Search

A search for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}$ oscillations has been conducted at the Los Alamos Meson Physics Facility using ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ from ${\ensuremath{\pi}}^{+}$ decay in flight. An excess in the number of beam-related events from the ${\ensuremath{\nu}}_{e}\stackrel{\ensuremath{\rightarrow}}{C}{e}^{\ensuremath{-}}X$ inclusive reaction is observed. The excess is too large to be explained by normal ${\ensuremath{\nu}}_{e}$ contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of $(2.6\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ is consistent with the previously reported ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillation evidence from LSND.

A search for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}$ oscillations has been conducted with the LSND apparatus using ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ from ${\ensuremath{\pi}}^{+}$ decay in flight. Two analyses observe a total of 40 beam-on high-energy (60--200 MeV) electron events consistent with the ${\ensuremath{\nu}}_{e}\mathrm{C}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}X$ inclusive reaction. This number is significantly above the $21.9\ifmmode\pm\else\textpm\fi{}2.1$ events expected from the ${\ensuremath{\nu}}_{e}$ contamination in the beam and the beam-off background. If interpreted as an oscillation signal, the observed oscillation probability of $(2.6\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ is consistent with the previously reported ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillation evidence from LSND.

Charged current scattering of ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ on ${}^{12}\mathrm{C}$ has been studied using a ${\ensuremath{\pi}}^{+}$ decay-in-flight ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ beam at the Los Alamos Meson Physics Facility. A sample of 56.8 $\ifmmode\pm\else\textpm\fi{}9.6$ events satisfying criteria for the exclusive reaction ${}^{12}{\mathrm{C}(\mathrm{\ensuremath{\nu}}}_{\mathrm{\ensuremath{\mu}}}{,\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}{)}^{12}{\mathrm{N}}_{\mathrm{g}.\mathrm{s}.}$ was obtained using a large liquid scintillator neutrino detector. The observed flux-averaged cross section $(6.6\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}41}{\mathrm{cm}}^{2}$ agrees well with reliable theoretical expectations. A measurement was also obtained for the inclusive cross section to all accessible ${}^{12}\mathrm{N}$ states ${}^{12}\mathrm{C}({\ensuremath{\nu}}_{\ensuremath{\mu}},{\ensuremath{\mu}}^{\ensuremath{-}})X.$ This flux-averaged cross section is $(11.2\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\pm\else\textpm\fi{}1.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}40}{\mathrm{cm}}^{2},$ which is approximately half of that given by a recent continuum random-phase approximation calculation.

Charged current reactions of nu_e on 12C have been studied using a mu decay-at-rest nu_e beam from the Los Alamos Meson Physics Facility. More than 500 events from the exclusive reaction 12C + nu_e -> e- + 12N g.s. were measured in a large Liquid Scintillator Neutrino Detector (LSND). The observed energy dependence of the cross section and the angular distribution of the outgoing electron agree well with theoretical expectations. Measurements are also presented for inclusive transitions to 12N excited states, 12C + nu_e -> e- + 12N and compared with theoretical expectations. Results are consistent with a recent Continuum Random Phase Approximation (CRPA) calculation.

A search for nu_bar_mu to nu_bar_e oscillations has been conducted at the Los Alamos Meson Physics Facility using nu_bar_mu from mu+ decay at rest. The nu_bar_e are detected via the reaction (nu_bar_e,p) -> (e+,n), correlated with the 2.2 MeV gamma from (n,p) -> (d,gamma). The use of tight cuts to identify e+ events with correlated gamma rays yields 22 events with e+ energy between 36 and 60 MeV and only 4.6 (+/- 0.6) background events. The probability that this excess is due entirely to a statistical fluctuation is 4.1E-08. A chi^2 fit to the entire e+ sample results in a total excess of 51.8 (+18.7) (-16.9) (+/- 8.0) events with e+ energy between 20 and 60 MeV. If attributed to nu_bar_mu -> nu_bar_e oscillations, this corresponds to an oscillation probability (averaged over the experimental energy and spatial acceptance) of 0.0031 (+0.0011) (-0.0010) (+/- 0.0005)., 57 pages, 34 figures, revtex, additional information available at http://nu1.lampf.lanl.gov/~lsnd/

