CalcHEP tutorial

In order to get on with CalcHEP, we'll see two examples one sightly more complicated than the other. Hopefully, by the end of this tutorial, you'll be able to use pretty every function of CalcHEP.

A simple leptonic process

Let's start with a simple process and try to compute matrix elements using CalcHEP. In this section, we'll study:

ee → μμττ

The process will be studied in the standard model framework.

Preparing HEPMDB

First, connect to HEPMDB and go in the Calculate section. In the left panel, in the CalcHEP section, check whether the Standard Model (CKM=1) line appears. If not, in the CalcHEP menu, choose Import model. When you find the line Standard Model (CKM=1) for CalcHEP (it should be the model 43), select it and click Select. The model should now be accessible in your environment.

This model is the complete standard model without CKM matrix. As a consequence, it is lighter and in situation where quarks' flavours don't matter, it gives the same results as the standard model.

A template batch file

Select the model and select Edit Full Batch File in the CalcHEP menu. A new window will appear. You will be able to edit the batch file which is the file describing the process of interest. Click on the button Load batch template.

You now have a template file with nearly every possible field already offered. You also have help in the comments. The comments are lines beginning with a #. They are ignored by CalcHEP and thanks to them, you can add any information you want for any body reading the batch file.

Model information

The first three fields are information about the model.

1. The first line gives the model to use. You shouldn't touch it as this is a relique of CalcHEP local batch mode.
2. The second option is Model changed. This tells CalcHEP that model files have been changed.
3. Finally, Gauge tells what gauge to use, Feynmann or unitary. Many model are built implictly assuming that the gauge is Feynmann's. So you shouldn't change this field either.

Defining the process

We have several lines to define our process. Remove every Process, Decay and Composite field. You can now write:

Process: e,E -> m,M,l,L

In the standard model of CalcHEP, e refers to electron, m to muons, l to taus and the capital letters to their anti-particle. The particle list with their names for the model is available at the View particles entry of the CalcHEP menu. You can't access it while editing the batch file however.

So we set up an electron-positron scattering with a pair of muons and a pair of taus outgoing. No further information is needed to describe the process itself.