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Register a new userNew results on atmospheric neutrinos from Soudan 2
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W. Anthony Mann
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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 nu_mu/nu_e ratio-of-ratios and find R = 0.68 pm 0.11 pm 0.06. Assuming this anomalously low R-value is the result of nu_mu flavor disappearance via nu_mu to nu_tau oscillation, we select samples of charged current events which offer good resolution, event-by-event, for L/Enu reconstruction. Oscillation-weighted Monte Carlo events are fitted to these data events using a chisq function summed over bins of log(L/E_nu). The region allowed in the (sin^2 2theta, Delta m^2) plane at 90% CL is obtained using the Feldman-Cousins procedure: 0.46 < sin^2 2theta < 1.0 and 2.2x10^-4 < Delta m^2 < 2.2x10^-2 ev^2. A small but relatively energetic sample of partially contained nu_mu events has also been isolated. Their distribution in log(L/E_vis) relative to null oscillation Monte Carlo is compatible with nu_mu to nu_tau oscillation scenarios within the parameter region allowed by our contained events.
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An updated measurement of the atmospheric nu_mu/nu_e ratio-of-ratios, 0.68+-0.11+-0.06, has been obtained using a 4.6-kty exposure of the Soudan-2 iron tracking calorimeter. The L/E distributions have been analyzed for effects of nu_mu -> nu_x oscillations, and an allowed region in the Delta m^2 vs. sin^2 2 theta plane has been determined.
A new measurement of the atmospheric numu/nue ratio-of-ratios, 0.61 +- 0.15 +- 0.05, has been obtained using a 3.2-kty exposure of the Soudan-2 underground detector. This measurement, based upon neutrino reactions in an iron tracking calorimeter of honeycomb-lattice geometry, is in agreement with the anomalously low value reported by the underground water detectors.
The atmospheric neutrino flavour ratio measured using a 1.52 kton-year exposure of Soudan 2 is found to be 0.72 +- 0.19 +0.05 -0.07 relative to the expected value from a Monte Carlo calculation. The possible background of interactions of neutrons and photons produced in muon interactions in the rock surrounding the detector has been investigated and is shown not to produce low values of the ratio.
New results from Super-Kamiokande, K2K and SNO not only have spurred on the interest in neutrino oscillation physics, but also have started to shift the interest from discovery to precision measurements. Future projects focusing on atmospheric neutrinos are reviewed in this context. Important contributions could be made in the precision determination of the oscillation parameters, in the observation of matter effects and in the determination of the neutrino mass hierarchy. Unfortunately, the probability that the projects discussed in this review will be running in the next ten years is rather small. The only project with a shorter time scale has not been funded.
Recent results from a 282 kiloton-year exposure of the Super-Kamiokande detector to atmospheric neutrinos are presented. The data when fit both by themselves and in conjunction with constraints from the T2K and reactor neutrino experiments show a weak, though insignificant, preference for the normal mass hierarchy at the level of ~1 sigma. Searches for evidence of oscillations into a sterile neutrino have resulted in limits on the parameters governing their mixing, |U_mu4}|^2 <0.041 and |U_tau4|^2 < 0.18 at 90% C.L. A similar search for an indication of Lorentz-invariance violating oscillations has yielded limits three to seven orders of magnitude more stringent than existing measurements. Additionally, analyses searching for an excess of neutrinos in the atmospheric data produced from the annihilation of dark matter particles in the galaxy and sun have placed tight limits on the cross sections governing their annihilation and scattering.
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