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Register a new userUnexplained Excess of Electron-Like Events From a 1-GeV Neutrino Beam
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MiniBooNE Collaboration
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The MiniBooNE Collaboration observes unexplained electron-like events in the reconstructed neutrino energy range from 200 to 475 MeV. With $6.46 times 10^{20}$ protons on target, 544 electron-like events are observed in this energy range, compared to an expectation of $415.2 pm 43.4$ events, corresponding to an excess of $128.8 pm 20.4 pm 38.3$ events. The shape of the excess in several kinematic variables is consistent with being due to either $ u_e$ and $bar u_e$ charged-current scattering or to $ u_mu$ neutral-current scattering with a photon in the final state. No significant excess of events is observed in the reconstructed neutrino energy range from 475 to 1250 MeV, where 408 events are observed compared to an expectation of $385.9 pm 35.7$ events.
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A beam line for electrons with energies in the range of 1 to 45 GeV, low contamination of hadrons and muons and high intensity up to 10^6 per accelerator spill at 27 GeV was setup at U70 accelerator in Protvino, Russia. A beam tagging system based on drift chambers with 160 micron resolution was able to measure relative electron beam momentum precisely. The resolution sigma_p p was 0.13% at 45 GeV where multiple scattering is negligible. This test beam setup provided the possibility to study properties of lead tungstate crystals (PbWO_4) for the BTeV experiment at Fermilab.
We study the dependence of neutral current (NC) neutrino-induced $pi^0$/photon production ($ u_mu + A to u_mu +1pi^0 / gamma + X$) on the atomic number of the target nucleus, A, at 4-momentum transfers relevant to the MiniBooNE experiment: $Delta$ resonance mass region. Our conclusion is based on experimental data for photon-nucleus interactions from the A2 collaboration at the Mainz MAMI accelerator. We work in the approximation that decays of $Delta$ resonance unaffected by its production channel, via photon or Z boson. $1pi^0+X$ production scales as A$^{2/3}$, the surface area of the nucleus. Meanwhile the photons created in $Delta$ decays will leave the nucleus, and that cross section will be proportional to the atomic number of the nucleus. Thus the ratio of photon production to $pi^0$ production is proportional to A$^{1/3}$. For carbon $^{12}$C this factor is $approx$2.3. MiniBooNE normalises the rate of photon production to the measured $pi^0$ production rate. The reduced neutral pion production rate would yield at least twice as many photons as previously expected, thus significantly lowering the number of unexplained electron-like events.
The T2K collaboration: reports evidence for electron neutrino appearance at the atmospheric mass splitting, |Delta m_{32}^2|=2.4x10^{-3} eV^2. An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0.6 GeV at the Super-Kamiokande (SK) detector 295 km from the beams origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of 3.3pm0.4(syst.) events is expected. The background-only hypothesis is rejected with a p-value of 0.0009 (3.1sigma), and a fit assuming u_{mu}-> u_e oscillations with sin^2(2theta_{23})=1, delta_{CP}=0 and |Delta m_{32}^2|=2.4x10^{-3} eV^2 yields sin^2(2theta_{13})=0.088^{+0.049}_{-0.039}(stat.+syst.).
The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3$sigma$ when compared to 4.92 $pm$ 0.55 expected background events. In the PMNS mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles $theta_{12}$, $theta_{23}$, $theta_{13}$, a mass difference $Delta m^2_{32}$ and a CP violating phase $delta_{mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|Delta m^2_{32}| = 2.4 times 10^{-3}$ $rm eV^2$, $sin^2 theta_{23} = 0.5$, and $Delta m^2_{32} >0$ ($Delta m^2_{32} <0$), a best-fit value of $sin^2 2 theta_{13}$ = $0.140^{+0.038}_{-0.032}$ ($0.170^{+0.045}_{-0.037}$) is obtained at $delta_{mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $theta_{13}$ from reactor experiments, some values of $delta_{mathrm{CP}}$ are disfavored at the 90% CL.
We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 t-y and an unprecedentedly low background rate of $76pm2$ events/(t y keV) between 1 and 30 keV, the data enables sensitive searches for solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4$sigma$ significance, and a 3D 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by $g_{ae}<3.8 times 10^{-12}$, $g_{ae}g_{an}^{eff}<4.8times 10^{-18}$, and $g_{ae}g_{agamma}<7.7times10^{-22} GeV^{-1}$, and excludes either $g_{ae}=0$ or $g_{ae}g_{agamma}=g_{ae}g_{an}^{eff}=0$. The neutrino magnetic moment signal is similarly favored over background at 3.2$sigma$ and a confidence interval of $mu_{ u} in (1.4,2.9)times10^{-11}mu_B$ (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by $beta$ decays of tritium at 3.2$sigma$ with a trace amount that can neither be confirmed nor excluded with current knowledge of its production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are reduced to 2.0$sigma$ and 0.9$sigma$, respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at ($2.3pm0.2$) keV (68% C.L.) with a 3.0$sigma$ global (4.0$sigma$ local) significance. We also consider the possibility that $^{37}$Ar may be present in the detector and yield a 2.82 keV peak. Contrary to tritium, the $^{37}$Ar concentration can be tightly constrained and is found to be negligible.
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