Overview

Teaching: 15 min
Exercises: 10 min

Questions

• How do we define geometry using DD4hep?

Objectives

• Know where standard geometries as stored in eic-shell.

• Understand the structure of a geometry description file.

Introduction

The ePIC geometry is described using the DD4hep toolkit.

DD4hep (Detector Description for High Energy Physics) is a toolkit which acts as a single, central source of detector information for simulation and reconstruction of simulated and experimental data. The core DD4hep geometry description is built around the ROOT geometry package while providing automatic conversions to other geometrical representaions, such as exporting CAD models, surfaces for acts reconstruction, material maps but most importany, Geant4 for carrying out simulations.

DD4hep additionally provides a much simplified wrapper around running Geant4 simulations, providing standardized output from sensitive detectors. In the case of the ePIC simulation, simulated particles and tracker/calorimeter hits are saved as collections in EDM4hep format (Event Data Model for High Energy Physics) built on podio (plain-old-data I/O).

Lesson

We start the discussion of the geometry definition with an overview of the locations of geometry files, and what is included in these files.

Location of standard geometries in eic-shell

Several standard geometry versions are included in eic-shell under the /opt/detector/ location. This includes (currently) at least the following:

$ ls -1 /opt/detector/
calibrations
dummyOutput.root
epic-23.10.0
epic-23.11.0
epic-23.12.0
epic-24.02.0
epic-24.02.1
epic-24.03.0
epic-24.03.1
epic-24.04.0
epic-main
fieldmaps
gdml
lib
share

Note: ls -1 lists the files with 1 file per line, i.e. in 1 column.

The versions avaliable in eic-shell are updated when tagged releases for the geometry are made each month or an update to dependancies installed in eic-shell removes back compatibility with older versions.

The epic-main directory contains the current ‘nightly build’ of the ePIC geometry, built from the epic repositories main branch every day.

$ ls -1 /opt/detector/epic-main/
bin
lib
share
$ ls -1 /opt/detector/epic-main/bin
thisepic.sh

You can load a geometry by ‘sourcing’ the bin/thisepic.sh file.

$ source /opt/detector/epic-main/bin/thisepic.sh

Both commands should have the same effect:

• your prompt should have changed to main> to indicate the geometry that is loaded,
• your shell environment will have the necessary variables loaded to work with the epic-main geometry.

You can verify the latter by investigating the values of several environment variables:

• DETECTOR is the name of the detector geometry that is loaded (epic),
• DETECTOR_VERSION is the version (i.e. GitHub branch or tag) that is loaded (main),
• DETECTOR_CONFIG is the detector configuration to use (i.e. whether to include MRICH or PFRICH, SciGlass or imaging ECAL),
• DETECTOR_PATH is the location that points to the geometry resources (/opt/detector/epic-main/share/epic).

Note: When working on the geometry in your own git branch you will need to source the setup.sh present in the local install directory.

Exercise:

• Load the standard ePIC geometry and verify (with e.g. echo $DETECTOR) that the environment variables are set.
• Load another geometry and verify that the environment variables are indeed different.

What is stored at those locations?

We will now take a look in the directory pointed to with the environment variable $DETECTOR_PATH, the location of the geometry resources:

$ ls $DETECTOR_PATH
calibrations                      epic_craterlake_tracking_only.xml        epic_lfhcal_with_insert.xml
compact                           epic_craterlake.xml                      epic_mrich_only.xml
dummyOutput.root                  epic_dirc_only.xml                       epic_pfrich_only.xml
epic_bhcal.xml                    epic_drich_only.xml                      epic_pid_only.xml
epic_calorimeters.xml             epic_forward_detectors_with_inserts.xml  epic_tof_endcap_only.xml
epic_craterlake_10x100.xml        epic_forward_detectors.xml               epic_tof_only.xml
epic_craterlake_10x275.xml        epic_full.xml                            epic_vertex_only.xml
epic_craterlake_18x110_Au.xml     epic_imaging_only.xml                    epic.xml
epic_craterlake_18x275.xml        epic_inner_detector.xml                  epic_zdc_lyso_sipm.xml
epic_craterlake_5x41.xml          epic_ip6_extended.xml                    epic_zdc_sipm_on_tile_only.xml
epic_craterlake_material_map.xml  epic_ip6.xml                             fieldmaps
epic_craterlake_no_bhcal.xml      epic_lfhcal_only.xml                     gdml

