Background studies for the CODEX-b experiment: measurements and simulation

This report presents results from a background measurement campaign for the CODEX-b proposal undertaken in August, 2018. The data were recorded in the DELPHI side of the LHCb cavern behind a 3.2~m concrete shield wall, during Run~2 proton-proton collisions with the goal of calibrating the simulation for the full CODEX-b detector. The maximum flux rate in the DELPHI side of the cavern was found to be around 0.6~mHz/cm$^2$ across a vertical plane just behind the shield wall, parallel to the beam line. A detailed simulation under development within the LHCb {\tt Gauss} framework is described. This includes shielding elements pertinent for CODEX-b's acceptance -- the LHCb detector, the shield wall and cavern infrastructure. Additional flux from tracks not in the line of sight from the interaction point, but bent by the magnetic fields, are incorporated. Overall, the simulation overestimates the background flux compared to the measurement. Several cross-checks and avenues for further investigations are described.