  We compute the electric dipole polarizability of 48Ca with an increased
precision by including more correlations than in previous studies. Employing
the coupled-cluster method we go beyond singles and doubles excitations and
include leading-order three-particle-three-hole (3p-3h) excitations for the
ground state, excited states, and the similarity transformed operator. We study
electromagnetic sum rules, such as the bremsstrahlung sum rule m_0 and the
polarizability sum rule alpha_D using interactions from chiral effective field
theory. To gauge the quality of our coupled-cluster approximations we perform
several benchmarks with the effective interaction hyperspherical harmonics
approach in 4He and with self consistent Green's function in 16O. We compute
the dipole polarizability of 48Ca employing the chiral interaction N2LOsat
[Ekstroem et al., Phys. Rev. C 91, 051301 (2015)] and the 1.8/2.0 (EM) [Hebeler
et al., Phys. Rev. C 83, 031301 (2011)]. We find that the effect of 3p-3h
excitations in the ground state is small for 1.8/2.0 (EM) but non-negligible
for N2LOsat. The addition of these new correlations allows us to improve the
precision of our 48Ca calculations and reconcile the recently reported
discrepancy between coupled-cluster results based on these interactions and the
experimentally determined alpha_D from proton inelastic scattering in 48Ca
[Birkhan et al., Phys. Rev. Lett. 118, 252501 (2017)]. For the computation of
electromagnetic and polarizability sum rules, the inclusion of leading-order
3p-3h excitations in the ground state is important, while less so for the
excited states and the similarity-transformed dipole operator.
