CMS goes DD4hep
Markus Frank (CERN), 20/08/2018
A visualisation of the CMS tracking detectors using the DD4hep toolkit.
The CMS collaboration announced that it plans to use the detector description toolkit DD4hep to provide the structure of the experiment to all their data processing applications. They also plan a full-scale test, including all subdetectors, for the end of 2019. AIDA-2020 physicists from CERN and DESY actively helped to bootstrap this decision by providing the necessary expertise to make this part of CMS software fit for the ambitious upgrade programme necessary to cope with the high luminosity phase of the Large Hadron Collider (LHC). This collaboration will also help to elaborate other features of DD4hep for future experiments.
The development of a coherent set of software tools for the description of high-energy physics detectors from a single source of information, as implemented in DD4hep, has been on the agenda of many experiments for decades. Providing appropriate and consistent detector views to simulation, reconstruction and analysis applications is crucial for the success of the experiments. Detector description in general includes not only the geometry and the materials used in the apparatus, but all parameters describing e.g. the detection techniques, constants required by alignment and calibration, description of the readout structures, conditions data and more. AIDA-2020 Work Package 3 takes care of all the advanced software needed for coherent designing, modelling, description and reconstruction processes.
Responding to requests from the linear collider community, the DD4hep detector description toolkit [1] was originally designed to fulfill these needs. The DD4hep toolkit fully describes and qualifies the detection apparatus and enables access to all information required to interpret event data recorded from particle collisions. Experience from the LHC experiments has shown that a view of the detector that is not limited to geometry is very beneficial for obtaining a coherent set of tools to interpret collision data. DD4hep supports the entire experiment life-cycle and is flexible enough to cope with the changing needs arising during the development of the detector concepts, detector optimisation procedures, prototyping and test beam studies, construction, and later operation of the experiment.
The initial phases are characterised by ideal detector descriptions, i.e. a relatively small number of parameters are sufficient to describe new designs. Once operational, the requirements change, the detector will be different from the ideal detector, and each part of the detector will require its own specific parameters and conditions.
DD4hep reuses existing software components, combines the functionalities and thus optimises the flexibility, minimising the efforts required by users to leverage the benefits. Reused components include elements of the ROOT geometry package [2] and the Geant4 simulation toolkit [3]. Figure 2 shows the architecture of the main components of the toolkit and their interfaces to the end-user applications, namely simulation, reconstruction, alignment and visualisation.
The components of the DD4hep detector geometry toolkit.
The central element of the toolkit is the so-called generic detector description model. Any detector is modeled as a tree of Detector Elements, which offers all applications an entry point to any part of the experiment and represents complete sub-detectors or any part thereof. It provides an in-memory model of the detector, including all geometrical, structural and functional aspects arising from the various applications used to handle data from particle collisions. These applications may require a specialised functionality not offered by the basic detector element. Based on common data, different functionalities may be implemented with specialised C++ classes, enhancing the functionality of the detector element. Such extensions, called Views, are an important means for code simplification and, depending on the functionality of these specialised components, they are able to either store additional data, expose additional behaviour, or both, and address the following use-cases:
Convenience Views provide higher-level abstractions and internally group complex calculations. Such views make end users’ lives easier.
Optimisation Views allow users to extend the data of the common detector element and store precomputed results, which would be expensive to obtain repeatedly.
Compatibility Views help to ensure smooth periods of software redesign – they either adapt new data designs to existing application code or expose new behaviour based on existing data.
Using these concepts, DD4hep intrinsically provides a set of basic subdetector designs as typically used for hermetic 4π detectors. Detector designers may base initial design studies of a planned experiment on a palette of pre-implemented subdetector concepts. DD4hep, with the sub package DDG4, supports the simulation of such designs, using the nowadays most prominent toolkit, Geant4. DD4hep provides automatic translation mechanisms and handles the simulation response of the active elements. The behaviour of the simulation package was designed to allow for, but does not require, user-defined extensions. The configuration is handled by using python scripting.
The support for alignment operations and other auxiliary information necessary to interpret data from particle collisions is crucial once an experiment is mature. The possibility to gauge the detector response, e.g. using environmental data, or to move hits of the detector response, with respect to their ideal positions, is necessary to obtain results at the most achievable precision. The two sub-packages DDCond and DDAlign support the management of such auxiliary data.
Over the time, the DD4hep package suite has gained increasing interest for all kinds of detector studies. It is the de-facto standard for detector description in the linear collider and FCC communities. Recently, the mature LHC experiments CMS and LHCb have also started evaluating the use of DD4hep for the upgrade programmes.
[1] DD4hep web page, http://dd4hep.cern.ch
[2] R.Brun, A.Gheata, M.Gheata, "The ROOT geometry package", Nuclear Instruments and Methods A 502 (2003) 676-680.
[3] S. Agostinelli et al., ”Geant4 - A Simulation Toolkit”, Nuclear Instruments and Methods A 506 (2003) 250-303.