The FASER W-Si High Precision Preshower Technical Proposal

The FASER detector is designed to search for light weakly interacting new particles decaying into charged final states at the LHC. While the first physics data will be taken at the start of Run 3 of the LHC program, an upgrade is already foreseen to enhance the sensitivity to long-lived particles decaying into photons. A high-precision preshower detector will be constructed within the next two years allowing to distinguish the predicted axion-like particles signature of two very closely spaced highly energetic photons. Profiting from recent developments in monolithic pixel silicon detectors, the FASER Collaboration plans to build instrumented silicon pixel detector planes with a granularity of 100 𝜇m interleaved with tungsten absorber planes. The addition of the new pre-shower detector will expand the physics search capability of FASER.