Characterization of Photomultiplier Tubes for the FASER Detector

FASER (ForwArd Search ExpeRiment) is a recently approved, small experiment to be installed in the Large Hadron Collider (LHC) in approximately one year from now before the start of Run 3. It is designed to detect light, weakly-interacting particles produced in the decay of neutral pions such as dark photons, dark Higgs bosons, sterile neutrinos, and axion-like particles. These particles would be produced near the ATLAS interaction point and travel roughly 480 m along the line-of-sight until decaying into standard model particles in the volume of the FASER detector situated in the unused TI12 tunnel. The detector will be quite small with a length of about 3.5 m and an aperture radius of 10 cm. Candidate particles entering the detector first pass through a double layer of scintillators that will veto charged particles, then travel into a dipole magnet decay volume where they will decay into observable particles like positron-electron pairs. The positron-electron pairs then continue through the spectrometer section which consists of two dipole magnets with three silicon strip tracking detectors at either end and in between the magnets to track position and determine momentum. Finally, the positron-electron pairs will end their flight in the electromagnetic calorimeter where we will measure their total energy. A very important part of assembling the detector is to characterize the photomultiplier tubes (PMTs) that will be used to measure the light produced in the scintillators and calorimeters. This includes measuring the gain as a function of voltage, measuring the linearity of the gain, and measuring the quantum efficiency as a function of wavelength. In addition to characterizing the PMTs, we also estimated the efficiency of a new bent light guide for our scintillators.