New Results on Atmospheric Neutrinos from Soudan 2 * 1. Detector; Data Exposure

Neutrino interactions recorded in a 5.1 fiducial kiloton-year exposure of the Soudan-2 iron tracking calorimeter are analyzed for effects of neutrino oscillations. Using contained single track and single shower events, we update our measurement of the atmospheric νµ/νe ratio-of-ratios and find R = 0.68 ± 0.11 ± 0.06. Assuming this anomalously low R-value is the result of νµ flavor disappearance via νµ to ντ oscillation, we select samples of charged current events which offer good resolution, event-by-event, for L/Eν reconstruction. Oscillation-weighted Monte Carlo events are fitted to these data events using a χ 2 function summed over bins of log(L/Eν). The region allowed in the (sin 2 2θ, ∆m 2) plane at 90% CL is obtained using the Feldman-Cousins procedure: 0.46 < sin 2 2θ ≤ 1.0 and 2.2 × 10 −4 < ∆m 2 < 2.2 × 10 −2 eV 2. A small but relatively energetic sample of partially contained νµ events has also been isolated. Their distribution in log(L/Evis) relative to null oscillation Monte Carlo is compatible with νµ to ντ oscillation scenarios within the parameters region allowed by our contained events. The Soudan-2 experiment is currently taking data using its fine-grained iron tracking calor-imeter of total mass 963 tons. This detector images non-relativistic as well as relativistic charged particles produced in atmospheric neutrino reactions. It is operating underground at a depth of 2100 meters-water-equivalent on level 27 of the Soudan Mine State Park in northern Minnesota (northwest of Sudbury). The calorimeter's modular design enabled data-taking to commence in April 1989 when the detector was one quarter of its full size; assembly of the detector was completed during 1993. Data-taking has continued with 85% live time, even though dynamite blasting has been underway nearby for the MINOS cavern excavation since Summer 1999. The data exposure as of this Conference is 5.40 fiducial kiloton-years (kty). Results presented here are based upon a 5.1 kty exposure. The tracking calorimeter operates as a slow-drift (0.6 cm/µs) time projection chamber. Its tracking elements are meter-long plastic drift tubes which are placed into the corrugations of steel sheets. The sheets are stacked to form a tracking lattice of honeycomb geometry. A stack is packaged as a calorimeter module and the detector is assembled building-block fashion using these modules [1]. The calorimeter is surrounded on all sides by a cavern-liner active shield array of two or three layers of proportional tubes [2]. Topologies for …