Muonless π0-Production by Muon-Neutrinos and -Antineutrinos

Muonless single π0 production by muon-neutrinos and -antineutrinos has been studied in an aluminium spark-chamber. The corresponding reactions with muon were used as a reference. The relative cross sections are found to be R.0304′0 = 0.40 ± 0.06 for neutrinos, and R′0 = 0.61 ± 0.10 for antineutrinos. The muonless π0 cross section ratio for anti-neutrinos and neutrinos is rNC = 0.50 ± 0.09. This result excludes parity conservation in this process at a level of 3.5 standard deviations. From rNC, and independently from R′0/R′0, we derive sin2 θ W = 0.37 ± 0.05.