5.1 Interactions and decays in the detector.

SGV follows the particles through the detector, either along a straight line (neutral particles) or a helix (charged particles). If the steering card GENBC is set to true, the most important electromagnetic interactions in the detector will be simulated : photon conversions into ${\rm e}^{+}{\rm e}^{-}$ pairs, and hard brems-strahlung from electrons. If such an interaction takes place, the created particles are added to the end of LUJETS, and will be treated when the loop reaches those lines. The original particle will be marked as decayed (K(i,1) set to 15), and the detector layers after the interaction are removed from the list of layers to be included in the measurement error calculation. In the case of brems-strahlung, the original electron will be kept as un-decayed if the energy loss is small, the rationale being that in that case the pattern recognition code in a real detector probably would not have detected that the interaction took place. The steering card PTLOSLIM controls at which loss of transverse momentum a new electron should be created after the bremsstrahlung. The created photon is, however, always added to LUJETS. The steering cards PMINBR and PMINPA specifies the minimum momentum of the electrons or photons, respectively, to generate the interactions.

Particles decaying in the detector are treated as follows : If the decaying particle is charged, the decay-vertex position and the momenta of the decay-products are rotated according to the magnetic field in the detector (which the event generation program knows nothing about). For both neutral and charged particles the total distance traveled before the decay is noted, and detectors after the decay are not included in the measurement. If a particle hits a calorimeter before decaying, the decay-products are remved from LUJETS (K(i,1) set to 0), and the particle is marked as stable (K(i,1) set to 1).

Photons and electrons reaching calorimeters are removed from layers further out, ie. they never 'punch through' a calorimeter. Note that that for a layer to be recognized as a calorimeter in this respect, it must be specified as such in the t r a c k i n g detector part of the geometry description. See chapter 6 for details ! Note that there will be a certain amount of double-counting of energy and momentum, due to decaying or interacting particles. Both the parent and the decay-products might be seen, and therefore counting the true energy of all seen particles might might yield more energy that in the initial state.