Electric dipole polarizability in 208Pb as a probe of the symmetry energy and neutron matter around ρ0/3

It is currently a big challenge to accurately determine the symmetry energy E-sym(rho) and the pure neutron matter equation of state E-PNM(rho) even their values around saturation density rho(0). We find that the electric dipole polarizability alpha(D) in Pb-208 can be determined uniquely by the magnitude of E-sym(rho) or almost equivalently E-PNM(rho) at subsaturation densities around rho(0)/3, shedding light upon the genuine correlation between alpha(D) and E-sym(rho). By analyzing the experimental data of alpha(D) in Pb-208 from the Research Center for Nuclear Physics using a number of nonrelativistic and relativistic mean-field models, we obtain very stringent constraints on E-sym(rho) and E-PNM(rho) around rho(0)/3. The obtained constraints are found to be in good agreement with the results extracted in other analyses. In particular, our results provide for the first time the experimental constraints on E-PNM(rho) around rho(0)/3, which are in harmony with the recent determination of E-PNM(rho) from microscopic theoretical studies and potentially useful in constraining the largely uncertain many-nucleon interactions in microscopic calculations of neutron matter.