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Register a new userPrecise Measurement of the Neutrino Mixing Parameter theta_{23} from Muon Neutrino Disappearance in an Off-axis Beam
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New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter theta_{23}. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to 6.57 x 10^{20} protons on target, T2K has fit the energy-dependent nu_mu oscillation probability to determine oscillation parameters. Marginalizing over the values of other oscillation parameters yields sin^2 (theta_{23}) = 0.514 +0.055/-0.056 (0.511 +- 0.055), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is Delta m^2_{32} = (2.51 +- 0.10) x 10^{-3} eV^2/c^4 (inverted hierarchy: Delta m^2_{13} = (2.48 +- 0.10) x 10^{-3} eV^2/c^4). Adding a model of multinucleon interactions that affect neutrino energy reconstruction is found to produce only small biases in neutrino oscillation parameter extraction at current levels of statistical uncertainty.
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The T2K collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to $3.01 times 10^{20}$ protons on target. In the absence of neutrino oscillations, $205 pm 17$ (syst.) events are expected to be detected and only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum assuming three neutrino flavors, normal mass hierarchy and $theta_{23}leq pi/4$ yields a best-fit mixing angle $sin^2(2theta_{23})=1.000$ and mass splitting $|Delta m^2_{32}| =2.44 times 10^{-3}$ eV$^2$/c$^4$. If $theta_{23}geq pi/4$ is assumed, the best-fit mixing angle changes to $sin^2(2theta_{23})=0.999$ and the mass splitting remains unchanged.
We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43 10**20 protons on target, we observe 31 fully-contained single muon-like ring events in Super-Kamiokande, compared with an expectation of 104 +- 14 (syst) events without neutrino oscillations. The best-fit point for two-flavor nu_mu -> nu_tau oscillations is sin**2(2 theta_23) = 0.98 and |Delta m**2_32| = 2.65 10**-3 eV**2. The boundary of the 90 % confidence region includes the points (sin**2(2 theta_23),|Delta m**2_32|) = (1.0, 3.1 10**-3 eV**2), (0.84, 2.65 10**-3 eV**2) and (1.0, 2.2 10**-3 eV**2).
This Letter reports new results on muon neutrino disappearance from NOvA, using a 14 kton detector equivalent exposure of $6.05times10^{20}$ protons-on-target from the NuMI beam at the Fermi National Accelerator Laboratory. The measurement probes the muon-tau symmetry hypothesis that requires maximal mixing ($theta_{23} = pi/4$). Assuming the normal mass hierarchy, we find $Delta m^2 = (2.67 pm 0.11)times 10^{-3}$ eV$^2$ and $sin^2 theta_{23}$ at the two statistically degenerate values $0.404^{+0.030}_{-0.022}$ and $0.624^{+0.022}_{-0.030}$, both at the 68% confidence level. Our data disfavor the maximal mixing scenario with 2.6 $sigma$ significance.
The T2K experiment observes indications of $ u_murightarrow u_e$ appearance in data accumulated with $1.43times10^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $|Delta m_{23}^2|=2.4times10^{-3}$ eV$^2$, $sin^2 2theta_{23}=1$ and $sin^2 2theta_{13}=0$, the expected number of such events is 1.5$pm$0.3(syst.). Under this hypothesis, the probability to observe six or more candidate events is 7$times10^{-3}$, equivalent to 2.5$sigma$ significance. At 90% C.L., the data are consistent with 0.03(0.04)$<sin^2 2theta_{13}<$ 0.28(0.34) for $delta_{rm CP}=0$ and a normal (inverted) hierarchy.
In study of muon neutrino disappearance at 810 km, the NOvA experiment finds flavor mixing of the atmospheric sector to deviate from maximal ($sin^2theta_{23} = 0.5$) by 2.6 $sigma$. The result is in tension with the 295-km baseline measurements of T2K which are consistent with maximal mixing. We propose that $theta_{23}$ is in fact maximal, and that the disagreement is harbinger of environmentally-induced decoherence. The departure from maximal mixing can be accounted for by an energy-independent decoherence of strength $Gamma = (2.3 pm 1.1) times 10^{-23}$ GeV.
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