Showing papers by "Petr Vogel published in 1999"

Angular distribution of neutron inverse beta decay, ν ¯ e + p → e + + n

Abstract: The reaction νe+p→e++n is very important for low-energy (Eν≲60 MeV) antineutrino experiments. In this paper we calculate the positron angular distribution, which at low energies is slightly backward. We show that weak magnetism and recoil corrections have a large effect on the angular distribution, making it isotropic at about 15 MeV and slightly forward at higher energies. We also show that the behavior of the cross section and the angular distribution can be well understood analytically for Eν≲60 MeV by calculating to O(1/M), where M is the nucleon mass. The correct angular distribution is useful for separating νe+p→e++n events from other reactions and detector backgrounds, as well as for possible localization of the source (e.g., a supernova) direction. We comment on how similar corrections appear for the lepton angular distributions in the deuteron breakup reactions νe+d→e++n+n and νe+d→e-+p+p. Finally, in the reaction νe+p→e++n, the angular distribution of the outgoing neutrons is strongly forward peaked, leading to a measurable separation in positron and neutron detection points, also potentially useful for rejecting backgrounds or locating the source direction.

Neutrino Magnetic Moments, Flavor Mixing, and the Super-Kamiokande Solar Data

Abstract: We find that magnetic neutrino-electron scattering is unaffected by oscillations for vacuum mixing of Dirac neutrinos with only diagonal moments and for Majorana neutrinos with two flavors. For Mikheyev-Smirnov-Wolfenstein mixing, these cases are again obtained, though the effective moments can depend on the neutrino energy. Thus, e.g., the magnetic moments measured with νe from a reactor and νe from the Sun could be different. With minimal assumptions, we find a new limit on μν using the 825-d Super-Kamiokande solar neutrino data: |μν|≤1.5×10^-10 μB at 90% CL, comparable to the existing reactor limit.

Can a supernova be located by its neutrinos

Abstract: A future core-collapse supernova in our Galaxy will be detected by several neutrino detectors around the world. The neutrinos escape from the supernova core over several seconds from the time of collapse, unlike the electromagnetic radiation, emitted from the envelope, which is delayed by a time of the order of hours. In addition, the electromagnetic radiation can be obscured by dust in the intervening interstellar space. The question therefore arises whether a supernova can be located by its neutrinos alone. The early warning of a supernova and its location might allow greatly improved astronomical observations. The theme of the present work is a careful and realistic assessment of this question, taking into account the statistical significance of the various neutrino signals. Not surprisingly, neutrino-electron forward scattering leads to a good determination of the supernova direction, even in the presence of the large and nearly isotropic background from other reactions. Even with the most pessimistic background assumptions, SuperKamiokande (SK) and the Sudbury Neutrino Observatory (SNO) can restrict the supernova direction to be within circles of radius 5° and 20°, respectively. Other reactions with more events but weaker angular dependence are much less useful for locating the supernova. Finally, there is the oft-discussed possibility of triangulation, i.e., determination of the supernova direction based on an arrival time delay between different detectors. Given the expected statistics we show that, contrary to previous estimates, this technique does not allow a good determination of the supernova direction.

Probing r-Process Production of Nuclei Beyond Bi209 with Gamma Rays

Abstract: We estimate gamma-ray fluxes due to the decay of nuclei beyond Bi209 from a supernova or a supernova remnant assuming that the r-process occurs in supernovae. We find that a detector with a sensitivity of about 10**(-7) photons/cm**2/s at energies of 40 keV to 3 MeV may detect fluxes due to the decay of Ra226, Th229, Am241, Am243, Cf249, and Cf251 in the newly discovered supernova remnant near Vela. In addition, such a detector may detect fluxes due to the decay of Ac227 and Ra228 produced in a future supernova at a distance of about 1 kpc. As nuclei with mass numbers A > 209 are produced solely by the r-process, such detections are the best proof for a supernova r-process site. Further, they provide the most direct information on yields of progenitor nuclei with A > 209 at r-process freeze-out. Finally, detection of fluxes due to the decay of r-process nuclei over a range of masses from a supernova or a supernova remnant provides the opportunity to compare yields in a single supernova event with the solar r-process abundance pattern.

The Palo Verde reactor neutrino oscillation experiment

Abstract: Our collaboration is operating a segmented scintillation detector, containing 11.3 tons of 0.1% Gd loaded liquid scintillator, at 750, 888 and 889 m distance from three nuclear power reactors. Goal of the experiment is to search for neutrino oscillations in disappearance mode. The anti-neutrino capture on the proton serves as detection reaction. The experiment is expected to reach a sensitivity of Δm 2 > 1.3 · 10 −3 eV 2 and sin 2 2Θ > 0.1. Our range of sensitivity is tuned to test the ν μ ↔ ν e solution of the atmospheric neutrino anomaly as reported by the Kamiokande experiment. Assembly of the detector in an underground lab, with 32 mw.e. effective overburden, is completed. Since June 1998 we are taking data a full reactor power. Detector design and performance are discussed.

Neutrino nucleus scattering

Abstract: The status of the theoretical description of neutrino-nuclear interaction for low and intermediate energies is reviewed and its result compared with the existing data. Particular emphasis is on $^{12}$C, the ingredient of liquid scintillator, and on $^{16}$ the main component of the water \v{C}erenkov detectors. First, I show that the data on the exclusive process populating the ground state of $^{12}$N are well reproduced by the theory. This is also the case for the excitation of the continuum with low energy neutrinos from the muon decay at rest and for the muon capture. However, for not yet understood reasons, the theory overestimates the cross section for higher energy neutrinos from the pion decay in flight, by up to 50%. I also show that the Continuum Random Phase Approximation and the Relativistic Fermi Gas model give very similar full and differential cross sections for the neutrino energies of several hundred MeV, thus checking one method against the other.

Weak reactions on 12C within the Continuum Random Phase Approximation with partial occupancies

Abstract: We extend our previous studies of the neutrino-induced reactions on 12C and muon capture to include partial occupation of nuclear subshells in the framework of the continuum random phase approximation. We find, in contrast to the work by Auerbach et al., that a partial occupation of the p1/2 subshell reduces the inclusive cross sections only slightly. The extended model describes the muon capture rate and the 12C(nu_e,e-)12N cross section very well. The recently updated flux and the improved model bring the calculated 12C(nu_mu,mu^-)12N cross section (~ 17.5 10^{-40} cm^2) and the data (12.4 +/- 0.3(stat.) +/- 1.8(syst.) 10^{-40} cm^2) closer together, but does not remove the discrepancy fully.

What might we learn from a future supernova neutrino signal

Abstract: Neutrinos from a future Galactic supernova will be detected by several large underground detectors, in particular by SuperKamiokande (SK) and the Sudbury Neutrino Observatory (SNO). If, as expected, the $ u_{\mu}$ and $ u_{\tau}$ neutrinos have somewhat higher energy on average that the electron neutrinos, they will dominate the neutral current response. The ways to separate the neutral and charged current signals will be discussed, and the best strategy to measure the possible time delay of the neutral current events will be outlined. Given the expected count rates, one will be able to measure in this way the $ u_{\tau}$ mass down to about 30 eV in SNO and to 50 eV in SuperKamiokande. Another application to be discussed is the supernova localization by the neutrino signal, prior to or independently of the electromagnetic signal. The accuracy with which this can be accomplished using the angular distributions of the reactions will be estimated. With two or more detectors one can, in principle, attempt triangulation based on the arrival time of the neutrinos. It will be argued that for realistic parameters this method will be very difficult and likely leads only to crude localization.