P. Kitching

We present measurements of ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ disappearance in K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment. One-hundred and twelve beam-originated neutrino events are observed in the fiducial volume of Super-Kamiokande with an expectation of ${158.1}_{\ensuremath{-}8.6}^{+9.2}$ events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muonlike events with reconstructed energies. The probability that the observations are explained by the expectation for no neutrino oscillation is 0.0015% ($4.3\ensuremath{\sigma}$). In a two-flavor oscillation scenario, the allowed $\ensuremath{\Delta}{m}^{2}$ region at ${sin}^{2}2\ensuremath{\theta}=1$ is between 1.9 and $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{eV}}^{2}$ at the 90% C.L. with a best-fit value of $2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{eV}}^{2}$.

We present results for ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ oscillation in the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment. K2K uses an accelerator-produced ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokande detector. We observed the energy-dependent disappearance of ${\ensuremath{\nu}}_{\ensuremath{\mu}}$, which we presume have oscillated to ${\ensuremath{\nu}}_{\ensuremath{\tau}}$. The probability that we would observe these results if there is no neutrino oscillation is 0.0050% ($4.0\ensuremath{\sigma}$).

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ in the pion momentum region $140<{P}_{\ensuremath{\pi}}<199\text{ }\text{ }\mathrm{MeV}/c$ in an exposure of $1.71\ifmmode\times\else\texttimes\fi{}{10}^{12}$ stopped kaons with an estimated total background of $0.93\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat}{)}_{\ensuremath{-}0.24}^{+0.32}(\mathrm{syst})$ events. This brings the total number of observed ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.73}_{\ensuremath{-}1.05}^{+1.15})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. An interpretation of the results for alternative models of the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}+nothing$ is also presented.

The beam-spin asymmetry in hard electroproduction of photons has been measured. The data have been accumulated by the HERMES experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen-gas target. The asymmetry in the azimuthal distribution of the produced photons in the angle $\ensuremath{\varphi}$ relative to the lepton scattering plane was determined with respect to the helicity state of the incoming positron beam. The beam-spin analyzing power in the $\mathrm{sin}\ensuremath{\varphi}$ moment was measured to be $\ensuremath{-}0.23\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.03(\mathrm{syst})$ in the missing-mass range below 1.7 GeV. The observed asymmetry is attributed to the interference of the Bethe-Heitler and deeply virtual Compton scattering processes.

Results are reported from the HERMES experiment at HERA on a measurement of the neutron spin structure function g1n(x, Q2) in deep inelastic scattering using 27.5 GeV longitudinally polarized positrons incident on a polarized 3He internal gas target. The data cover the kinematic range 0.023 < × < 0.6 and 1 (GeV/c)2 < Q2 < 15 (GeV/c)2. The integral ∫0.60.023 g1n(x) dx evaluated at a fixed Q2 of 2.5 (GeV/c)2 is −0.034 ± 0.013(stat.)±0.005(syst.). Assuming Regge behavior at low x, the first moment Гn1 = ∫10 g1n(x)dx is −0.037 ± 0.013(stat.)±0.005(syst.)±0.006(extrapol.).