Search for Coherent Charged Pion Production in Neutrino-Carbon Interactions

We report the result from a search for charged-current coherent pion production induced by muon neutrinos with a mean energy of 1.3 GeV. The data are collected with a fully active scintillator detector in the K2K long-baseline neutrino oscillation experiment. No evidence for coherent pion production is observed, and an upper limit of $0.60\times {10}^{-2}$ is set on the cross section ratio of coherent pion production to the total charged-current interaction at 90% confidence level. This is the first experimental limit for coherent charged pion production in the energy region of a few GeV.

Figure 1

Distribution of $E_{\mathrm{vtx}}$ for (a) a non-QE-pion sample and (b) a QE sample. Black circles: observed data; histograms: MC expectation with breakdown of interaction modes. The statistical ${\chi }^2/\mathrm{DOF}$ in the selected region of (a), indicated by a vertical line, is $30.1/7$ ($9.8/7$) with (without) CC coherent pion production.

Figure 2

The $q_{\mathrm{rec}}^2$ distributions for the (a) one-track, (b) QE, (c) non-QE-proton, and (d) non-QE-pion samples. The statistical ${\chi }^2/\mathrm{DOF}$ in the region $q_{\mathrm{rec}}^2<0.10(\mathrm{GeV}/c{)}^2$ of (c) and (d) are $7.2/2$ ($2.7/2$) and $32.3/2$ ($1.2/2$) with (without) CC coherent pion production.

Figure 3

The reconstructed $q^2$ distribution in the final sample.

Figure 4

Top: The neutrino energy spectra for (a) the coherent pion and (b) total CC events. The hatched histograms show the selected events. Bottom: The efficiencies as a function of neutrino energy for (c) the coherent pion and (d) total CC events. All of them are estimated by the MC simulation.