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Simulation Framework

In order to optimize the design of the components of the SHiP Experiment, the simulation framework FairShip was developed. It is based on the ROOT framework and the FairRoot framework which was initially developed for the experiments at the FAIR facility in Darmstadt, Germany. Since the SHiP Experiment is searching for extremely rare events, an optimization of the design of the experiment is crucial.

FairShip combines several simulation tools to perform the following tasks:

  • Definition and optimization of the geometry of the experiment
  • Simulation of the particle interactions and decays in the experiment
  • Simulation of the detector response to the particles
  • Reconstruction of the particle tracks and decays from the detector response
  • Analysis of the reconstructed events to determine the sensitivity of the experiment to the hidden sector particles

This allows for an easy comparison of different design choices since the resulting sensitivity to the hidden sector particles can be determined and compared.

Simulation Tools

Under the hood, FairShip uses several simulation tools to perform the tasks mentioned above1.

Pythia8

was used as an event generator to simulate the proton-target-interactions at the SPS beam dump with an impact energy of \(400\,\text{GeV}\). It accounted for the production of hadrons, charmed and beauty hadrons, mesons, muons and exotic particles like "Heavy Neutral Leptons" (HNLs). The resulting particles were then passed to the simulation tool Geant4.

Pythia6

was also used as an event generator, focusing on muon deep inelastic scattering (DIS) events. It was used to simulate the interactions of muons with the detector material, which could mimic a hidden sector particle signal, and was used to optimize the design of the Muon Shield and the Upstream Background Tagger.

GENIE MC

is a Neutrino event generator. Since the SHiP experiment contains a neutrino detector, GENIE was used to simulate neutrino interactions, including Neutrino-Nucleon interactions, Resonance production, Deep Inelastic Scattering (DIS) and Quasi-Elastic (QE) scattering. The resulting particles were then passed to Geant4.

OPERA-3D

was used to simulate the magnetic field of all the magnets involved (Muon Shield and Tracker Magnet). Instead of an analytical approximation, the magnetic field was calculated from the exact magnet geometry, which was necessary to optimize the design of the Muon Shield. It allowed for a precise simulation of the deflected tracks.

Geant4

is a toolkit for the simulation of the passage of particles through matter. It simulated the transport of each particle through the Target, the Hadron Absorber , the Muon Shield, the Scattering and Neutrino Detector and the whole Hidden Sector Decay Spectrometer, including the Tracker, the Calorimeters and the Muon System. It can take several physical processes into account, including electromagnetic and hadronic interactions, decays, multiple scattering, energy loss, the production of secondary particles and many more. The results were important for the optimization of the material thickness, geometry, detector acceptance and background suppression of all the components of the SHiP experiment.

FLUKA

was added to the simulation framework in order to account for the radiation levels within the experiment environment. Since SHiP is a fixed target experiment, the proton beam will produce a large amount of radiation in the Target and Hadron Absorber. With FLUKA the radiation dose and the activation of the materials can be simulated, leading to an optimization of the radiation protection, the shielding and the cooling of the components.

FairShip

The results of all previous tools were combined by FairShip to simulate the detector response and the reconstruction of the events. Afterwards it analysed the signal- and background efficiency for a given design approach. It then analysed the signal and background efficiencies for a given design approach. Based on these results, the shape of the Target, the length of the decay volume, the geometry and strength of the Muon Shield, the position of the Trackers and the design of the Surround- and Upstream Background Tagger were optimized to maximize the sensitivity and minimize the background.

The following diagram summarizes the simulation framework and the tools used in FairShip:

flowchart LR

    subgraph Generators["Event Generation"]
        A["PYTHIA8<br>PYTHIA6"]
        B[GENIE]
    end

    subgraph Simulation
        C[Geant4]
        D[OPERA 3D]
        E[FLUKA]
    end

    subgraph Framework["FairShip Framework"]
        F["Detector Response<br>Event Reconstruction<br>Signal & Background Analysis"]
        G[Optimization]
    end

    A --> C
    B --> C
    D --> F
    C --> F
    E --> F
    F --> G
    G -. Design Updates .-> C

  1. https://arxiv.org/pdf/1703.03612, Section 1 ↩