Low-energy photon production in neutrino neutral-current interactions

The search for ${\nu }_{\mu }\to {\nu }_e$ oscillations by the MiniBooNE Collaboration at Fermilab has revealed a low-energy signal which could be due either to electrons produced by ${\nu }_e$ 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 ${\epsilon }^{\mu \nu \kappa \lambda }{Z}_{\mu }{\omega }_{\nu }{F}_{\kappa \lambda }$. This term is normalized with the help of the known rates for the processes $f_1\to \rho \gamma$ and $\tau \to {\nu }_{\tau }{a}_1$. 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 $\mathrm{\Delta }$ excitation and decay), the present estimate reduces its strength even further.

Figure 1

Processes sensitive to WZW anomalies: (a) Coherent reaction $\nu A\to \nu \gamma A$ on a nucleus of atomic number $A$; (b) Induced $f_1\to \gamma \rho$ decay; (c) Coherent $K_L\to K_S$ regeneration on a nucleus of atomic number $A$.