Difference between revisions of "HOWTO Use EvtGen to simulate particle decays in GlueX"
(→Configuration Files) |
|||
Line 11: | Line 11: | ||
== Configuration Files == | == Configuration Files == | ||
+ | In the standard GlueX software environment, the source code and configuration files are found in the directory given by the environmental variable <code>EVTGENDIR</code>. 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 <code>$EVTGENDIR/evt.pdl</code>, and an example of several lines is shown below, including a header that shows what each column means: | ||
+ | <pre> | ||
+ | * name id mass/GeV width/GeV max_Dm/GeV 3*charge 2*spin lifetime*c/mm PythiaId | ||
+ | add p Particle K_4*+ 329 2.0450000e+00 1.9800000e-01 2.0000000e-01 3 8 0.0000000e+00 0 | ||
+ | add p Particle h_b(2P) 110553 1.0255000e+01 0.0000000e+00 0.0000000e+00 0 2 0.0000000e+00 110553 | ||
+ | add p Particle b 5 5.0000000e+00 0.0000000e+00 0.0000000e+00 -1 1 0.0000000e+00 5 | ||
+ | add p Particle anti-nu_e -12 0.0000000e+00 0.0000000e+00 0.0000000e+00 0 1 0.0000000e+00 -12 | ||
+ | add p Particle D_2*0 425 2.4626000e+00 4.9000000e-02 3.0900000e-01 0 4 0.0000000e+00 425 | ||
+ | add p Particle Upsilon 553 9.4603000e+00 5.4020000e-05 5.0000000e-04 0 2 0.0000000e+00 553 | ||
+ | </pre> | ||
+ | Particles are identified by both a name string and an id, which generally follows the PDG MC numbering scheme for clarity, if possible. New particles can be added by adding new lines to the file. | ||
+ | |||
+ | The standard decay definition file can be found in <code>$EVTGENDIR/DECAY.DEC</code>, 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 π<sup>0</sup>. This is defined as: | ||
+ | <pre> | ||
+ | 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 | ||
+ | </pre> | ||
+ | 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 [https://evtgen.hepforge.org/doc/models.html in this page]. In the example here, PHSP is a multi-particle phasespace model, while PI0_DALITZ is a particular model of the π<sup>0</sub> → e<sup>+</sup>e<sup>+</sup>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 <code>evt.pdl</code> and <code>DECAY.DEC</code> can be specified using the environment variables <code> EVTGEN_DECAY_FILE </code> and <code> EVTGEN_PARTICLE_DEFINITIONS </code>, 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 == | == decay_evtgen == |
Revision as of 14:17, 23 June 2021
Contents
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 =