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This page contains a few packages in C++ (tested with gcc) useful for determining the mass of the dark matter particle at the LHC. It is assumed that each event has two identical decay chains, i.e., the masses of the invisible particles and all intermediate particles are the same for the two decay chains. The strategy is to use mass shell constraints and the measured missing transverse momenta to constrain the mass space. Depending on how many visible particles per decay chain, the detailed method is different.
When each decay chain contains three visible particles, the 4-momenta of the missing particles, and therefore the masses of all particles can be directly solved for by combining two events. The references are
The code for solving the system is here. Use “tar -xvzf <filename>” to unzip. ROOT is needed for compiling. See the “README” file for instructions on how to run the code. A description on how to run a few examples is given below. A Mathematica notebook used to symbolically reduce the equation system to a univariate polynomial equation is kept here (This file only–remove the “.gz” extension before using it. It is not zipped. The extension is only used to avoid conflict with the Wiki), though it is not needed to run the code.
When each decay chain contains two visible particles, the missing particles' momenta can be obtained event by event if the masses are known. The solution is physical only for a restricted region in the mass space. One can utilize this fact to estimate the unknown masses. See the following references:
We provide code that solve for the missing particles' momenta with given masses, which is kept in the same place as (a), but the two can be compiled independently, and ROOT is not needed in this case. See the “README” file for instructions on how to run the code. A description on how to run a few examples is given below.
In this case, there are two unknown masses: the mass of the dark matter particle and that of its mother particle. The kinematic constraints divide the 2-dimensional masses space to an allowed region and a forbidden region, which are separated by the variable "MT2". See the following for a reference, which also describes a fast algorithm for calculating MT2.
The code for calculating MT2 using the algorithm is available here.
The essential code in the “src” directory, “topology22.cpp” and “topology33.cpp”, is given as subroutines without a “main” program. Examples of such main programs can be found in the “example” directory. To test these programs, you need to do the following (in a linux type environment with gcc installed):
1. Download the source file {:wimpmass.tar.gz|wimpmass.tar.gz}}. Do “tar -xvzf wimpmass.tar.gz”. A directory called “WIMPMASS” will be created.
2. Assuming ROOT is installed, go to the directory “WIMPMASS/src” and “WIMPMASS/examples”, modify the Makefile to point to you ROOT directory.
3. Go to the directory “WIMPMASS/”, do “make”.
4. Go to the directory “WIMPMASS/examples”. There should be two executable files generated: “example22” and “example33” —-
If you have any questions regarding the code or find any bug, please contact Zhenyu Han at zhenyuhan@physics.ucdavis.edu
Last Updated: May 7, 2009
