Observation of light production by charged particles in WLS fibers

Wavelength shifting (WLS) fibers are widely used in particle physics for light collection from scintillators. Light production by charged particles directly in WLS fibers is traditionally ignored. In this study, light produced by charged particles in WLS fibers is clearly observed. The light yield of different batches of Y11(200) 1 mm diameter WLS fibers is as large as 23 $\pm$ 2% with respect to the light yield of the Bicron BCF-12 1 mm diameter scintillating fiber. In clear fibers of the same diameter, no scintillation light is produced, while Cherenkov light is clearly seen at the 45-degree crossing angle. The observed amount of light produced by charged particles in the WLS fibers is not small and should be taken into account in advanced detector simulations.