From simulation to physics results

What are scientists doing with all these data ? How are the results of these large simulation programs and campaigns used ? These are very relevant and recurrent questions ATLAS physicist are trying to address in a growing set of blog entries.



Let's assume that a theory or model predicts that such and such particles can be created in proton-proton collisions, with such and such energy and angular characteristics. As these are most probably unstable, the model also has to say how they will decay into other particles, which will decay again... until it ends up with only stable ones. The so called "generator program" builds what is called "events": each of them contains the list of produced particles and their characteristics at the emission point. The next step is to predict how such event will look like in our detector ? We do know every detail of the ATLAS detector and how particles behave when they cross it. The simulation program contains all the geometrical description, all the interaction probabilities and is able to simulate, step by step, the fate of each particle attached to the "event". We are ready to see what known and new physics will look like...



This step is common to simulated and real data. Sensor labels correspond to positions in space, digits are converted back into energy using a long set of calibration constants. Advanced algorithms allow to reconstruct trajectories and total energies, and then identify the type of particle (electron, photon, proton, neutron, muon and even neutrinos via missing energy).



All the ingredients are available to compare the type and characteristics of the simulated events with real data, and finally tell if the theory or model hypotheses are validated or not ! Of course, the ability to draw a conclusion depends on the precision and knowledge of the detector (this is called systematic uncertainty) and on the number of events (which drives the statistical uncertainty).




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