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Low-energy photon production in neutrino neutral-current interactions

The search for $ν_μ\to ν_e$ oscillations by the MiniBooNE Collaboration at Fermilab has revealed a low-energy signal which could be due either to electrons produced by $ν_e$ or photons produced by the interaction of the weak neutral current on the target nucleus. One contribution to the latter is a Wess-Zumino-Witten anomaly leading to a term in the Lagrangian proportional to $ε^{μνκλ} Z_μω_νF_{κλ}$. This term is normalized with the help of the known rates for the processes $f_1 \to ργ$ and $τ\to ν_τa_1$. A rate of about 1/4 of that employed in several previous estimates is obtained. As the anomaly term had already been found to play a subdominant role in photon production (e.g., in comparison with $Δ$ excitation and decay), the present estimate reduces its strength even further.