Calculating with CalcHEP

CalcHEP is a package for calculation of Feynman diagrams and integration over multi-particle phase space. The author is Alexander Pukhov and it can be downloaded from the CalcHEP Home Page

The main idea in CalcHEP is to enable one to go directly from the Lagrangian to the cross sections and distributions effectively, with the high level of automation. The package can be compiled on any Unix platform.

To install the program follow the instructions on the web page. To use it, you should make an UserDir in your account, following the instructions on the web page. Once that is done the program is fairly easy to use. In the following we will assume that the program is installed in /usr/local/lib/calchep and that the UserDir is /home/user/calchep

Then you should go to that directory and type

./calchep

A graphics interface will appear and you should just navigate around. The program can make plots of distributions, however I find it more useful to create a numerical program compiled in C. For this end follow the steps:

1. Choose the option Make n_calchep.

2. The program n_calchep will be located in /home/user/calchep/results. You should change to that directory, run the program and follow the navigation tabs.

3. Alternatively I find it more useful to start the calculation and then to quit. This will create a file session.dat in the same subdirectory results of your CalcHEP home directory. This file has all the information about the process, including the values of the SM constants.

4. I have created a few scripts (scripts for CalcHEP ), that use this file and can create output by making a variation over cycles. Particularly useful are Ecm_cycle and p1Lab_cycle. If you type them without parameters you will get an output that explains the usage, for instance for Ecm_cycle you get,

   DESCRIPTION: Makes a cycle of Ecm values. All quantities in GeV. 
   
   USAGE: Ecm_cycle  StartValue Step NumberPoints m1 m2
   

   where m1 and m2 are the masses of the initial particles. To send the output to a file you just do, for instance,

    Ecm_cycle 1 2 20 0 0 > output.dat
   

   These scripts should be placed in a directory in your PATH. This usualy is /home/user/bin, but you have to check. This way they can run in any place in your area. One important thing is that these scripts assume that the main directory of CalcHep is /usr/local/lib/calchep. If you have CalcHEP in another directory, you should edit the scripts and change the line

   CALCHEP=/usr/local/lib/calchep
   

   to the location in your installation. Included is also a script called NewProject for cleaning old files before we start a new project. Also included is a script cleanup that removes the text from the output of the cycle scripts, making them useful for plotting. The file cleanup.sed must be in the same directory.

You should play with these programs and reproduce, for a well known process, the results. An example is $e^- e^+ \rightarrow \mu^- \mu^+$. You should reproduce this plot where the red line is the result for QED (only photon exchange).