Measurement of the quasielastic axial vector mass in neutrino interactions on oxygen

The weak nucleon axial-vector form factor for quasielastic interactions is determined using neutrino interaction data from the K2K Scintillating Fiber detector in the neutrino beam at KEK. More than 12 000 events are analyzed, of which half are charged-current quasielastic interactions ${\nu }_{\mu }n\to {\mu }^{-}p$ occurring primarily in oxygen nuclei. We use a relativistic Fermi gas model for oxygen and assume the form factor is approximately a dipole with one parameter, the axial-vector mass $M_A$, and fit to the shape of the distribution of the square of the momentum transfer from the nucleon to the nucleus. Our best fit result for $M_A=1.20\pm 0.12\mathrm{GeV}$. Furthermore, this analysis includes updated vector form factors from recent electron scattering experiments and a discussion of the effects of the nucleon momentum on the shape of the fitted distributions.

Figure 1

The arrangement of the near neutrino detectors at KEK. The beam comes in from the right and continues to Super-Kamiokande, 250 km away to the left. The SciFi detector is on a stand in the middle.

Figure 2

A schematic diagram of the SciFi detector.

Figure 3

The distribution of $\Delta \theta$, the difference between the predicted second track angle and the observed angle for K2K-I data. The histogram shows the Monte Carlo prediction, while the hatched region shows the QE fraction. In this analysis, $\Delta \theta =25°$ is used to separate a QE enhanced sample. The inset diagram shows the definition of $\Delta \theta$.

Figure 4

Muon momentum distribution for all K2K-1 one-track and two-track events. The QE fraction, estimated from the MC simulation, is shown as the shaded region. The errors on the data are statistical only.

Figure 5

Muon angle distribution for all K2K-I one-track and two-track events. The QE fraction, estimated from the MC simulation, is shown as the shaded region. Only statistical errors are shown.

Figure 6

Distribution of $\cos ({\theta }_{\mu })$ for data and MC, showing the data at small angle, which is also the low $Q^2$ region. Left to right are the one-track, two-track QE enhanced, and two-track non-QE enhanced samples. The dashed line is with charged-current coherent pion, the solid line without. The shaded region is the QE fraction, estimated from the MC simulation. The distributions are normalized to the total number of events when all three samples are combined.

Figure 7

The data and the best fit $Q_{\mathrm{rec}}^2$ distributions for K2K-1 data (left) and K2K-IIa data (right) for the one-track, two-track QE enhanced, and two-track non-QE enhanced samples. The shaded region shows the QE fraction of each sample, estimated from the MC. The contribution from each energy region is summed for each plot. The lowest two data points in each plot are not included in the fit, due to the large uncertainty in the effects of the nucleus.

Figure 8

Fit values obtained for different values of the low $Q^2$ cut. Only statistical errors are shown. The horizontal line is the combined best fit. The vertical line is the systematic errors.

Figure 9

Fit values obtained separately from the shape of the $Q^2$ distribution for each neutrino energy. The horizontal line indicates the combined best fit value.

Figure 10

Effect of the nucleon momentum on the shape of the $Q^2$ distribution. The comparison is between the free nucleon and a uniform Fermi gas model. The effect of Pauli blocking is seen at low $Q^2$, the tail of the momentum distribution at high $Q^2$, an overall suppression, and a slight change in the slope in the middle region. The calculated quasielastic cross sections for 1.0 GeV neutrinos on oxygen are on the left, and the ratio $(\mathrm{\text{Fermi}}\mathrm{\text{gas}})/(\mathrm{\text{free}}\mathrm{\text{neutron}})$.