Discovery of New Physics in Radiative Pion Decays?

Recently a strong indication for a deviation from the standard model (SM) has been obtained by PIBETA Collaboration. Namely, SM fails to describe the energy distribution and the branching ratio of the radiative decays of the positive pions at rest in the high-E_gamma/low-E_e kinematic region. The previous experiment at ISTRA facility, testing the radiative decays of negative pions in flight in a wide kinematic region has alarmed about the same effect, although statistically less significant. The present PIBETA result indicates a deficit of the branching ratio of the radiative pion decay in the specified kinematic region at 8 sigma level in comparison with SM prediction, while in the other kinematic regions both the branching ratios and the energy distributions are compatible with the V-A interaction. We argue that this effect can result only from a small admixture of new tensor interactions. They may arise due to an exchange of new spin one chiral bosons which interact anomalously with matter.