Event processing

The processing phase is triggered by a call to the subroutine GRUN which, for each event to be processed, gives control to the subroutines:

GTRIGI

initialise event processing and create the event header bank JHEAD;

GTRIG

process one event;

GTRIGC

clean up the portion of memory used by the event and check that enough time is left for the next event ( [BASE200]).

GUKINE

generates or reads ( [IOPA]) the kinematics of the event and stores it in the data structures JVERTX and JKINE ( [KINE]);

GUTREV

calling GTREVE which performs the following operations for each vertex in turn:

1. moves all the particles attached to the vertex from the permanent stack JKINE to the temporary stack JSTAK;
2. controls the propagation of each particle though the setup by calling GUTRAK/GTRACK ( [TRAK]); each particle is tracked in turn and when a sensitive detector is traversed, the user may store any useful information in the data structure JHITS via the routines described in the section [HITS];

The JSTAK data structure is a LIFO (Last In -- First Out) stack. Secondary products generated by the current particle transported are processed before proceeding to the next particle. It is very important to understand that by default GEANT does not follow the secondary particles generated. It is the responsibility of the user to indicate which particles should be followed via the routines GSKING/GSKPHO.

The data structure JXYZ, containing the coordinates of space points along the tracks, can be filled by the user during tracking ( [TRAK]).

GUDIGI

simulate the detector responses for the event, using the information recorded in the data structure JHITS during particle transport, and store the results in the data structure JDIGI ( [HITS]);

GUOUT

perform all the processing at the end of the event and output the relevant data structures ( [IOPA]).

Other routines called during the tracking phase triggered by GTREVE should be mentioned for completeness:

• hadronic interactions can be simulated via either the GHEISHA [] or FLUKA [,,,,] hadronic shower generator. In the subroutines GUPHAD and GUHADR ( [TRAK]) the user may select the hadronic shower generation program to be used. The default for GEANT is GHEISHA;
• after each tracking step along the track, control is given to the subroutine GUSTEP. From the information available in labelled common blocks the user is able to take the appropriate action, such as storing a hit or transferring a secondary product either in the stack JSTAK or in the event structure JVERTX/JKINE via the subroutine GSKING. In the subroutine GSSTAK, called by GSKING, a user routine GUSKIP is called which permits skipping any unwanted track before entering it in the stack for subsequent transport;
• the subroutine GUSWIM is called by the the routines which transport charged particles when in a magnetic field; it selects and calls the appropriate routine to transport the particle. Although formally a user routine, the default version provided by GEANT is usually appropriate for most situations. The magnetic field, unless it is constant along the Z axis, has to be described via the subroutine GUFLD.