Given the extremely high output rate foreseen at LHC and the general-purpose
nature of ATLAS experiment, an efficient and flexible way to select events
in the High Level Trigger is needed. An extremely flexible solution is
proposed that allows for early rejection of unwanted events and an easily
configurable way to choose algorithms and to specify the criteria for
trigger decisions. It is implemented in the standard ATLAS object-oriented
software framework, Athena. The early rejection is achieved by breaking the
decision process down into sequential steps. The configuration of each step
defines sequences of algorithms which should be used to process the data,
and 'trigger menus' that define which physics signatures must be satisfied
to continue on to the next step, and ultimately to accept the event. A
navigation system has been built on top of the standard Athena transient
store (StoreGate) to link the event data together in a tree-like structure.
This is fundamental to the seeding mechanism, by which data from one step is
presented to the next. The design makes it straightforward to utilize
existing off-line reconstruction data classes and algorithms when they are
suitable
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