This thesis presents the development, layout optimisation, and validation of a high-granularity tungsten–silicon (W–Si) preshower detector for the FASER experiment at the LHC, designed to extend sensitivity to long-lived, weakly interacting particles produced in the very forward region along the beam axis. To enable detection of those particles when decaying into highly collimated, high-energy photon pairs, the original scintillator-based system was replaced with planes of tungsten absorbers interleaved with monolithic silicon pixel sensors, providing the spatial resolution required to resolve closely spaced photon showers.
Geant4 simulations compared three candidate configurations to guide the detector design. The selected geometry, featuring six asymmetrically arranged tungsten–silicon planes, maximises early photon conversion and di-photon reconstruction efficiency while keeping the material budget low enough to preserve downstream calorimeter performance, achieving a total photon conversion probability of ~98.6% (98.4% by the third plane) and a di-photon reconstruction efficiency above 80% for symmetric events with Eγ1 = Eγ2 > 500 GeV and δγγ = 500 μm. These results also defined the technical specifications for the custom ASIC.
The second part of the thesis reports the experimental characterisation of two ASIC prototypes. The first explored several levels of in-pixel front-end integration. Two configurations offered the best balance of performance and layout compactness: one combining the preamplifier and driver, and another additionally including the discriminator. Both were carried forward into the pre-production ASIC, used to validate the front-end electronics, the Analog-to-Digital Converter (ADC), and the Time-to-Digital Converter (TDC). These results directly informed the final ASIC design submitted for production.
This work establishes the technical foundation for the upgraded W–Si preshower detector, significantly broadening FASER's sensitivity to unexplored regions of parameter space in the search for physics beyond the Standard Model. Installed in February 2025, the detector is currently operational.
