Computing the dipole polarizability of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Ca</mml:mi><mml:mprescripts/><mml:none/><mml:mn>48</mml:mn></mml:mmultiscripts></mml:math> with increased precision

We compute the electric dipole polarizability of $^{48}\mathrm{Ca}$ with an increased precision by including more correlations than in previous studies. Employing the coupled-cluster method we go beyond single and double 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 ${\ensuremath{\alpha}}_{D}$ 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 $^{4}\mathrm{He}$ and with self consistent Green's function in $^{16}\mathrm{O}$. We compute the dipole polarizability of $^{48}\mathrm{Ca}$ employing the chiral interaction ${\mathrm{N}}^{2}{\mathrm{LO}}_{\mathrm{sat}}$ [Ekstr\"om 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 ${\mathrm{N}}^{2}{\mathrm{LO}}_{\mathrm{sat}}$. The addition of these new correlations allows us to improve the precision of our $^{48}\mathrm{Ca}$ calculations and reconcile the recently reported discrepancy between coupled-cluster results based on these interactions and the experimentally determined ${\ensuremath{\alpha}}_{D}$ from proton inelastic scattering in $^{48}\mathrm{Ca}$ [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 it is less so for the excited states and the similarity-transformed dipole operator.

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