# Event Tag System

## Event Tag System

### Event Tag System

In many experiments, the analysis framework only allows the user to read all events in a file one after the other. In order to select the wanted events a user has to apply a filter after the event data has been read into the memory. This is an inefficient use of the computer resources since the time to read the events typically dominates the overall execution of the analysis.
ALICE decided to implement a system, called event tag system, that will reduce the time needed to perform an analysis by providing to each analysis code just the events of interest as they are selected by the users themselves. This event tag system is designed to provide fast pre-selection of events with the desired characteristics. This task will be performed first of all by imposing event selection criteria within the analysis code and then by interacting with software that is designed to provide a file transparent event access for analysis. The latter is an evolution of the procedure that has already been implemented by the STAR collaboration.

### Structure of the Event Tag System

The event tag system that has been built, intends to provide a summary of the most useful physics information that describe each ESD to the user. It consists of four levels of information:

• Run Level: Fields that describe the run conditions and configurations and are retrieved from Detector Control System (DCS), Data Acquisition system (DAQ) and offline.
• LHC Level: Fields that describe the LHC condition per ALICE run which are retrieved from the DCS.
• Detector Level: Fields that describe the detector configuration per ALICE run which are retrieved from the Experimental Control system (ECS).
• Event Level: Fields that describe each event - mainly physics related information and are retrieved by both offline and the grid file catalog.

The corresponding classes that form this system have already been included in AliRoot's framework. The output tag files will be root files having a tree structure, the names of which will be: Run$RunNumber.Event$First_$Last.$Counter.ESD.tag.root

### Event meta data tags

The classes that deal with all these levels of information can be found under the STEER module of \$ALICE_ROOT. The names of them are AliRunTag, AliLHCTag, AliDetectorTag and AliEventTag for the Run, LHC, Detector and Event level information respectively The fields that are listed on each level can be found in RUN, LHC, DETECTOR and EVENT.