A search for ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillations has been conducted at the Los Alamos Meson Physics Facility by using ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ from ${\ensuremath{\mu}}^{+}$ decay at rest. The ${\overline{\ensuremath{\nu}}}_{e}$ are detected via the reaction ${\overline{\ensuremath{\nu}}}_{e}p\ensuremath{\rightarrow}{e}^{+}n$, correlated with a $\ensuremath{\gamma}$ from $\mathrm{np}\ensuremath{\rightarrow}d\ensuremath{\gamma}$ ( $2.2\mathrm{MeV}$). The use of tight cuts to identify ${e}^{+}$ events with correlated $\ensuremath{\gamma}$ rays yields 22 events with ${e}^{+}$ energy between 36 and $60\mathrm{MeV}$ and only $4.6\ifmmode\pm\else\textpm\fi{}0.6$ background events. A fit to the ${e}^{+}$ events between 20 and $60\mathrm{MeV}$ yields a total excess of ${51.0}_{\ensuremath{-}19.5}^{+20.2}\ifmmode\pm\else\textpm\fi{}8.0$ events. If attributed to ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillations, this corresponds to an oscillation probability of ( $0.31\ifmmode\pm\else\textpm\fi{}0.12\ifmmode\pm\else\textpm\fi{}0.05$)%.

A search for ${\overline{\ensuremath{\nu}}}_{e}$'s in excess of the number expected from conventional sources has been made using the Liquid Scintillator Neutrino Detector, located 30 m behind the Los Alamos Meson Physics Facility beam stop. The ${\overline{\ensuremath{\nu}}}_{e}$ are detected via ${\overline{\ensuremath{\nu}}}_{e}p\ensuremath{\rightarrow}{e}^{+}n$ with ${e}^{+}$ energy between 36 and $60\mathrm{MeV}$, followed by a $\ensuremath{\gamma}$ ray from $\mathrm{np}\ensuremath{\rightarrow}d\ensuremath{\gamma}$ ( $2.2\mathrm{MeV}$). Using strict cuts to identify $\ensuremath{\gamma}$ rays correlated with ${e}^{+}$ yields 9 events with only $2.1\ifmmode\pm\else\textpm\fi{}0.3$ background expected. A likelihood fit to the entire ${e}^{+}$ sample results in a total excess of ${16.4}_{\ensuremath{-}8.9}^{+9.7}\ifmmode\pm\else\textpm\fi{}3.3$ events. If attributed to ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillations, this corresponds to an oscillation probability of ( ${0.34}_{\ensuremath{-}0.18}^{+0.20}\ifmmode\pm\else\textpm\fi{}0.07$)%.

The proton Compton effect has been studied in the region between the threshold for pion photoproduction and the \ensuremath{\Delta}(1232). The measurements were performed using bremmstrahlung from the high duty-factor electron beam available at the Saskatchewan Accelerator Laboratory. Elastically scattered photons were detected with an energy resolution of approximately 1.5% using a large NaI total absorption scintillation detector. Differential cross sections were measured for photon energies in the range 136 MeV\ensuremath{\le}${\mathit{E}}_{\ensuremath{\gamma}}$\ensuremath{\le}289 MeV and for angles in the range 25\ifmmode^\circ\else\textdegree\fi{}${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$135\ifmmode^\circ\else\textdegree\fi{}. The angular distributions and the excitation functions derived from these data are in agreement with recent theoretical analyses. The results were interpreted within a formalism based in part on dispersion relations to obtain model-dependent estimates of the electric and magnetic polarizabilities, \ensuremath{\alpha}\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\beta}\ifmmode\bar\else\textasciimacron\fi{}. We find, subject to the dispersion sum rule constraint \ensuremath{\alpha}\ifmmode\bar\else\textasciimacron\fi{}+\ensuremath{\beta}\ifmmode\bar\else\textasciimacron\fi{}=(14.2\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$, that \ensuremath{\alpha}\ifmmode\bar\else\textasciimacron\fi{}=(9.8\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$ and \ensuremath{\beta}\ifmmode\bar\else\textasciimacron\fi{}=(4.4\ensuremath{\mp}0.4\ensuremath{\mp}1.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$, which are consistent with the best previous measurements.

