# DD4hep plugin¶

The DD4hepPlugin allows building of a Acts::TrackingGeometry from DD4hep input. DD4hep uses ROOT TGeo as the underlying geometry model.

## General¶

The basic input for building the detector are a detector description in the XML file format and a corresponding detector constructor written in C++. These two components have to be changed accordingly. Detector constructors use these XML files as an input and construct a detector in the DD4hep geometry format. The whole detector is segmented into different detector parts, e.g. barrels and endcaps which describe different sub-detectors. Since these sub-detectors are built differently, they need different detector constructors. In this way one detector description in XML can use various detector constructors, needed for the different kinds of detector parts.

In the detector description model of DD4hep any detector is a tree of instances of the so-called DetElement class. This DetElement class provides all needed detector information, e.g. readout segmentation, geometrical information, environmental conditions. This tree is parallel to the volume tree, which provides the TGeoVolumes and their placements. The relation between these two trees is one-directional, i.e. every volume can be accessed via its corresponding DetElement, but not vice versa. Not every supporting material will be declared as a detector element, hence, the geometrical tree can have a deeper hierarchy structure. In order to access both, detector specific and geometrical information, the conversion to the tracking geometry navigates through the detector tree. The DetElement can also be extended, to add specific features or to access information. This extension mechanism is used during the translation process.

## Acts extension¶

DD4hep provides a special extension mechanism for the DetElement which allows to add custom features. In Acts this functionality is used for the conversion from DD4hep into Acts. The extensions are used to indicate certain volumes, e.g. if a DetElement is the beam pipe or if a DetElement is a layer carrying the sensitive modules. In addition the extensions are used in order to distinguish if a sub detector is a barrel (described as a cylinder volume in Acts) or an endcap (which is described as a disc volume in Acts). Furthermore the extensions are used to hand over specific information needed for tracking, e.g. paramters for material mapping.

## DD4hepDetectorElement¶

In Acts the surfaces describing the sensitive modules of a detector are directly linked to these of the initial geometry input. In the case of DD4hep the Acts::DD4hepDetectorElement was introduced which is the direct link of Acts to DD4hep. In the case for tracking relevant parameters in the DD4hep geometry description are changed (e.g. alignment) it will be automatically changed in Acts.

## Build¶

The DD4hepPlugin is only build on demand. The DD4hepPlugin depends on the TGeoPlugin therefore both plugins need to be installed. During the cmake configuration the flags ACTS_BUILD_PLUGIN_DD4HEP=on and ACTS_BUILD_PLUGIN_TGEO=on need to be set. In addition ROOT and DD4hep installations need to be available to cmake.

## Prerequisites¶

To guarantee a working translation from DD4hep input to Acts geometry the following conditions need to be met:

• The detector needs to have a barrel-endcap structure: Every hierarchy of subdetectors (e.g. PixelDetector, StripDetector,…) needs to be decomposed into

1. {barrel}

2. {barrel + 2 endcaps}

3. {2 endcaps} - in case there is no barrel at this stage (e.g. forward end caps)

• If a hierachy is not only a single barrel but is decomposed of a barrel and its corresponding endcaps they need to be grouped together in an assembly using the DD4hep_SubdetectorAssembly constructor which is provided by DD4hep. Example of usage in xml file (where Barrel0, nEndCap0 and pEndCap0 are sub detectors defined in the file PixelTracker.xml):

<include ref="PixelTracker.xml"/>
<detectors>
<detector id="1" name="PixelTracker"type="DD4hep_SubdetectorAssembly"vis="BlueVisTrans">
<shape name="PixelEnvelope" type="Tube" rmin="Env0_rmin"rmax="Env0_rmax"dz="Env0_dz" material="Air"/>
<composite name="Barrel0"/>
<composite name="nEndCap0"/>
<composite name="pEndCap0"/>
</detector>
</detectors>


If a user wants to create his/her own constructor to group these volumes together the type needs to be set to “compound”.

• Since the translation walks trough the DetElement tree the following objects need to be declared as a DD4hep DetElement:

• The subvolumes e.g. barrel, endcap, beampipe (they are usually build with different DD4hep constructors and are therefore DD4hep DetElement’s per default).

• Layers when containing sensitive material and/or the layer should carry material (which will be mapped on the layer if indicated), or the layer is sensitive itself.

Note

the layer does not need to be a direct child of the volume (barrel or endcap),it an be nested in substructures

• Sensitive detector modules

Note

The sensitive detector modules need to be placed in a layer however it can be nested in substructures (can be a component of a modules) i.e. it does not need to be a direct child of the layer

• The Tracking geometry needs to be built from bottom to top to ensure navigation. Therefore the different hierarchies need to be sorted ascending. Per default the sub detectors are sorted by the id of their DetElement. In case another sorting needs to be applied, the users can provide their own function.

• The Acts::ActsExtension’s need to be used during the detector construction indicating if a DetElement

• is a barrel

• is an endcap

• is the beampipe

• is a layer

There are two modes building the layers around the sensitive detector modules:

• The DetElement containing the sensitive modules have a geometrical shape.

The boundaries of the layers in Acts are taken directly from the given shape.

• The DetElement containing the sensitive modules have no specific shape (assembly).

The boundaries of the layers are calculated automatically by adding a tolerance to the geometric extension of the contained surfaces. The tolerances in r and z need to be set for every DetElement representing layer using envelopeR and envelopeZ in Acts::ActsExtension.

The volumes are automatically build around the layers:

• The boundaries for the volumes are calculated automatically by adding a tolerance to the geometric extension of the contained layers. The tolerance parameters layerEnvelopeR and layerEnvelopeZ need to be set in the Acts::convertDD4hepDetector() function.

Furthermore parameters can be handed over for material mapping or the axes orientation of modules.

Summing up the DetElement tree in DD4hep should have the following structure:

It is also possible to translate a very simple detector geometry, which just consists of cylindrical (for a barrel) or disc (for endcaps) layers which either have material, or, are declared sensitive in dd4hep themselves without containing any detector modules.

## Usage¶

To receive the Acts::TrackingGeometry the user should use the global function Acts::convertDD4hepDetector(), where he/she needs to hand over the DD4hep world DetElement. For a valid translation the user needs to make sure, that all prerequisites described above are met and that the right Acts::ActsExtension’s are added during the DD4hep construction.