Achim Schwenk

Neutron matter at next-to-next-to-next-to-leading order in chiral effective field theory

Neutron matter presents a unique system for chiral effective field theory because all many-body forces among neutrons are predicted to next-to-next-to-next-to-leading order (N(3)LO). We present the first complete N(3)LO calculation of the neutron matter energy. This includes the subleading three-nucleon forces for the first time and all leading four-nucleon forces. We find relatively large contributions from N(3)LO three-nucleon forces. Our results provide constraints for neutron-rich matter in astrophysics with controlled theoretical uncertainties.

Renormalization group approach to neutron matter: quasiparticle interactions, superfluid gaps and the equation of state

Renormalization group methods can be applied to the nuclear many-body problem using the approach proposed by Shankar. We start with the two-body low momentum interaction Vlowk and use the RG flow from the particle–hole channels to calculate the full scattering amplitude in the vicinity of the Fermi surface. This is a new straightforward approach to the many-body problem which is applicable also to condensed matter systems without long-range interactions, such as liquid 3He. We derive the one-loop renormalization group equations for the quasiparticle interaction and the scattering amplitude at zero temperature. The RG presents an elegant method to maintain all momentum scales and preserve the antisymmetry of the scattering amplitude. As a first application we solve the RG equations for neutron matter. The resulting quasiparticle interaction includes effects due to the polarization of the medium, the so-called induced interaction of Babu and Brown. We present results for the Fermi liquid parameters, the equation of state of neutron matter and the 1S0 superfluid pairing gap.

Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers

Neutron-star (NS) merger simulations are conducted for 38 representative microphysical descriptions of high-density matter in order to explore the equation-of-state dependence of the postmerger ring-down phase. The formation of a deformed, oscillating, differentially rotating very massive NS is the typical outcome of the coalescence of two stars with 1.35 M⊙ for most candidate EoSs. The oscillations of this object imprint a pronounced peak in the gravitational-wave (GW) spectra, which is used to characterize the emission for a given model. The peak frequency of this postmerger GW signal correlates very well with the radii of nonrotating NSs, and thus allows to constrain the highdensity EoS by a GW detection. In the case of 1.35-1.35 M⊙ mergers the peak frequency scales particularly well with the radius of a NS with 1.6 M⊙, where the maximum deviation from this correlation is only 60 meters for fully microphysical EoSs which are compatible with NS observations. Combined with the uncertainty in the determination of the peak frequency it appears likely that a GW detection can measure the radius of a 1.6 M⊙ NS with an accuracy of about 100 to 200 meters. We also uncover relations of the peak frequency with the radii of nonrotating NSs with 1.35 M⊙ or 1.8 M⊙, with the radius or the central energy density of the maximum-mass TolmanOppenheimer-Volkoff configuration, and with the pressure or sound speed at a fiducial rest-mass density of about twice nuclear saturation density. Furthermore, it is found that a determination of the dominant postmerger GW frequency can provide an upper limit for the maximum mass of nonrotating NSs. The effect of variations of the binary setup are investigated and corresponding functional dependences between the peak frequency and radii of nonrotating NSs are derived. With higher total binary masses, correlations are tighter for radii of nonrotating NSs with higher masses. The prospects for a detection of the postmerger GW signal and a determination of the dominant GW frequency are estimated to be in the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO detector.

Low-momentum nucleon–nucleon interaction and Fermi liquid theory

Abstract We use the induced interaction of Babu and Brown to derive two novel relations between the quasiparticle interaction in nuclear matter and the unique low-momentum nucleon–nucleon interaction V low k in vacuum. These relations provide two independent constraints on the Fermi liquid parameters of nuclear matter. We derive the full renormalization group equations in the particle–hole channels from the induced interaction. The new constraints, together with the Pauli principle sum rules, define four combinations of Fermi liquid parameters that are invariant under the renormalization group flow. Using empirical values for the spin-independent Fermi liquid parameters, we are able to compute the major spin-dependent ones by imposing the new constraints and the Pauli principle sum rules. The effects of tensor forces are discussed.

Asymmetric nuclear matter based on chiral two- and three-nucleon interactions

We calculate the properties of isospin-asymmetric nuclear matter based on chiral nucleon-nucleon (NN) and three-nucleon (3N) interactions. To this end, we develop an improved normal-ordering framework that allows to include general 3N interactions starting from a plane-wave partial-wave-decomposed form. We present results for the energy per particle for general isospin asymmetries based on a set of different Hamiltonians, study their saturation properties, the incompressibility, symmetry energy, and also provide an analytic parametrization for the energy per particle as a function of density and isospin asymmetry.

Instanton Contribution to the Pion Electro-Magnetic Formfactor at Q 2 > 1 GeV 2

We study the effects of instantons on the charged pion electromagnetic form factor at intermediate momenta. In the single instanton approximation (SIA), we predict the pion form factor in the kinematic region

Large-scale nuclear structure calculations for spin-dependent WIMP scattering with chiral effective field theory currents

{Q}^{2}=2char21{}10{mathrm{GeV}}^{2}.

Shell-model interactions from chiral effective field theory

By developing the calculation in a mixed time-momentum representation, it is possible to maximally reduce the model dependence and to calculate the form factor directly. We find the intriguing result that the SIA calculation coincides with the vector dominance monopole form, up to a surprisingly high momentum transfer

New results for medium and heavy-mass nuclei

{Q}^{2}ensuremath{sim}10{mathrm{GeV}}^{2}.

Ab initio treatment of fully open-shell medium-mass nuclei with the IM-SRG

This suggests that vector dominance for the pion holds beyond low energy nuclear physics.