Handling of the generation of primary events in Gauss, the LHCb simulation framework

The LHCb simulation application. Gauss, consists or two independent phases, the generation of the primary event and the tracking of particles produced in the experimental setup. For the LHCh experimental program it is particularly important to model B meson decays: the EvtGcn code developed in CLEO and BaBah has been chosen and customized for non-coherent B production as occurring in pp collisions at the LHC, The initial proton-proton collision is provided by a different generator engine, currently PYTHIA 6 for massive production of signal and generic pp collisions events. Beam gas events, background events originating from proton halo, cosmics and calibration events for different detectors can be generated in addition to pp collisions. Different generator packages as available in the physics community or specifically developed in LHCb are used for the different purposes. Running conditions affecting the events generated such as the size of the luminous region, the number of collisions occuring in a bunch crossing and the number of spill-over events from neighbouring bunches are modeled via dedicated algorithms appropriately configured. The design of the generator phase of Gauss will be described: a modular structure with well defined interfaces specific to the various tasks, e.g. pp collisions, particles' decays, selections, etc. has been chosen. Different implementations are available for the various tasks allowing selecting and combining them as most appropriate at run time as in the case of Pythia 6 im pp collisions or HIJING for beam gas. The advantages of such structure, allowing for example to adopt transparently new generators packages will be discussed.