Tutorial - Part 1: Our First Model Point Test

The Ingredients for CheckMATE

As we have seen, the gluino and the lightest stop are the lightest colored SUSY particles and hence p p > ~glu ~glu and p p > ~t1 ~t1 should be the dominant production modes that could happen at the LHC in our model.

Event Generation - Start

To use CheckMATE, one has to simulate these processes by using an event generator, in our case Herwig++^[3]. In simple words, these generators use the given model parameters to calculate the probability to find a certain final state configuration when two protons with given energy collide. They then use this information to simulate a finite Monte-Carlo-sample and process this further by decaying instable particles and hadronising partons. The collection of all stable final state objects with their respective momenta is then stored in a so called event file, usually in .hepmc or .hep format.

Fortunately, to keep this part of the tutorial as simple as possible, the whole setup for the simulation is provided: Within the virtual machine, open a terminal and go to the folder /home/yeti/CheckMATETutorial/part1. You should find two .run-files, which have been created assuming the cMSSM with our considered model point. These files can be used by Herwig++ to start the event generation.

Cross Section Calculation

Furthermore, CheckMATE needs the total cross section of the process simulated above. These in principal could be taken from Herwig++ itself. However, especially in the case that colored particles are involved, more exact next-to-leading-order calculations or beyond are needed to get reliable results. There are various tools on the market to calculate these for a given SUSY parameter setup. In our case, we want to calculate the total cross sections using NLLFast^[4]. This tool is a very fast cross section estimator that uses large grids with precalculated cross sections for some production modes. The program takes these grids to find the cross section for a given set of masses by interpolation.

On your virtual machine, the program has already been installed for you under /home/yeti/CheckMATETutorial/tools/NLLFast. It should be run as follows:
For gluino pair production: ./nllfast gg cteq [average squark mass in GeV] [gluino mass in GeV] For stop pair production: ./nllfast st cteq [stop mass in GeV]

cteq is a famous parameterization of the probabilities to find a given quark flavour with a specific mass inside a proton, called parton density function (PDF). We need to take a look again at the slha file to find the masses.

Note that one could use the theory errors NLLFast returns within CheckMATE to estimate how these weaken the limits. However, we are not going to do this within this tutorial.

Event Generation - Finish

Have a look at your other terminal window and (if it is not ready yet) wait until both Herwig runs have succeeded. Among other files, there should be now two .hepmc files in the same folder. Also, there should be two .out files which contain the cross sections Herwig++ calculated.

With event files and cross sections at hand, we can now run CheckMATE to test the model point.
Let's Go!