class AliGenPileup: public AliGenCocktail

                          Class AliGenPileup
   This is a generator of beam-beam pileup.
   It generates interactions within 3 orbits (+-1) around
   the trigger event. The trigger event itself is chosen
   randomly among the bunch crossings within the central orbit.
   The user can decide whenever to include in the simulation the
   "trigger" interaction or not. This is handled by the
   GenerateTrigInteraction(Bool_t flag) method.
   In the case the trigger interaction is included, it is
   generated using the same settings (vertex smear for example) as
   the pileup events.
   In case the trigger simulation is not included, the user can make
   a cocktail of generator used to produce the trigger interaction and
   AliGenPileup. In this case in order to avoid a fake increase of the rate around the
   trigger, the number of background events within the bunch
   crossing of the trigger is readuced by one.
   The beam profile (the list of the active bunch crossings) can be
   controlled via the SetBCMask(const char *mask) method. The syntax
   follows the one in AliTriggerBCMask class. For example:
   "3564H" would mean that all the bunch corssings within the orbit
   are aloowed (which is of course unphysical). In case one wants to simulate
   one-bunch-crossing-per-orbit scenario, the way to do it is to put something like:
   "1H3563L" or similar.
   The SetGenerator(AliGenerator *generator, Float_t rate) method is
   used in order to define the generator to be used. The second argument is the pileup
   rate in terms of #_of_interactions/bunch-crossing = sigma_tot * luminosity.
   The pileup generation time window can be set via
   AliGenerator::SetPileUpTimeWindow(Float_t pileUpTimeW) method. By the default the
   window is set to 88micros (= TPC readout window).

 cvetan.cheshkov@cern.ch  9/12/2008