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Register a new userThe Search for $ u_mu to u_e$ Oscillations at MiniBooNE
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H.A. Tanaka
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MiniBooNE (Mini Booster Neutrino Experiment) searches for the $ u_muto u_e$ oscillations with $Delta m^2 sim 1 eV^2/c^4$ indicated by the LSND experiment. The LSND evidence, when taken with the solar and atmospheric neutrino oscillations, suggests new physics beyond the Standard Model. However, this evidence has not been confirmed by other experiments. MiniBooNE has completed its first $ u_muto u_e$ oscillation search using a sample of $sim 1$ GeV neutrino events obtained with $5.58times 10^{20}$ protons delivered to the Booster Neutrino Beamline. The analysis finds no significant excess of $ u_e$ events in the analysis region of 475-3000 MeV.
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The MiniBooNE experiment at Fermilab reports results from a search for $bar u_mu rightarrow bar u_e$ oscillations, using a data sample corresponding to $5.66 times 10^{20}$ protons on target. An excess of $20.9 pm 14.0$ events is observed in the energy range $475<E_ u^{QE}<1250$ MeV, which, when constrained by the observed $bar u_mu$ events, has a probability for consistency with the background-only hypothesis of 0.5%. On the other hand, fitting for $bar{ u}_{mu}rightarrowbar{ u}_e$ oscillations, the best-fit point has a $chi^2$-probability of 8.7%. The data are consistent with $bar u_mu rightarrow bar u_e$ oscillations in the 0.1 to 1.0 eV$^2$ $Delta m^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.
The MiniBooNE experiment at Fermilab reports results from an analysis of $bar u_e$ appearance data from $11.27 times 10^{20}$ protons on target in antineutrino mode, an increase of approximately a factor of two over the previously reported results. An event excess of $78.4 pm 28.5$ events ($2.8 sigma$) is observed in the energy range $200<E_ u^{QE}<1250$ MeV. If interpreted in a two-neutrino oscillation model, $bar{ u}_{mu}rightarrowbar{ u}_e$, the best oscillation fit to the excess has a probability of 66% while the background-only fit has a $chi^2$-probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the $0.01 < Delta m^2 < 1.0$ eV$^2$ range and have some overlap with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector (LSND). All of the major backgrounds are constrained by in-situ event measurements so non-oscillation explanations would need to invoke new anomalous background processes. The neutrino mode running also shows an excess at low energy of $162.0 pm 47.8$ events ($3.4 sigma$) but the energy distribution of the excess is marginally compatible with a simple two neutrino oscillation formalism. Expanded models with several sterile neutrinos can reduce the incompatibility by allowing for CP violating effects between neutrino and antineutrino oscillations.
The MiniBooNE experiment at Fermilab reports results from an analysis of the combined $ u_e$ and $bar u_e$ appearance data from $6.46 times 10^{20}$ protons on target in neutrino mode and $11.27 times 10^{20}$ protons on target in antineutrino mode. A total excess of $240.3 pm 34.5 pm 52.6$ events ($3.8 sigma$) is observed from combining the two data sets in the energy range $200<E_ u^{QE}<1250$ MeV. In a combined fit for CP-conserving $ u_mu rightarrow u_e$ and $bar{ u}_{mu}rightarrowbar{ u}_e$ oscillations via a two-neutrino model, the background-only fit has a $chi^2$-probability of 0.03% relative to the best oscillation fit. The data are consistent with neutrino oscillations in the $0.01 < Delta m^2 < 1.0$ eV$^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector (LSND).
A first result of the search for umu $rightarrow$ ue oscillations in the OPERA experiment, located at the Gran Sasso Underground Laboratory, is presented. The experiment looked for the appearance of ue in the CNGS neutrino beam using the data collected in 2008 and 2009. Data are compatible with the non-oscillation hypothesis in the three-flavour mixing model. A further analysis of the same data constrains the non-standard oscillation parameters $theta_{new}$ and $Delta m^2_{new}$ suggested by the LSND and MiniBooNE experiments. For large $Delta m^{2}_{new}$ values ($>$0.1 eV$^{2}$), the OPERA 90% C.L. upper limit on sin$^{2}(2theta_{new})$ based on a Bayesian statistical method reaches the value $7.2 times 10^{-3}$.
We performed an improved search for $ u_mu to u_e$ oscillation with the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment, using the full data sample of $9.2 times 10^{19}$xspace protons on target. No evidence for a $ u_e$ appearance signal was found, and we set bounds on the $ u_mu to u_e$ oscillation parameters. At $Delta m^2$ = $2.8 times 10^{-3} mathrm{eV}^2$, the best fit value of the K2K $ u_mu$ disappearance analysis, we set an upper limit of $sin^2 2theta_{mu e}$ $<$ 0.13 at 90% confidence level.
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