First measurement of the muon neutrino charged current single pion production cross section on water with the T2K near detector

The T2K off-axis near detector, ND280, is used to make the first differential cross section measurements of muon neutrino charged current single positive pion production on a water target at energies $\sim 0.8\mathrm{GeV}$. The differential measurements are presented as a function of the muon and pion kinematics, in the restricted phase space defined by $p_{{\pi }^{+}}>200\mathrm{MeV}/c$, $p_{\mu }>200\mathrm{MeV}/c$, $\cos ({\theta }_{{\pi }^{+}})>0.3$ and $\cos ({\theta }_{\mu })>0.3$. The total flux integrated ${\nu }_{\mu }$ charged current single positive pion production cross section on water in the restricted phase space is measured to be $⟨\sigma {⟩}_{\varphi }=4.25\pm 0.48(\mathrm{stat})\pm 1.56(\mathrm{syst})\times {10}^{-40}{\mathrm{cm}}^2/\text{nucleon}$. The total cross section is consistent with the NEUT prediction ($5.03\times {10}^{-40}{\mathrm{cm}}^2/\text{nucleon}$) and $2\sigma$ lower than the GENIE prediction ($7.68\times {10}^{-40}{\mathrm{cm}}^2/\text{nucleon}$). The differential cross sections are in good agreement with the NEUT generator. The GENIE simulation reproduces well the shapes of the distributions, but overestimates the overall cross section normalization.

Figure 1

Schematic view of FGD2 and its fiducial volume (FV) delimited by the red line. The first upstream scintillator layer is not included in the FV.

Figure 2

Reconstructed pion kinematics (top), muon kinematics (middle), $\cos ({\theta }_{(\mu ,{\pi }^{+})})$ (bottom left) and neutrino energy (bottom right) distributions of the events in the ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ water-enhanced sample. The NEUT Monte Carlo prediction is separated into the ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ interactions in water, ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ interactions in scintillator, ${\nu }_{\mu }$ CC-non-$1{\pi }^{+}$ interactions in water, ${\nu }_{\mu }$ CC-non-$1{\pi }^{+}$ interactions in scintillator, non-${\nu }_{\mu }$ CC interactions, and interactions outside of the FGD2 FV. The last bin in the $p_{{\pi }^{+}}$, $p_{\mu }$ and $E_{\nu }^{\text{rec}}$ distributions contains all the overflow events.

Figure 3

Reconstructed pion momentum distributions of the events in the two external samples: $\mathrm{CC}1{\pi }^{+}$ scintillator (left) and $\mathrm{CC}1{\pi }^{+}\mathrm{n}\pi$ water-enhanced (right). The last bin contains all the overflow events.

Figure 4

Unfolded ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ differential cross sections as a function of pion kinematics (top), muon kinematics (center), $\cos ({\theta }_{(\mu ,{\pi }^{+})})$ (bottom left) and $E_{\nu }^{\text{rec}}$ (bottom right) in the reduced phase space of $p_{{\pi }^{+}}>200\mathrm{MeV}/c$, $p_{\mu }>200\mathrm{MeV}/c$, $\cos ({\theta }_{{\pi }^{+}})>0.3$ and $\cos ({\theta }_{\mu })>0.3$. For the $E_{\nu }^{\text{rec}}$, the $\sigma (E)$ is presented as a model dependent result. The inner (outer) error bars show the statistical (total) uncertainty on the data. The dashed (solid) line shows the NEUT, version 5.1.4.2, (GENIE, version 2.6.4) prediction.

Figure 5

Total ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ cross section on water in the reduced phase space of $p_{{\pi }^{+}}>200\mathrm{MeV}/c$, $p_{\mu }>200\mathrm{MeV}/c$, $\cos ({\theta }_{{\pi }^{+}})>0.3$ and $\cos ({\theta }_{\mu })>0.3$. The T2K data point is placed at the ${\nu }_{\mu }$ flux mean energy. The vertical error represents the total uncertainty, and the horizontal bar represents 68% of the flux each side of the mean. The T2K flux distribution is shown in gray. The NEUT (version 5.1.4.2) and GENIE (version 2.6.4) predictions are the total ${\nu }_{\mu }$ $\mathrm{CC}1{\pi }^{+}$ predictions as a function of neutrino energy. The NEUT and GENIE averages are the flux-averaged predictions.