l/A-event Generator

This is the Argonne generic l/A-event generator (lAger), a flexible MC generator system to simulate electro- and photo-production off nucleons and nuclei.

Below you can find an overview of the release versions, as well as a short tutorial and copyright notice. If you use lAger to generate data used in a presentation or an article in a scientific publication, please cite:

S. Joosten, Argonne l/A-event Generator (2021), GitLab repository, https://eicweb.phy.anl.gov/monte_carlo/lager

Versions

• v3.6.x Tweaked VMD formalism and better examples for DVMP at EIC
• v3.5.x New container/installer setup
• v3.4.x Add support for psi prime
• v3.3.x Adds photoproduction generators based on JPAC pomeron and baryon resonance calculations
• v3.2.x Adds HepMC3 and fermi momentum and misc fixes
• v3.1.x First stable relase of lAger

Tutorial

Simple Installation

1. Create a local directory that you want to work in, e.g $HOME/lager, and go into this directory.

mkdir $HOME/lager
cd $HOME/lager

2. Execute the following line in your terminal to setup your environment in this directory to install the latest stable container

curl https://eicweb.phy.anl.gov/monte_carlo/lager/-/raw/master/install.sh | bash

3. The launchers are installed in the bin directory. You can start lager by using the installed bin/lager launcher script. There is also a lager-shell launcher that opens a shell within the singularity container, useful for expert usage. Finally, some example configuration files are downloaded into the share/lager-examples directory.

4. You can now explore the command line flags for lager using the -h flag.

lager -h # if you have <PREFIX>/bin in your $PATH
<PREFIX>/bin/lager -h # if you do not have the prefix in your $PATH

Command-line interface

These instructions assume you are working from the singularity container, but mostly also apply if you are using the docker container or a local build.

The main command line options for lager are:

• configuration json file: -c your_configuration.json
• run number (also random seed, e.g. 1): -r 1
• output directory: -o $HOME/some_output_directory/

The generator will write 3 output files for each run into this directory.

• A ROOT file with the generator output
• A log file with the terminal output from the generator run. This will contain have useful statistics such as total number of generated events and total integrated cross section.
• A copy of the input JSON file with optional overrides from the command line added in, to enable easy determination of the exact configration used for a specific generated output file.

The file names will contain the run number and desired luminosity/number of events to generate, so it is safe the use the same output directory when running a large amount of processes in parallel.

JSON configuration file

lager is mainly configured through a JSON file. You can find examples under examples. The entire configuration is placed under the mc key. As example, let's discuss solid.ep-2gluon.json.

Top-level configuration

  "type" : "solid",
  "tag"  : "",
  "lumi" : "1000",
  "generator" : {},
  "detector": {},
  "reconstruction: {}

These are the 6 main fields to setup lAger:

1. type: This can be any name you want to use to identify this simulation. Here we use solid to identify this as a SoLID simulation. The first part of the output file name will be this type key.
2. tag: An additional tag to identify this simulation. Usefull to distinguish multiple very similar configurations, e.g. when changing beam energies.
3. lumi or events: Either the desired luminosity (in fb^-1), or the desired number of events. The lumi key is given precedence to the events key in case both are specified.
4. generator: The actual generator configuration, the most important component.
5. detector: Optional simple geometric acceptance components barrel, spectrometer, composite (multiple barrels/spectrometers) or the default null/4pi (detect everything).
6. reconstruction: Optional requirement that certain particles were detected. Will only write out events that fit the reconstruction requirements.

Generator configuration

The main appeal of lager lies into the flexibility of the generator as it is split in smaller sub-generators that can be mixed and matched together to form a very powerful generation tool. Below you can see the generator setup for a solid-ep simulation performed for the SoLID pCDR document.

"generator" : {
  "type" : "ep-2gluon",
  "vertex" : {"type" : "linear", "range" : [ "-7.5", "7.5" ]},
  "beam" : {
    "lepton": {
      "type" : "constant",
      "particle_type" : "e-",
      "dir" : [ "0", "0", "1" ],
      "energy" : "11.0"
    }, 
    "ion": {
      "type" : "constant",
      "particle_type" : "proton",
      "dir" : [ "0", "0", "-1" ],
      "energy" : "0.9382721"
    }
  },
  "target" : {"type": "primary"},
  "photon" : {"type" : "vphoton", "y_range" : [ "0.6", "1" ]},
  "process_0" : {
    "type" : "brodsky_2vmX"
    ...
  }
}

The generator is modular, and expects 6 keys to be present:

1. type: some name you want to use to refer to this combination of detector elements
2. vertex: your vertex definition, e.g. linear or origin
3. beam: Lepton and ion beam definitions. For a normal "constant" beam, it requires particle type, beam 3-vector and beam energy.
4. target: Actual target we use. In this case we use the primary ion beam as target (primary).
5. photon: Real or virtual photon intensity. Here it is a virtual photon with y between 0.6 and 1. Can be set to "flat" in case you do not want to fold in a photon intensity. For some processes (e.g. DVCS/BH) you may want to work around the photon.
6. process_0: Your process, can be any of the supported processes. Most processes currently give sensible results for both electro- and photo-production. You can in principle use process_0 through process_9 but make sure not to specify more than a single process at a time - process mixing is not (yet) supported.

Running an example

To run an example using the solid.ep-2gluon.json configuration, go into the examples directory and execute lAger (make sure to fill in your desired output directory).

cd examples
lager -c solid.ep-2gluon.json -r 1 -o <YOUR_OUTPUT_DIRECTORY>

Please use the issue tracker on https://eicweb.phy.anl.gov/monte_carlo/lager to file bug reports.

Copyright

lAger: General Purpose l/A-event Generator Copyright (C) 2016-2021 Sylvester Joosten sjoosten@anl.gov

lAger is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Shoftware Foundation, either version 3 of the License, or (at your option) any later version.

lAger is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with lAger. If not, see https://www.gnu.org/licenses/.