Simultaneous measurement of the muon neutrino charged-current cross section on oxygen and carbon without pions in the final state at T2K

This paper reports the first simultaneous measurement of the double differential muon neutrino charged-current cross section on oxygen and carbon without pions in the final state as a function of the outgoing muon kinematics, made at the ND280 off-axis near detector of the T2K experiment. The ratio of the oxygen and carbon cross sections is also provided to help validate various models’ ability to extrapolate between carbon and oxygen nuclear targets, as is required in T2K oscillation analyses. The data are taken using a neutrino beam with an energy spectrum peaked at 0.6 GeV. The extracted measurement is compared with the prediction from different Monte Carlo neutrino-nucleus interaction event generators, showing particular model separation for very forward-going muons. Overall, of the models tested, the result is best described using local Fermi gas descriptions of the nuclear ground state with RPA suppression.

Figure 1

Schematic showing an exploded view of the ND280 off-axis detector. Each subdetector is labeled using the acronyms given in the text. FGD1 is placed upstream of FGD2. The neutrino beam enters from the left of the figure.

Figure 2

Schematic view of the FGD1 (top) and FGD2 (bottom) structure. Green vertical and horizontal bars represent the X and Y layers respectively, while blue larger vertical modules in the bottom figure represent the water modules. The red shaded rectangular areas indicate the fiducial volume for each subdetector. The neutrino beam enters from the left of the figure.

Figure 3

Schematic view of the FGD2 and of the technique employed to select oxygen-enhanced and carbon-enhanced samples based on the reconstructed muon track’s start position. Yellow stars represent the true interaction position, while orange diamonds represent the reconstructed position. Interactions happening on water, are mainly reconstructed in the X (Y) layers if the muon track is forward- (backward-) going.

Figure 4

Scheme representing the signal sample selection. Samples are additionally divided in FGD1, FGD2X and FGD2Y sub-samples, depending on the starting position of the reconstructed muon track.

Figure 5

Data events per subsample, as enumerated in Table 3, compared with the T2K MC predictions broken down per interaction and target nucleon type. In the legend, OOFV means out of fiducial volume events.

Figure 6

Distribution of events in the sample I for FGD1 (top), FGD2X (middle) and FGD2Y (bottom) as a function of the reconstructed muon momentum (left) and the muon angle (right) depending on the true final state topology and target. The last bin of the reconstructed muon momentum distributions contains all the events with momentum greater than $5\mathrm{GeV}/c$. Histograms are stacked. The MC has been normalized to $5.73\times {10}^{20}$ POT, the equivalent number of POT collected for the data. The legends show also the fraction for each component. In the legend, OOFV means out of fiducial volume events.

Figure 7

Signal selection efficiency as a function of true muon kinematics using the binning adopted for the analysis (Table 4) for oxygen (black solid) and carbon (red dashed) events. For readability purposes, the last momentum bin is cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 8

Distribution of all signal samples events in the reconstructed analysis binning. Only the statistical error is shown on data. The MC prediction before (dashed) and after (solid) the fit (with regularization) are also shown. For display purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 9

Regularized oxygen (full dots) and carbon (empty dots) double differential cross sections per nucleon. Error bars include statistical and systematics uncertainties. Dots for carbon have been manually shifted to higher momentum values for display purposes.

Figure 10

Regularized double differential oxygen cross sections per nucleon. Data results (points with error bars) are compared with NEUT 5.4.1 LFG (brown), GENIE v3—SuSAv2 (green), NuWro SF (magenta) and GiBUU (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 11

Regularized double differential carbon cross sections per nucleon. Data results (points with error bars) are compared with NEUT 5.4.1 LFG (brown), GENIE v3—SuSAv2 (green), NuWro SF (magenta) and GiBUU (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 12

Ratio of the regularized double differential cross sections per nucleon on oxygen and carbon. Data results (points with error bars) are compared with NEUT 5.4.1 LFG (brown), GENIE v3—SuSAv2 (green), NuWro SF (magenta) and GiBUU (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 13

Regularized double differential oxygen cross sections per nucleon. Data results (points with error bars) are compared with NEUT 5.4.0 SF (brown), GENIE v3 LFG (green), NuWro LFG (magenta) and $\mathrm{RMF}(1\mathrm{p}1\mathrm{h})+\mathrm{SuSAv}2(2\mathrm{p}2\mathrm{h})$ (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 14

Regularized double differential carbon cross sections per nucleon. Data results (points with error bars) are compared with NEUT 5.4.0 SF (brown), GENIE v3 LFG (green), NuWro LFG (magenta) and $\mathrm{RMF}(1\mathrm{p}1\mathrm{h})+\mathrm{SuSAv}2(2\mathrm{p}2\mathrm{h})$ (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 15

Ratio of the regularized double differential cross sections per nucleon on oxygen and carbon. Data results (points with error bars) are compared with NEUT 5.4.0 SF (brown), GENIE v3 LFG (green), NuWro LFG (magenta) and $\mathrm{RMF}(1\mathrm{p}1\mathrm{h})+\mathrm{SuSAv}2(2\mathrm{p}2\mathrm{h})$ (light blue) predictions. The values in bracket represent the ${\chi }^2$ as obtained from Eq. (9). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$.

Figure 16

Breakdown of the neutrino interactions contribution to the $\mathrm{CC}0\pi$ channel for the NEUT 5.4.1—LFG predictions for oxygen (left) and carbon (right). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$ and, in the last panels, the cross section values have been multiplied by 200.

Figure 17

Summary of the uncertainties for the oxygen (first six panels) and carbon (last six panels) cross sections as obtained over 1000 toys with regularization. The statistical error is in black for oxygen and red for carbon. Systematic errors are then sequentially added in quadrature starting with all of those addressed via the prior variation propagation method (light blue), followed by proton FSI (violet) and backward migration (green).

Figure 18

Summary of the uncertainties for the oxygen over carbon cross section ratio as obtained over 1000 toys with regularization. The statistical error is in blue. Systematic errors are then sequentially added in quadrature starting with all of those addressed via the prior variation propagation method (light blue), followed backward migration (green). As described in the text, proton FSI errors are considered to be fully correlated between oxygen and carbon and thus canceled out in the ratio.

Figure 19

Relative covariance (left) and correlation (right) matrices for regularized results for oxygen (first 29 bins) and carbon (last 29 bins) cross section bins. The bins are ordered in slices of increasing $\cos {\theta }_{\mu }$ which each contain several bins of muon momentum, as shown in Table 4.

Figure 20

L-curve as for $p_{\theta }^{reg}$ (left) obtained on the data with $p_p^{reg}$ fixed at 4 and for $p_p^{reg}$ (right) obtained with $p_{\theta }^{reg}$ fixed at 7. From left to right, the regularization strength for each point are: 0.2, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 10, 15, 20, 25, 40, 50, 60. The selected best values for the regularization parameters are: $p_p^{reg}=4$ and $p_{\theta }^{reg}=7$ (red points). These values are very similar to those obtained for the mock data samples. The plot is obtained when using the nominal MC as prior. The value of the unregularized total ${\chi }^2$ is 1184.8.

Figure 21

Correlation matrices for regularized (left) and unregularized (right) results for oxygen (first 29 bins) and carbon (last 29 bins) cross section bins. The bins are ordered in slices of increasing $\cos {\theta }_{\mu }$ which each contain several bins of muon momentum, as shown in Table 4.

Figure 22

Regularized (dots) and unregularized (line) results for oxygen (left) and carbon (right). For readability purposes, the last momentum bins are cut at $5\mathrm{GeV}/\mathrm{c}$ and, in the last panels, the cross section values have been multiplied by 200.