Nuclear matter as a liquid phase of spontaneously broken semi-classical $SU(2)_L \times SU(2)_R$ chiral perturbation theory: Static chiral nucleon liquids

Abstract

We study effective field theories (EFT) of nuclear structure based on spontaneously broken global $SU(2)_L\times SU(2)_R$ chiral symmetry of QCD with two massless quarks, i.e. $SU(2)\chi PT$. For ground-state nuclei, this EFT enables expansion and truncation in inverse powers of $\Lambda_{\chi SB}\simeq 1 GeV$, with analytic operators renormalized to all loop orders. We derive the EFT Lagrangian to order $\Lambda^0_{\chi SB}$. We show that $SU(2)\chi PT$ of protons, neutrons and pions admits a semi-classical "Static Chiral Nucleon Liquid" (Static$\chi$NL) phase and that "Pion-less" $SU(2)\chi PT$ emerges in this liquid: far-infrared pions decouple from Static$\chi$NL, vastly simplifying the derivation of saturated nuclear matter (the infinite liquid phase) and of finite microscopic liquid drops (ground-state nuclides). Static$\chi$NL are made entirely of nucleons with even parity, total spin zero, and even $Z$ and $N$; local expectation values for spin and momenta vanish. They explain the power of pion-less $SU(2)\chi PT$ to capture experimental ground-state properties of certain nuclides, this explanation following directly from the global symmetries of QCD with two massless quarks. Mean-field Static$\chi$NL non-topological solitons are true solutions of $SU(2)\chi PT$'s semi-classical symmetries: they obey all CVC and PCAC conservation laws and they have zero internal and external pressure. The nuclear liquid-drop model and the semi-empirical mass formula emerge -- with correct nuclear density and saturation and asymmetry energies -- in an explicit Thomas-Fermi construction. We relate our work to compatible and complementary work in pionless and in halo/ cluster EFTs, also composed entirely of nucleons and applied to light ($A\leq 6$) nuclei, which might provide important (<12.5%) corrections to Static$\chi NL$.