HOWTO Use EvtGen to simulate particle decays in GlueX

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Introduction

Much of the event generator work to this point in GlueX has focused on simulating the production of various states and the decays of broad resonances. EvtGen is an event generator widely used in HENP to simulate the decay of particles and narrow resonances, such as the η, φ, and J/ψ. The models for these decays are implemented in an easily extensible framework, can handle multiple sequential decays, and are calculated based on decay helicity amplitudes. More official documentation can be found at this page.

This document gives a brief introduction to the use of EvtGen in the GlueX software framework.

Simulating Events

Configuration Files

In the standard GlueX software environment, the source code and configuration files are found in the directory given by the environmental variable EVTGENDIR. There are two required configuration files, one of particle definitions and one of decay definitions, and one optional file of decay definitions which override the standard definitions. The format and use of these files is shown in the following.

The standard event definition file can be found in $EVTGENDIR/evt.pdl, and an example of several lines is shown below, including a header that shows what each column means:

*                  name                  id      mass/GeV    width/GeV     max_Dm/GeV    3*charge   2*spin    lifetime*c/mm    PythiaId   
add  p Particle  K_4*+                  329  2.0450000e+00  1.9800000e-01  2.0000000e-01     3     8  0.0000000e+00          0
add  p Particle  h_b(2P)             110553  1.0255000e+01  0.0000000e+00  0.0000000e+00     0     2  0.0000000e+00     110553
add  p Particle  b                        5  5.0000000e+00  0.0000000e+00  0.0000000e+00    -1     1  0.0000000e+00          5
add  p Particle  anti-nu_e              -12  0.0000000e+00  0.0000000e+00  0.0000000e+00     0     1  0.0000000e+00        -12
add  p Particle  D_2*0                  425  2.4626000e+00  4.9000000e-02  3.0900000e-01     0     4  0.0000000e+00        425
add  p Particle  Upsilon                553  9.4603000e+00  5.4020000e-05  5.0000000e-04     0     2  0.0000000e+00        553

Particles are identified by both a name string and an id, which generally follows the PDG MC numbering scheme for clarity, if possible. New particles can be added by adding new lines to the file.

The standard decay definition file can be found in $EVTGENDIR/DECAY.DEC, and has a substantially more complicated format. Let's look at the most important part, the definition of a particle decay, in this case the π0. This is defined as:

Decay pi0
0.988228297 gamma   gamma                                   PHSP; #[Reconstructed PDG2011]
0.011738247 e+      e-      gamma                           PI0_DALITZ; #[Reconstructed PDG2011]
0.000033392 e+      e+      e-      e-                      PHSP;  #[New mode added] #[Reconstructed PDG2011]
0.000000065 e+      e-                                      PHSP;  #[New mode added] #[Reconstructed PDG2011]
Enddecay

Each decay is defined by a "Decay [particle name]" header and an "Enddecay" footer. A particle can have multiple decay modes. Each decay mode is defined by the form: "[probability] [decay particles] [decay model]". The number and ordering of the decay particles depends on the decay model. The default models are given in the EvtGen documentation, and some are also described in this page. In the example here, PHSP is a multi-particle phasespace model, while PI0_DALITZ is a particular model of the π0</sub> → e+e+γ decay. If the probabilities of all the decays for a particle do not add up to 100%, they are all rescaled so that this condition is met.


In the standard GlueX generators, the location of evt.pdl and DECAY.DEC can be specified using the environment variables EVTGEN_DECAY_FILE and EVTGEN_PARTICLE_DEFINITIONS , respectively. You may particularly want to use a local copy of evt.pdl if you are simulating particles which are not defined in the standard definition file, or are varying some resonance parameters.

decay_evtgen

gen_schannel

genEtaRegge

= Adding New Models =