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
pairs,
and hard brems-strahlung from electrons. If such an interaction takes
place, the created particles are added to the end of PYJETS, 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 PYJETS (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.