Simulated data are most often used to determine the detector efficiency (acceptance), so that the yield of measured data can be adjusted to compensate for uneven detector efficiency (e.g. when particles are not detected because they enter the gaps between the drift chambers or the calorimeters, or due to limitations of the physical properties of the detector or the reconstruction algorithms).
The term 'Monte Carlo' is used to mean simulated data when there is some element of randomness in the event generation. It refers to casinos.
There are several steps in the production of simulated data.
- Event generators (bggen, gen_amp, etc) produce a set of events as a list of particle track properties - particle id, mass, momentum and angle - for the photon-nucleon interaction event. The reactions included and the momenta of the particles produced are determined by the generator using its configuration file. Bggen produces typical hadronic background events, whereas the other generators mimic specific reactions, with mass and angular distributions described by the user. Within those distributions, random numbers are used to determine the 4-momenta of the primary particle and its decay products. A 'flat' distribution of decay products has a uniform mass and angular distribution.
- HDGeant4 uses a model of the materials making up each part of the GlueX detector (referred to as the detector geometry and material map) to simulate the interaction of the generated particles as they pass through the detector. It uses Geant4 to model track scattering and energy loss, and produces a list of the time, position and energy deposition of hits that would be measured for each track.
- The particle gun is a part of HDGeant4 that can be used instead of the event generators to specify the particle, momentum and track angles. This can be useful to investigate detector efficiency features.
- Mcsmear models the resolution of the detector, and adjusts the time, position and energy of the hit quantities so that the output data become less exact and look like a genuine measurement made using the GlueX detector.
- 'Random trigger' events are collected during experimental runs, using a separate trigger parallel to the primary physics trigger which is generated at regular time intervals (from a pulser). This records the hits that are in the detector at that time. These can be added to the simulated data to mimic the 'noise' hits from extra tracks through the detector which were not associated with hadronic production events and electronic noise.
Once the simulated data have been processed in this way, it can be analyzed in the same way as data collected with the real GlueX detector.
Also see Simulations