Cosmic ray background estimations in large liquid argon detectors for proton decay via p to K+ anti_nu and nu_e appearance from nu_mu beams

Abstract

In recent years caverns deep underground at Sanford Underground Research Facility (SURF) near Lead, SD, have been proposed to house a large-scale liquid argon detector. This was once intended to be part of the ``Deep Underground Science and Engineering Laboratory" (DUSEL) project to build extensive laboratory space for multidisciplinary scientific research, well-shielded from cosmic rays. The ``Long Baseline Neutrino Experiment" (LBNE) later proposed such a detector at SURF and in 2015 LBNE evolved into the ``Deep Underground Neutrino Experiment" (DUNE). All of these proposals have involved a detector suitable for the study of neutrino oscillation parameters, primarily using a nu_mu / anti_nu_mu beam generated at Fermilab in Batavia, IL. The large target mass also enables a nucleon decay search, as well as the study of other phenomena. Cosmic-ray muons are capable of penetrating deep below the surface of the Earth and causing events that mimic rare signal events. Monte Carlo simulations of the cosmic muon-induced background for proton decay via the channel p \rightarrow K+ anti_nu have been performed, where the signal is the observation of a single K+ track. Cuts have been developed to reject the background K+/K- observed, which has been estimated for a 10 kton (fiducial mass) LBNE/DUNE detector at a depth of 4850 ft and a 40 kton (fiducial mass) detector at 800 ft suitable for a DUSEL cavern. For the former the annual rate of background events for p \rightarrow K+ anti_nu after cuts is <2.3 * 10^{-3} kton^{-1} yr^{-1} (at 90% C.L.). For the latter this is <0.026 kton^{-1} yr^{-1}. The cosmic ray-induced background for nu_e appearance from the nu_mu beam has also been estimated for a near-surface level detector (10 kton fiducial mass) once proposed for LBNE. The signal in this instance is an electromagnetic (EM) shower. After cuts the background has been estimated as 2.88 +/- 0.25 yr^{-1} due to EM showers that start with gamma \rightarrow e+ e-. The upper limits of the annual rates of EM showers starting with K^0_L \rightarrow pi+/pi- e-/e+ anti_nu_e/nu_e in muon, proton and neutron-induced events are < 0.21 yr^{-1}, < 0.01 yr^{-1} and < 0.002 yr^{-1} (at 90% C.L.) after cuts. The upper limits of the annual rates of EM showers starting with pi0 \rightarrow e+ e- gamma in muon, proton and neutron-induced events are < 0.18 yr^{-1}, < 0.01 yr^{-1} and < 0.03 yr^{-1} (at 90% C.L.) after cuts