Tutorial - Part 1: Our First Model Point TestThe Ingredients for CheckMATEAs 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 - StartTo 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 learnt how to use Pythia to generate events. Fortunately, to keep this part of the tutorial as simple as possible, the whole setup for the simulation is already provided for you.
You should find two exectuables which start with generate_ and the two C++ source files which have been used to create these via Pythia8. You also find the same cMSSM SLHA file shown before.
Cross Section CalculationFurthermore, 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[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: 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 - FinishHave a look at your other terminal window and (if it is not ready yet) wait until both Pythia runs have succeeded. Among other files, there should be now two .hepmc files in the same folder. Also, Pythia should have produced lots of output on screen, most importantly with its calculated cross section at the end.
With event files and cross sections at hand, we can now run CheckMATE to test the model point. |