We report the first exclusive ({gamma},{pi}{sup {minus}}{ital p}) measurements on a complex nucleus. The {sup 16}O({gamma},{pi}{sup {minus}}{ital p}) reaction was measured at pion laboratory angles of 64{degree} and 120{degree}. Coincident protons were detected over the quasifree angular correlation range using a vertical array of seven plastic scintillator detectors spanning {plus minus}33{degree} about the scattering plane. The cross sections are compared to factorized distorted-wave impulse approximation calculations; these provide a good description of the backward angle data, but are in serious disagreement with the forward angle data.

Elastic photon scattering cross sections have been measured on {sup 4}He in the region of the giant dipole resonance (GDR) using the tagged-photon technique. These data place stringent constraints on the total photoabsorption {sigma}{sub {gamma}} at the peak of the GDR, which to very good approximation is the sum of the ({gamma},{ital p}) and ({gamma},{ital n}) cross sections. We find the peak cross section to be {sigma}{sub 0}=2.86{plus minus}0.12 mb. This result is inconsistent with the sum of the most recent measurements of the ({gamma},{ital p}) and ({gamma},{ital n}) cross sections. Thus any conclusion about charge symmetry based on the ratios of these cross sections is suspect.

Elastic photon scattering cross sections on {sup 16}O have been measured in the energy range 27{endash}108 MeV. These data are inconsistent with a conventional interpretation in which the electric and magnetic polarizabilities of the bound nucleon are unchanged from the free values and the meson-exchange seagull amplitude is taken in the zero-energy limit. Agreement with the data can be achieved by invoking either strongly modified polarizabilities or a substantial energy dependence to the meson-exchange seagull amplitude. It is argued that these seemingly different explanations are experimentally indistinguishable and probably physically equivalent. {copyright} {ital 1996 The American Physical Society.}

A search for neutrino oscillations of the type nu_bar_mu to nu_bar_e has been conducted at the Los Alamos Meson Physics Facility using nu_bar_mu from muon decay at rest. Evidence for this transition has been reported previously. This paper discusses in detail the experimental setup, detector operation and neutrino source, including aspects relevant to oscillation searches in the muon decay-at-rest and pion decay in flight channels., 27 pages, 26 figures, revtex and psfig, additional information available at http://nu1.lampf.lanl.gov/~lsnd/

We have measured the Compton-scattering cross section on hydrogen at 60\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{} using monochromatic tagged photons spanning the energy between 32 and 72 MeV. These data, when analyzed with a low-energy expansion of the scattering cross-section formula, provides a determination of values for the electric and magnetic polarizabilities of the proton. We find, respectively, \ensuremath{\alpha}\ifmmode\bar\else\textasciimacron\fi{}=(10.9\ifmmode\pm\else\textpm\fi{}2.2\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$ and \ensuremath{\beta}\ifmmode\bar\else\textasciimacron\fi{}=(3.3\ensuremath{\mp}2.2\ensuremath{\mp}1.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$, assuming the model-independent constraint \ensuremath{\alpha}\ifmmode\bar\else\textasciimacron\fi{}+\ensuremath{\beta}\ifmmode\bar\else\textasciimacron\fi{}=14.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{fm}}^{3}$.

Elastic photon scattering cross sections have been measured for 209Bi in the region of the GDR. The enhancement to the TRK sum rule is extracted and shown to be similar to that of neighboring nuclei. The mesonic contribution to the nucleon orbital magnetic moment, as determined from the GDR contribution to this enhancement, is found to be smaller than previously reported.