# How does CERN simulate and interpret a simulated SUSY event of the Atlas detector?

1. Why do these histograms correspond to a simulated SUSY event? What kind of superparticles do we actually see here? We observe 3 histograms, but what do they tell us?

2. How do they simulate such things? What kind of Software/Framework? Is it a computationally heavy simulation to execute? Can I run it by myself? Let us take this a step at a time.

The LHC experiments are about the discovery of new particles. Let us take the example of the Higgs discovery. The decay channels of the Higgs boson are given by the theory, and the experiment looked at the mass where it might appear.

Simulated events at LHC are generated by monte carlo programs where the scattering amplitude of the standard model is used to generate channels, for example :proton proton going to gamma gamma + anything events, where the mass of the Higgs is set very high so that a uniform background appears.

This is compared to the data (Take it as an illustration, as I notice they use a simple background extension instead of monte carlo simulation for this figure, but they could have used mc events)

The decay modes of the Higgs are given by the theory, and the excess over the monte carlo simulated events gives the signal.

This needs the complete panoply of the standard model and a single event means nothing more than that it is consistent with being a Higgs to gamma gamma. Here is a single Higgs to gamma gamma event, A monte carlo event would look the same. It is the statistical accumulation that establishes a signal.

Now to search for supersymmetric events one can use the same method as for the Higgs boson, look for high mass resonances , but it is necessary to know what are the decay channels for supersymmetric particles to start with,

Monte carlo events are generated introducing the generation and decay amplitudes of channels for possible decay modes of sypersymmetric particles so as to see the limits of the variables allowed and the type of events expected from a supersymmetric particle decay. These decays are given by a specific supersymmetric theory and the ones you discuss are examples of the cuts needed on the data sample as to isolate possible candidates. These are special monte carlo simulations with decaying supersymmetric particles of a specific theory and masses will be varied . They do not need a large number of events, they just need to see how the decay products appear in the detectors.

For example the one above shows two high p_transerse muons,two high p_t electrons ( the short ones) and some jets.these cuts will reduce the number of candidates so that the invariant mass plots of the data can give a chance to show an enhancement in a particular mass.

This one event means nothing, it is the statistical accumulation of such topology events in the data that will show up a signal or allow for a limit calculation .

With this background we come to your questions:

Why these histograms correspond to simulated SUSY event? What kind of superparticles do we actually see here? We see 3 histograms, but what do they tell us?

The histograms are the accumulation of hadronic energy in the hadronic calorimeter, jets. They tell us it is an event with muons, leptons and three jets. The decay channel was introduced in the generation of events from the theory used.

How do they simulate such things? What kind of SW/Framework? Is it a computationally heavy simulation to perform?

It is a monte carlo simulation

Can I run it by myself?

If you become a graduate student writing your thesis on supersymmetric events in the experiment, yes. It needs a lot of elbow grease.

• these are special mc simulations with decaying supersymmetric particles of a specific theory and masses will be varied . They do not need a large number of events, they just need to see how the decay products appear in the detectors. I think not yer for open source see opendata.cern.ch/?ln=en – anna v Nov 28 '17 at 15:52
• It's quite crazy that we observe matter by measuring missing energy in a transverse direction. – 0x90 Nov 29 '17 at 2:09
• The transverse direction is used for enhancing data samples because its topology is invariant to Lorenz transformations, and supper symmetric particles will decay with large angles to their direction of motion (due to their large mass and decaying into few particles). Thus the perpendicular frame is a snapshot of a particular direction in the center of mass system of the proton proton collision. The full dimensions are used in the fits for the events studied. – anna v Nov 29 '17 at 5:24

Why do these histograms correspond to a simulated SUSY event? What kind of superparticles do we actually see here? We observe 3 histograms, but what do they tell us?

Just a quick note. These are not "histograms", what you see is just the visualization of a simulated event in the detector. My guess is that you refer to the three groups of "towers" in the picture. These show the deposition of energy by the particles on subdetectors of the experiment.

How do they simulate such things? What kind of Software/Framework? Is it a computationally heavy simulation to execute? Can I run it by myself?

I must say that it's extremely heavy computationally and extremely complex to prepare. Simulation of geometry and materials goes in, theoretical calculations of the probabilities of interactions and the structure of the colliding particles, not to mention calculations stemming from the hypothesized new particles, and some statistics for good measure ;p I think Atlas' framework is called Athena. The underlying software package is GEANT4. Their preparation is the collective work of hundreds of people from different fields. But as anna wrote if you become a student or otherwise member of such experiments, you'll run them.

(On a more positive note, GEANT4 is open software and one can rather easily learn how to set up simulations of small experiments and play with them.)