Experimental M1 response of Ar40 as a benchmark for neutrino-nucleus scattering calculations

The excitation of atomic nuclei via magnetic dipole transitions is closely related to the inelastic neutral-current neutrino-nucleus (NC-$\ensuremath{\nu}\mathrm{A}$) scattering process due to the similarity of the transition operators. NC-$\ensuremath{\nu}\mathrm{A}$-scattering serves for the detection of supernova neutrinos and poses a significant source of background in modern liquid-argon based high-energy neutrino detection experiments. To enable tests of the reliability of predictions for neutrino-nucleus scattering, the magnetic dipole response of $^{40}\mathrm{Ar}$ below 7.7 MeV was characterized in a nuclear resonance fluorescence experiment using quasimonoenergetic $\ensuremath{\gamma}$-ray beams. The linear polarization of the beams allowed for assignments of electric or magnetic character to previously known dipole excitations. A total magnetic dipole strength of $0.{36}_{\ensuremath{-}0.05}^{+0.04}\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{N}^{2}$ was identified in the energy range of the present experiment. Combined with data from previous measurements, the full magnetic dipole strength of $^{40}\mathrm{Ar}$ below the neutron separation threshold was investigated. Due to the low background in the energy range within the bandwidth of the $\ensuremath{\gamma}$-ray beams, the previous sensitivity limit was improved. A large-scale nuclear shell model calculation in the $sd\text{\ensuremath{-}}fp$ space satisfactorily agrees with the data in terms of excitation energies and strengths of the observed ${1}^{+}$ states.