Observation of neutrino and antineutrino induced coherent neutral pion production off Al27

Abstract The angular distribution of isolated π 0 ,s, produced by ν μ ′s and ν μ ′s in the Aachen-Padova spark chamber, shows a peak in the forward direction which is not explicable by resonance production. A similar effect is seen in the acoplanarity distribution of the decay photons, which shows an excess of nearly coplanar events. A control sample containing a recoil proton shows no such peaks and is well reproduced by a resonance model. The observed effects are consistent with neutral current (NC) induced coherent single π 0 production off the entire Al 27 nucleus. The coherent cross sections are σ ν =(29±10)×10 −40 cm 2 /nucleus and σ ν =(25±7)×10 −40 cm 2 / nucleus . They determine the isovector axial NC coupling constant s =0.93±0.12. No η 0 mesons were observed, which limits the corresponding isoscalar coupling constant to δ