NA49 simulation environment

The NA49 simulation environment is a collection of several computer programs, each of which performs a specific task and which are run sequentially as a chain that is shown as a flowchart in the figure below.
Simulation input
The input to the simulation may be of two kinds: a simple event generator producing a file in ASCII format or a model like Venus, UrQMD or Fritiof. The output from such models as they are implemented in the NA49 library is in DSPACK format.

GEANT
The CERN-developed GEANT software simulates the transport of particles through a detector apparatus. The NA49 GEANT implementation is designed to model the physical layout of the NA49 experiment. The event is defined by a list of particles at the target vertex and their initial momentum vectors. Interactions with detector material, decays of unstable particles, and the production of secondary particles are all accounted for by Monte Carlo techniques. The NA49 GEANT (GNA49) models nearly every component of the NA49 apparatus. The magnetic field is based on the same map used by the reconstruction software.

MTSIM
The TPC simulation program, MTSIM, reads in the GNA49 output and creates data in the form of digitized output of the TPC electronics. The same reconstruction software used to analyze the experimental data can therefore be used for the simulated data as well. MTSIM takes the GNA49 TPC MC points and produces digitized data based on the properties of the TPC gas and readout electronics. The signal threshold cuts and compression algorithm of the data aquisition system are also applied to complete the simulation.

Embedding
A simulated event can be superimposed on an actual experimental event with GTEMBED. This provides a realistic background so that the embedded event appears to the reconstruction software as being practially identical to the experimental data. The process of track embedding requires adding ADC signals in each pad-timeslice pixel from both data sets.

Reconstruction
The next step is the reconstruction. It is operated in a manner identical to that of the experimental event reconstruction and the performance of the reconstruction program should be the same in both cases.

Matching
The reconstructed data, in the form of TPC points and tracks, are compared to the GEANT MC data with an evaluation tool called GTEVAL. The user can then examine the simulation results to determine which of the originally generated MC particles were actually reconstructed. The matching procedure starts with point matching, which is based on the spatial proximity of the GNA49 point to the reconstructed point. Then the track information associated with each point is compiled to make a list of possible matches between tracks. Specialized data arrays are used by GTEVAL to record the point and track matches, which may not necessarily be one-to-one.

Conversion to ROOT MINI-DST
If an user wants to use ROOT MINI-DST format he must convert the output from DSPACK. After that he is ready to analize Monte Carlo data. The components of the simulation will be discussed in the following sections: