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Achim Schwenk

Publications and source records attributed to Achim Schwenk.

71 records · Page 4Linked to original sources
arXiv2016

Analysis strategies for general spin-independent WIMP-nucleus scattering

We propose a formalism for the analysis of direct-detection dark-matter searches that covers all coherent responses for scalar and vector interactions and incorporates QCD constraints imposed by chiral symmetry, including all one- and two-body WIMP-nucleon interactions up to third order in chiral effective field theory. One of the free parameters in the WIMP-nucleus cross section corresponds to standard spin-independent searches, but in general different combinations of new-physics couplings are probed. We identify the interference with the isovector counterpart of the standard spin-independent response and two-body currents as the dominant corrections to the leading spin-independent structure factor, and discuss the general consequences for the interpretation of direct-detection experiments, including minimal extensions of the standard spin-independent analysis. Fits for all structure factors required for the scattering off xenon targets are provided based on state-of-the-art nuclear shell-model calculations.

hep-ph↗
arXiv2016

Measuring the neutron star equation of state using X-ray timing

One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. The flux we observe from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star's rotation, giving rise to a pulsation. Information about mass and radius is encoded into the pulse profile via relativistic effects, and tight constraints on mass and radius can be obtained. The second technique involves characterising the spin distribution of accreting neutron stars. The most rapidly rotating stars provide a very clean constraint, since the mass-shedding limit is a function of mass and radius. However the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasi-periodic oscillations in X-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state. We illustrate how these complementary X-ray timing techniques can be used to constrain the dense matter equation of state, and discuss the results that might be expected from a 10m$^2$ instrument. We also discuss how the results from such a facility would compare to other astronomical investigations of neutron star properties. [Modified for arXiv]

astro-ph.HE↗
arXiv2015

Chiral power counting of one- and two-body currents in direct detection of dark matter

We present a common chiral power-counting scheme for vector, axial-vector, scalar, and pseudoscalar WIMP-nucleon interactions, and derive all one- and two-body currents up to third order in the chiral expansion. Matching our amplitudes to non-relativistic effective field theory, we find that chiral symmetry predicts a hierarchy amongst the non-relativistic operators. Moreover, we identify interaction channels where two-body currents that so far have not been accounted for become relevant.

hep-ph↗
arXiv2014

Neutrino-pair bremsstrahlung from nucleon-$α$ versus nucleon-nucleon scattering

We study the impact of the nucleon-$α$ P-wave resonances on neutrino-pair bremsstrahlung. Because of the non-central spin-orbit interaction, these resonances lead to an enhanced contribution to the nucleon spin structure factor for temperatures $T \lesssim 4$ MeV. If the $α$-particle fraction is significant and the temperature is in this range, this contribution is competitive with neutron-neutron bremsstrahlung. This may be relevant for neutrino production in core-collapse supernovae or other dense astrophysical environments. Similar enhancements are expected for resonant non-central nucleon-nucleus interactions.

nucl-th↗
arXiv2012

Three-body forces and shell structure in calcium isotopes

Understanding and predicting the formation of shell structure from nuclear forces is a central challenge for nuclear physics. While the magic numbers N=2,8,20 are generally well understood, N=28 is the first standard magic number that is not reproduced in microscopic theories with two-nucleon forces. In this Letter, we show that three-nucleon forces give rise to repulsive interactions between two valence neutrons that are key to explain 48Ca as a magic nucleus, with a high 2+ excitation energy and a concentrated magnetic dipole transition strength. The repulsive three-nucleon mechanism improves the agreement with experimental binding energies.

nucl-th↗
arXiv2011

Chiral three-nucleon forces and pairing in nuclei

We present the first study of pairing in nuclei including three-nucleon forces. We perform systematic calculations of the odd-even mass staggering generated using a microscopic pairing interaction at first order in chiral low-momentum interactions. Significant repulsive contributions from the leading chiral three-nucleon forces are found. Two- and three-nucleon interactions combined account for approximately 70% of the experimental pairing gaps, which leaves room for self-energy and induced interaction effects that are expected to be overall attractive in nuclei.

nucl-th↗
arXiv2010

Three-body forces and the limit of oxygen isotopes

The limit of neutron-rich nuclei, the neutron drip-line, evolves regularly from light to medium-mass nuclei except for a striking anomaly in the oxygen isotopes. This anomaly is not reproduced in shell-model calculations derived from microscopic two-nucleon forces. Here, we present the first microscopic explanation of the oxygen anomaly based on three-nucleon forces that have been established in few-body systems. This leads to repulsive contributions to the interactions among excess neutrons that change the location of the neutron drip-line from $^{28}$O to the experimentally observed $^{24}$O. Since the mechanism is robust and general, our findings impact the prediction of the most neutron-rich nuclei and the synthesis of heavy elements in neutron-rich environments.

nucl-th↗
arXiv2010

Nucleon-Nucleon Scattering in a Harmonic Potential

The discrete energy-eigenvalues of two nucleons interacting with a finite-range nuclear force and confined to a harmonic potential are used to numerically reconstruct the free-space scattering phase shifts. The extracted phase shifts are compared to those obtained from the exact continuum scattering solution and agree within the uncertainties of the calculations. Our results suggest that it might be possible to determine the amplitudes for the scattering of complex systems, such as n-d, n-t or n-alpha, from the energy-eigenvalues confined to finite volumes using ab-initio bound-state techniques.

nucl-th↗
arXiv2009

Partial-wave contributions to pairing in nuclei

We present a detailed study of partial-wave contributions of nuclear forces to pairing in nuclei. For T=1, J=0 pairing, partial waves beyond the standard 1S0 channel play an interesting role for the pair formation in nuclei. The additional contributions are dominated by the repulsive 3P1 partial wave. Their effects, and generally spin-triplet nuclear forces between paired nucleons, are influenced by the interplay of spin-orbit partners. We explore the impact of including partial waves beyond the 1S0 channel on neutron-neutron pairing gaps in semi-magic isotopic chains. In addition, we show that nuclear forces favor T=1, J=0 over T=0, J=1 pairing, except in low-j orbitals. This is in contrast to the free-space motivation that suggests the formation of deuteron-like T=0 pairs in N=Z nuclei. The suppression of T=0 pairing is because the 3S1 strength is distributed on spin-orbit partners and because of the effects of the repulsive 1P1 channel and of D waves.

nucl-th↗
arXiv2009

Finite-size and confinement effects in spin-polarized trapped Fermi gases

We calculate the energy of a single fermion interacting resonantly with a Fermi sea of different-species fermions in asymmetric traps, and show that finite particle numbers and the trap geometry impact the phase structure and the critical polarization. Our findings contribute to understanding some experimental discrepancies in spin-polarized Fermi gases as finite-size and confinement effects.

cond-mat.other↗
arXiv2007

Neutron matter from low-momentum interactions

We present a perturbative calculation of the neutron matter equation of state based on low-momentum two- and three-nucleon interactions. Our results are compared to the model-independent virial equation of state and to variational calculations, and we provide theoretical error estimates by varying the cutoff used to regulate nuclear interactions. In addition, we study the dependence of the BCS $^1$S$_0$ superfluid pairing gap on nuclear interactions and on the cutoff. The resulting gaps are well constrained by the nucleon-nucleon scattering phase shifts, and the cutoff dependence is very weak for sharp or sufficiently narrow smooth regulators with cutoffs $\lm > 1.6 \fmi$.

nucl-th↗
arXiv2006

Induced P-wave Superfluidity in Asymmetric Fermi Gases

We show that two new intra-species P-wave superfluid phases appear in two-component asymmetric Fermi systems with short-range S-wave interactions. In the BEC limit, phonons of the molecular BEC induce P-wave superfluidity in the excess fermions. In the BCS limit, density fluctuations induce P-wave superfluidity in both the majority and the minority species. These phases may be realized in experiments with spin-polarized Fermi gases.

cond-mat.other↗
arXiv2004

Low-momentum interactions for nuclei

We show how the renormalization group is used to construct a low-momentum nucleon-nucleon interaction V_{low k}, which unifies all potential models used in nuclear structure calculations. V_{low k} can be directly applied to the nuclear shell model or to nucleonic matter without a G matrix resummation. It is argued that V_{low k} parameterizes a high-order chiral effective field theory two-nucleon force. We use cutoff dependence as a tool to assess the error in the truncation of nuclear forces to two-nucleon interactions and introduce a low-momentum three-nucleon force, which regulates A=3,4 binding energies. The adjusted three-nucleon interaction is perturbative for small cutoffs. In contrast to other precision interactions, the error due to missing many-body forces can be estimated, when V_{low k} and the corresponding three-nucleon force are used in nuclear structure calculations and the cutoff is varied.

nucl-th↗
arXiv2004

Nuclear interactions from the renormalization group

We discuss how the renormalization group can be used to derive effective nuclear interactions. Starting from the model-independent low-momentum interaction V_{low k}, we successively integrate out high-lying particle and hole states from momentum shells around the Fermi surface as proposed by Shankar. The renormalization group approach allows for a systematic calculation of induced interactions and yields similar contributions to the scattering amplitude as the two-body parquet equations. We review results for the 1S0 and 3P2 superfluid pairing gaps as well as the spin dependence of effective interactions in neutron matter. Implications for the cooling of neutron stars are discussed.

nucl-th↗
arXiv2004

Low-momentum interaction in few-nucleon systems

The effective low-momentum interaction Vlowk is applied to three- and four-nucleon systems. We investigate the 3H, 3He and 4He binding energies for a wide range of the momentum cutoffs. By construction, all low-energy two-body observables are cutoff-independent, and therefore, any cutoff dependence is due to missing three-body or higher-body forces. We argue that for reasonable cutoffs Vlowk is similar to high-order interactions derived from chiral effective field theory. This motivates augmenting Vlowk by corresponding three-nucleon forces. The set of low-momentum twoand three-nucleon forces can be used in calculations of nuclear structure and reactions.

nucl-th↗
arXiv2002

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 V_{low k} 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.

nucl-th↗