Contribution of the light-collection non-uniformity to the energy resolution for the spaghetti-type calorimeter modules

Spaghetti-type calorimeters (SpaCal) are being considered as a potential solution for experiments at the High-Luminosity Large Hadron Collider (HL-LHC). SpaCal modules consist of an absorber block with a matrix of holes filled with scintillating fibres. This geometry offers a flexible granularity. However, the total number of scintillating fibres per channel could exceed the available photocathode surface area, necessitating a light guide to efficiently collect and register the scintillating light from the scintillating fibres to a photomultiplier. The non-uniformities in the light collection would impact the energy resolution of the detector. In this study, the impact of the light collection non-uniformity on the energy resolution is estimated for the various geometries of light guides using optical simulations~(GEANT4 simulation with optical photons). The study performed assumed a SpaCal prototype with $30\times 30$~mm$^2$ cell size and photomultipliers featuring realistic entrance window shapes and sizes, and explores different light guide designs.