You will see many xml files, all of which are entry points to the geometry in certain configurations. For example, epic_drich_only.xml includes the geometry that has only the dual RICH or dRICH. epic_ip6.xml includes the beampipe geometry and the auxillary far-forward and backward detectors but no components of the central detector. The default configuration, epic.xml, is typically the configuration you will want to use, this is the value that DETECTOR_CONFIG will be set to by default.

Note: The current nightly eic-shell build contains only configurations for the craterlake detector setup.

Let’s take a look in the default entry point file, pointed at by the DETECTOR_CONFIG environment variable. This is the file epic.xml:

$ less $DETECTOR_PATH/$DETECTOR_CONFIG.xml

(Note: Use q to exit less, or use any editor you prefer.)

The xml file includes several blocks, but look in particular for the following lines:

• <include ref="${DETECTOR_PATH}/compact/definitions.xml"/>: This line includes the overall detector parametrization file (think of this as a detector parameter table similar to what the EIC Menagerie provides).
• <include ref="${DETECTOR_PATH}/compact/tracking/vertex_barrel.xml"/>: This line includes one of the tracker subsystems; there are other include lines that load other tracking subsystems, or even other types of subsystems.
• <include ref="${DETECTOR_PATH}/compact/far_forward/default.xml"/>: This line includes the far forward subsystems.

These included files (e.g. far_forward/default.xml) can include further nested inclusion of even more files (e.g. far_forward/ZDC.xml).

Note: The xml files are parsed in order, so the definitions.xml file needs to be included before any file which needs to access the defined parameters.

Let’s now take a look at a particular detector subsystem end point file (which does not include any more files), namely tracking/vertex_barrel.xml.

$ less $DETECTOR_PATH/compact/tracking/vertex_barrel.xml

You will notice that the detector is described by parameters in a define block, such as (abridged):

  <define>
    <constant name="VertexBarrelMod_length"             value="VertexBarrel_length"/>
    <constant name="VertexBarrelMod_rmin"               value="VertexBarrel_rmin"/>

    <constant name="SiVertexSensor_thickness"           value="40*um"/>
  </define>

which can either use another parameter defined previously in the included files, or which can be defined in this file itself. A best practices is to define detailed parameters of each subsystem in the end point file, but to defer to the central definitions in the definitions.xml for quantities such as the overal size and location of the subsystem, or interfaces with other subsystems.

The parameters are then used in the detector block to define the detector itself (much abridged):

  <detectors>
    <detector
      id="VertexBarrel_0_ID"
      name="VertexBarrel"
      type="epic_VertexBarrel"
      readout="VertexBarrelHits"
      insideTrackingVolume="true">
      <dimensions
        rmin="VertexBarrelLayer1_rmin"
        rmax="VertexBarrelLayer3_rmax"
        length="VertexBarrelEnvelope_length" />
    </detector>
  </detectors>

The parametrization of the entire detector, down to the subsystems, is defined in these xml files. But where are the volumes created? The key here is the type field, which points to the detector type plugin that interprets the parametrization (here the type is epic_VertexBarrel). A well-written detector plugin can support many different detector configurations and parametrizations without the need to ever touch a line of code.

Exercise:

• Identify which subsystem or detector you are interested in.
• Take a look in the epic.xml file and locate where this detector is included.
• Locate the end point file that defines the parameters that describe this file.
• Identify the detector plugin that is used for this detector.

Key Points

• Compact XML files are used to store parameters which are used by compiled plugins.