Test setup for LDMX HCal readout

Abstract: It is evident from cosmological observations such as the Cosmic Microwave Background, and the motion of galaxies, that the universe consists of approximately 5 times more mass than visible matter can account for. The Light Dark Matter eXperiment (LDMX) will take a unique approach in its hunt for dark matter; it will look for missing momentum signals in electron collisions with a Tungsten target, and will be looking in the sub GeV energy range for evidence of dark matter interactions. To achieve this, a sensitive hadronic calorimeter system will be required to detect the resulting particles from events that need to be ignored, or vetoed. The basic physical processes that need to be considered for hadronic calorimetry in general, as well as for the hadronic calorimeter in LDMX experiment specifically, are discussed. The experimental setup is presented, consisting of a Styron 665 W Polystyrene based plastic scintillator bar[1], with Wavelength Shifting Fibre (WLS) at the centre, and a Photomultiplier Tube (PMT) at each end. Light output was optimized by grinding the ends of the fibres down, and by increasing the fibre volume inside the bar. Light-tightness was maximised by implementing a simple plastic coupling insert for the fibre, made using a 3D printer. After beta particles failed to produce signals, minimally ionizing cosmic ray muons were used as events to measure the scintillation light in the bar. To achieve this, an area of ≈ 1dm2 was covered with overlapping scintillator paddles, which then triggered an oscilloscope to measure the output at the fibre ends. The measured amplitudes were compared to those of apparent single photo-electrons measured by the PMTs. A basic Python simulation was constructed to attempt to identify parameters that could be optimized for photon collection by the fibre, in slightly different configurations, based on the amount of light that was obtained from cosmic events. Though there was not sufficient light yield for resolving energies, there was some measurable time resolution. A rough estimate of the event location along the bar axis could be made, which indicated the effective mid point was closer to one end of the bar at ≈ 650mm. Finally, an outlook on what the next objectives could be is discussed.