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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.

Why don't we take also the 'mixed' process p p > ~glu ~t1 into account?
[Show Answer]

Why don't we consider the production of other, even lighter SUSY particles in the spectrum, e.g. the charginos ~chi+ with 750 GeV mass?
[Show Answer]

Which other processes we should still consider for the model?
[Show Answer]

Event Generation - Start

To use CheckMATE, one has to simulate these processes by using an event generator. Such an event generator, in simple words, uses the given model parameters to calculate the probability to find a certain final state configuration when two protons with given energy collide. The program then uses this information to simulate a finite Monte-Carlo-sample and processes this further by decaying instable particles and hadronising partons. Normally, 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. These files can then be passed to CheckMATE.

As you have seen during the workshop, there are many different event generators on the market. It is up to the user to decide which one to use for a specific task and for this part of the tutorial, we use the well-known event generator Pythia 8[3]. CheckMATE has the possibility to either take an event file as input or to use Pythia 8 to generate events on the fly. We will do both in this tutorial, starting with the the external event generation. Note that we use Pythia8 here, but you could easily provide events using Herwig[4], CalcHEP[5], MG5_aMC@NLO[6], Sherpa[7] or Whizard[8].

During this workshop, you should in principle already have learned how to use Pythia to generate events. In this part of the tutorial we will use Pythia within CheckMATE to perform event generation.

Cross Section Calculation

Furthermore, CheckMATE needs the total cross section of the process simulated above. These in principle could be taken from Pythia 8 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[5]. 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/checkmate/tools/NLLFast-2.1. It should be run as follows:
For gluino pair production: ./nllfast gg cteq [average squark mass in GeV] [gluino mass in GeV] For squark-squark and squark-antisquark pair production: ./nllfast ss cteq [average squark mass in GeV] [gluino mass in GeV]
./nllfast sb cteq [average squark mass in GeV] [gluino mass in GeV]
For squark-gluino production: ./nllfast sg cteq [average squark mass in GeV] [gluino mass in GeV]

We need to take a look again at the slha file to find the masses.

If we only consider the first two generations, what is the average squark mass?
[Show Answer]

Use NLLFast to find the cross sections for gluino and for stop pair production. (We are interested in the NLL+NLO values)
[Show Answer]

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.

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