Neutrino-induced pion production from nuclei at medium energies

We present a fully relativistic formalism for describing neutrino-induced $\Delta$-mediated single-pion production from nuclei. We assess the ambiguities stemming from the $\Delta$ interactions. Variations in the cross sections of over 10% are observed, depending on whether or not magnetic-dipole dominance is assumed to extract the vector form factors. These uncertainties have a direct impact on the accuracy with which the axial-vector form factors can be extracted. Different predictions for $C_5^A(Q^2)$ induce up to 40-50% effects on the $\Delta$-production cross sections. To describe the nucleus, we turn to a relativistic plane-wave impulse approximation (RPWIA) using realistic bound-state wave functions derived in the Hartree approximation to the $\sigma$-$\omega$ Walecka model. For neutrino energies larger than 1 GeV, we show that a relativistic Fermi-gas model with appropriate binding-energy correction produces comparable results as the RPWIA which naturally includes Fermi motion, nuclear-binding effects and the Pauli exclusion principle. Including $\Delta$ medium modifications yields a 20 to 25% reduction of the RPWIA cross section. The model presented in this work can be naturally extended to include the effect of final-state interactions in a relativistic and quantum-mechanical way. Guided by recent neutrino-oscillation experiments, such as MiniBooNE and K2K, and future efforts like MINER$\nu$A, we present $Q^2$, $W$, and various semi-inclusive distributions, both for a free nucleon and carbon, oxygen and iron targets.