Delphes, a framework for fast simulation of a generic collider experiment

TL;DR

Delphes addresses the need for rapid, phenomenology-friendly detector simulations by offering a fast, parametrized framework that maps generator-level events to detector-level observables. It models tracking in a magnetic field, calorimeter response, and high-level reconstruction, including jet finding, lepton identification, MET, trigger emulation, and very forward detectors, with outputs suitable for ROOT analyses and optional LHCO formatting. The tool supports CMS-like configurations while remaining adaptable to other detectors and future colliders, and it is complemented by visualization via FROG and beamline transport via Hector. Validation against CMS/ATLAS benchmarks demonstrates realistic jet and MET resolutions, making Delphes well-suited for feasibility studies and rapid analyses preceding full GEANT-based simulations.

Abstract

This paper presents a new C++ framework, DELPHES, performing a fast multipurpose detector response simulation. The simulation includes a tracking system, embedded into a magnetic field, calorimeters and a muon system, and possible very forward detectors arranged along the beamline. The framework is interfaced to standard file formats (e.g. Les Houches Event File or HepMC) and outputs observables such as isolated leptons, missing transverse energy and collection of jets which can be used for dedicated analyses. The simulation of the detector response takes into account the effect of magnetic field, the granularity of the calorimeters and subdetector resolutions. A simplified preselection can also be applied on processed events for trigger emulation. Detection of very forward scattered particles relies on the transport in beamlines with the HECTOR software. Finally, the FROG 2D/3D event display is used for visualisation of the collision final states.

Figures (13)

• Figure 1: Flow chart describing the principles behind Delphes. Event files coming from external Monte Carlo generators are read by a converter stage (top). The kinematics variables of the final-state particles are then smeared according to the tunable subdetector resolutions. Tracks are reconstructed in a simulated solenoidal magnetic field and calorimetric cells sample the energy deposits. Based on these low-level objects, dedicated algorithms are applied for particle identification, isolation and reconstruction. The transport of very forward particles to the near-beam detectors is also simulated. Finally, an output file is written, including generator-level and analysis-object data. If requested, a fully parametrisable trigger can be emulated. Optionally, the geometry and visualisation files for the 3D event display can also be produced. All user parameters are set in the Detector/Smearing Card and the Trigger Card.
• Figure 2: Profile of layout of the generic detector geometry assumed in Delphes. The innermost layer, close to the interaction point, is a central tracking system (pink). It is surrounded by a central calorimeter volume (green) with both electromagnetic and hadronic sections. The outer layer of the central system (red) is muon system. In addition, two end-cap calorimeters (blue) extend the pseudorapidity coverage of the central detector. Additional forward detectors are not depicted.
• Figure 3: Default segmentation of the calorimeters in the $(\eta,\phi)$ plane. Only the central detectors (ECAL, HCAL) and FCAL are considered. $\phi$ angles are expressed in radians.
• Figure 4: Illustration of the identification of $\tau$-jets ($1-$prong). The jet cone is narrow and contains only one track. The small cone serves to apply the electromagnetic collimation, while the broader cone is used to reconstruct the jet originating from the $\tau$-decay.
• Figure 5: Distribution of the electromagnetic collimation $C_\tau$ variable for true $\tau$-jets, normalised to unity. This distribution is shown for associated $WH$ photoproduction bib:whphotoproduction, where the Higgs boson decays into a $W^+ W^-$ pair. Each $W$ boson decays into a $\ell \nu_\ell$ pair, where $\ell = e, \mu, \tau$. Events generated with MadGraph/MadEvent bib:mgme. Final state hadronisation is performed by Pythiabib:pythia. Histogram entries correspond to true $\tau$-jets, matched with generator-level data.