Evidence for dipole nature of the low-energy $gamma$ enhancement in $^{56}$Fe

The $gamma$-ray strength function of $^{56}$Fe has been measured from proton-$gamma$ coincidences for excitation energies up to $approx 11$ MeV. The low-energy enhancement in the $gamma$-ray strength function, which was first discovered in the ($^3$He,$alphagamma$)$^{56}$Fe reaction, is confirmed with the ($p,p^primegamma$)$^{56}$Fe experiment reported here. Angular distributions of the $gamma$ rays give for the first time evidence that the enhancement is dominated by dipole transitions.