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Geant4 - Composite calorimeter example
README
CompositeCalorimeter is an example of a test-beam simulation used by the CMS Collaboration to validate Geant4 against real data taken (in 1996) in a CMS Hadron calorimeter test-beam. The name "Composite" for this example emphasizes that, although the test-beam had the goal of studying the hadronic calorimeter response, part of the data was taken with the presence of the electromagnetic crystal calorimeter in front of the hadronic calorimeter, to better reproduce the situation as in the real CMS experiment. The geometry of the simulation has been setup in such a way to allow very easily, at run time (therefore without need of changing any code; see below for the details) the inclusion or exclusion of the electromagnetic calorimeter part. Although some important aspects, for a detailed comparison between test-beam data and simulation, like beam profile, noise, and digitization, have been omitted here (to avoid too many technical details), nevertheless, this example is able to reproduce the main features of most of the relevant observables as measured in the real test-beam. The output of this example consists of a set of histograms and one ntuple which are stored on a ROOT file (default). In our opinion, the most original "lesson" which is offered by this advanced example for the Geant4 user is to show how the Geometry and the Sensitive/Hit part of the simulation is treated in a big experiment. Although the details of how this is done vary from experiment to experiment (it is worth, for instance, to compare with the Atlas-based advanced example lAr_calorimeter), the main driving needs and goals are quite general: to have consistency, but avoiding duplications and couplings as much as possibile, between Simulation, Reconstruction, and Visualization. Notice that the solution offered in this example by CMS could appear "overdone" for the sake of simulating only a relatively simple test-beam setup; but it should be kept in mind that the same approach is used also for the full CMS detector simulation, as well as for any subdetector.

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