if MSTJ(22) does not have its default value 1, checks are also made on the lifetime of a particle before it is allowed to decay. In the simplest alternative, MSTJ(22) = 2, the comparison is based on the average lifetime, or rather , measured in mm. Thus if the limit PARJ(71) is (the default) 10 mm, then decays of , , , , , and are all switched off, but charm and bottom still decay. No values below 1 m are defined. With the two options MSTJ(22) = 3 or 4, a spherical or cylindrical volume is defined around the origin, and all decays taking place inside this volume are ignored.
Whenever a particle is in principle allowed to decay, i.e. MSTJ(21) and MDCY on, an proper lifetime is selected once and for all and stored in V(I,5). The K(I,1) is then also changed to 4. For MSTJ(22) = 1, such a particle will also decay, but else it could remain in the event record. It is then possible, at a later stage, to expand the volume inside which decays are allowed, and do a new PYEXEC call to have particles fulfilling the new conditions (but not the old) decay. As a further option, the K(I,1) code may be put to 5, signalling that the particle will definitely decay in the next PYEXEC call, at the vertex position given (by you) in the V vector.
This then allows the PYTHIA decay routines to be used inside a detector simulation program, as follows. For a particle which did not decay before entering the detector, its point of decay is still well defined (in the absence of deflections by electric or magnetic fields), eq. (). If it interacts before that point, the detector simulation program is left to handle things. If not, the V vector is updated according to the formula above, K(I,1) is set to 5, and PYEXEC is called, to give a set of decay products, that can again be tracked.
A further possibility is to force particles to decay into specific decay channels; this may be particularly interesting for charm or bottom physics. The choice of channels left open is determined by the values of the switches MDME(IDC,1) for decay channel IDC (use PYLIST(12) to obtain the full listing). One or several channels may be left open; in the latter case effective branching ratios are automatically recalculated without the need for your intervention. It is also possible to differentiate between which channels are left open for particles and which for antiparticles. Lifetimes are not affected by the exclusion of some decay channels. Note that, whereas forced decays can enhance the efficiency for several kinds of studies, it can also introduce unexpected biases, in particular when events may contain several particles with forced decays, cf. section .