Measurement of the Muon Flux at the Sanford Underground Research Facility with the LUX-ZEPLIN Dark Matter Detector

Abstract

High-energy cosmic-ray muons reaching deep underground laboratories can cause events in detectors that mimic signals expected from dark matter particles, neutrinos, or rare decays. Knowledge of the muon flux and energy spectrum is important for evaluating the background rate caused by muons and their secondaries. In this paper, we report the measurement of the cosmic-ray muon flux in the Davis Campus of the Sanford Underground Research Facility with the LUX-ZEPLIN detector. Using 366.4~days of exposure, the muon rate through the detector was measured as $10.94\pm0.17_\textrm{stat.}~\textrm{day}^{-1}$ with energy thresholds of 20~MeV in the inner xenon detector and 8 MeV in the outer liquid scintillator detector. This rate corresponds to a muon flux of $(5.09\pm0.08_\textrm{stat.}\pm0.10_\textrm{sys.})\times10^{-9}~\textrm{cm}^{-2}\textrm{s}^{-1}$ in the Davis Cavern.

Figures (4)

• Figure 1: Energy spectrum (a) and zenith angular distribution (b) of simulated muons at SURF’s Davis cavern, generated with the MUSUN model at the surface of the box enclosing the cavern boundary (see text for details).
• Figure 2: Waveform output of a typical muon event captured by the LZ data acquisition system. The amplitude is measured in single photons registered by PMTs per 10 ns sample. The individual waveform traces from the TPC, Skin, and OD are shown in the top three panels. The bottom panel shows a magnified view of the pulses in all detectors overlaid at the time the muon initially passes through the experiment to demonstrate the contribution of pulse timing to the muon event selection. The TPC pulse corresponds to the solid red line, the OD pulse corresponds to the dashed green line, and the Skin pulse corresponds to the dotted blue line.
• Figure 3: (a) Muon rate as a function of the energy threshold in the TPC after analysis cuts have been applied to simulations and data. (b) The ratio of data-to-simulated muon event rates as a function of the energy threshold in the TPC.
• Figure 4: Spectra of energy depositions from muon events in the OD in data and simulations. Energy deposited in the OD in data was obtained from the total pulse area using a single scaling factor to match simulated spectrum. The shape of the spectra is due to the path dependence of the muon through the complex OD tanks geometry.