A pointing electromagnetic calorimeter for the SHiP experiment
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Abstract
Feebly interacting particles (FIPs) are a broad class of beyond-Standard-Model candidates for dark matter and the origin of neutrino masses. Their defining feature is a very weak coupling to Standard Model fields, which suppresses their production cross-sections and results in macroscopic decay lengths, making high-luminosity beam-dump experiments at the CERN SPS a great discovery environment. Axion-like particles (ALPs) are a minimal, model-independent subclass of FIPs arising from the spontaneous breaking of approximate global symmetries, experimentally accessible via an $ALP \to \gamma\gamma$ signature. Both experiments studied in this thesis, SHADOWS and SHiP, target neutral final states with displaced vertices, requiring an electromagnetic calorimeter (ECAL) that reconstructs the energy as well as the direction of photon showers. An evaluation of the ALP decay kinematics in the boosted regime relevant at the SPS shows that the propagated uncertainty of the invariant mass is dominated by the angular resolution, making pointing resolution the primary figure of merit throughout this work.
For SHADOWS, the off-axis geometry and moderate final-state energies set granularity as the key design parameter. A channel pitch of $1\,\text{cm}$ removes any granularity-induced reconstruction bias at small angles for showers above $2\,\text{GeV}$. Diphoton vertex and invariant-mass reconstruction are demonstrated for two ALP benchmark scenarios, and the energy resolution of ${\sim}11\%/\sqrt{E/\text{GeV}}$ meets the requirements. For the 2024 approved on-axis experiment SHiP, the harder and more forward final states impose stricter angular resolution requirements. A longitudinal air gap is introduced as the central design element, increasing the pointing lever arm without adding active channels or absorber.
A prototype was built to validate the pointing concept experimentally and to provide the data needed to tune the simulation framework. The prototype was tested at the CERN SPS at energies from $3$ to $288\,\text{GeV}$, establishing that the plastic scintillator strip-based concept performs calorimetric pointing under realistic conditions. A simulation tuned to the measured detector response reproduces all test-beam observables and is extended to a full detector module for the SHiP ECAL optimisation.
Systematic studies of the gap depth and position with single photons identify the optimal detector geometry and channel structure: a gap with a depth of $100\,\text{cm}$ at $3\,X_0$ combined with an alternating strip layer configuration. The conservative energy resolution estimate of $14.4\%/\sqrt{E/\text{GeV}}$ meets the SHiP requirement. Beyond the optimal configuration, the study establishes a systematic understanding of which design parameters control which performance observables, providing the foundation for well-informed decisions in the subsequent design stages and for studies with more sophisticated figures of merit.