We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy range. We measured the rate of pep solar neutrino interactions in Borexino to be (3.1±0.6stat±0.3syst) counts/(day·100 ton) and provided a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day·100 ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. This unprecedented sensitivity was achieved by adopting novel data analysis techniques for the rejection of cosmogenic 11 C, the dominant background in the 1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6±0.3)×10 8 cm −2 s −1 and <7.7×10 8 cm −2 s −1 (95% C.L.), respectively, in agreement with the Standard Solar Model. These results represent the first measurement of the pep neutrino flux and the strongest constraint of the CNO solar neutrino flux to date.PACS numbers: 13.35. Hb, 14.60.St, 26.65.+t, 95.55.Vj, 29.40.Mc Over the past 40 years solar neutrino experiments [1][2][3][4][5] have proven to be sensitive tools to test both astrophysical and elementary particle physics models. Solar neutrino detectors have demonstrated that stars are powered by nuclear fusion reactions. Two distinct processes, the main pp fusion chain and the sub-dominant CNO cycle, are expected to produce solar neutrinos with different energy spectra and fluxes. Until now only fluxes from the pp chain have been measured:7 Be, 8 B, and, indirectly, pp. Experiments involving solar neutrinos and reactor anti-neutrinos [6] have shown that solar neutrinos undergo flavor oscillations.Results from solar neutrino experiments are consistent with the Mikheyev-Smirnov-Wolfenstein Large Mixing Angle (MSW-LMA) model [7], which predicts a transition from vacuum-dominated to matter-enhanced oscillations, resulting in an energy dependent ν e survival probability, P ee . Non-standard neutrino interaction models formulate P ee curves that deviate significantly from MSW-LMA, particularly in the 1-4 MeV transition region, see e.g. [8]. The mono-energetic 1.44 MeV pep neutrinos, which belong to the pp chain and whose StanarXiv:1110.3230v1 [hep-ex] 14 Oct 2011 2 dard Solar Model (SSM) predicted flux has one of the smallest uncertainties (1.2%) due to the solar luminosity constraint [9], are an ideal probe to test these competing hypotheses.The detection of neutrinos resulting from the CNO cycle has important implications in astrophysics, as it would be the first direct evidence of the nuclear process that is believed to fuel massive stars (>1.5M ). Furthermore, its measurement may help to resolve the solar metallicity problem [9,10]. The energy spectrum of neutrinos from the CNO cycle is the sum of three continuous spectra with end point energies of 1.19 ( 13 N), 1.73 ( 15 O) and 1.74 MeV ( 17 F), close to the pep neutrino energy. The total CNO flux is similar to that of the pep neutrinos but its predicted value is strongly dependent on th...

We propose an evolution of the Mu2e experiment, called Mu2e-II, that would leverage advances in detector technology and utilize the increased proton intensity provided by the Fermilab PIP-II upgrade to improve the sensitivity for neutrinoless muon-to-electron conversion by one order of magnitude beyond the Mu2e experiment, providing the deepest probe of charged lepton flavor violation in the foreseeable future. Mu2e-II will use as much of the Mu2e infrastructure as possible, providing, where required, improvements to the Mu2e apparatus to accommodate the increased beam intensity and cope with the accompanying increase in backgrounds.✝ Inquiries should be directed to Mu2e-II-contacts@fnal.gov

Background: Heavy charged particles after nuclear muon capture are an important nuclear physics background to the muon-to-electron conversion experiments Mu2e and COMET, which will search for charged lepton flavor violation at an unprecedented level of sensitivity. Purpose: The AlCap experiment aimed to measure the yield and energy spectra of protons, deuterons, tritons, and α particles emitted after the nuclear capture of muons stopped in Al, Si, and Ti in the low-energy range relevant for the muon-to-electron conversion experiments. Methods: Individual charged particle types were identified in layered silicon detector packages and their initial energy distributions were unfolded from the observed energy spectra. Results: The proton yields per muon capture were determined as Y p (Al) = 26.64(28 stat.)(77 syst.) × 10 −3 and Y p (Ti) = 26.48(35)(80) × 10 −3 in the energy range 3.5-20.0 MeV, and as Y p (Si) = 52.5(6)(18) × 10 −3 in the energy range 4.0-20.0 MeV. Detailed information on yields and energy spectra for all observed nuclei are presented in the paper. Conclusions: The yields in the candidate muon stopping targets, Al and Ti, are approximately half of that in Si, which was used in the past to estimate this background. The reduced background allows for less shielding and a better energy resolution in these experiments. It is anticipated that the comprehensive information presented in this paper will stimulate modern theoretical calculations of the rare process of muon capture with charged particle emission and inform the design of future muon-to-electron conversion experiments.

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