SHADOWS (Search for Hidden And Dark Objects With the SPS): Letter of Intent

We propose a new proton beam-dump experiment, SHADOWS, to search for a large variety of feebly-interacting particles possibly produced in the interactions of a 400 GeV proton beam with a high-Z material dump. SHADOWS will use the 400 GeV primary proton beam extracted from the CERN SPS currently serving the NA62 experiment in the CERN North area. SHADOWS will take data off-axis concurrently to the HIKE experiment when the P42 beam line is operated in beam-dump mode to accumulate up to 5 · 10^19 protons on target in 4 years of operation. This document describes the main achievements with respect to the Expression of Interest and represents an intermediate step towards the Proposal.