arXiv subjects

H. -W. Hammer

Publications and source records attributed to H. -W. Hammer.

At least 109 records · Page 6Linked to original sources

arXiv2009

Resonant Atom-Dimer Relaxation in Ultracold Atoms

Three-body systems with large scattering length display universal phenomena associated with a discrete scaling symmetry. These phenomena include resonant enhancement of three-body loss rates when an Efimov three-body resonance is at the scattering threshold. In particular, there can be resonant peaks in the atom-dimer relaxation rate for large positive scattering length. We improve upon earlier studies and calculate the atom-dimer relaxation rate as a function of temperature using a Bose-Einstein distribution for the thermal average. As input, we use calculations of the atom-dimer scattering phase shifts from effective field theory.

cond-mat.other↗

arXiv2009

On the scattering of D and D* mesons off the X(3872)

Both the mass (just below the D^{*0}Dbar^0 threshold) and the likely quantum numbers (J^{PC}=1^{++}) of the X(3872) suggest that it is either a weakly-bound hadronic ``molecule'' or a virtual state of charmed mesons. Assuming the X(3872) is a weakly-bound molecule, the scattering of neutral D and D* mesons off the X(3872) can be predicted from the X(3872) binding energy. We calculate the phase shifts and cross section for scattering of D^0 and D^{*0} mesons and their antiparticles off the X(3872) in an effective field theory for short-range interactions. This provides another example of a three-body process, along with those in nuclear and atomic systems, that displays universal properties. It may be possible to extract the scattering within the final state interactions of B_c decays and/or other LHC events.

hep-ph↗

arXiv2009

Resonant Dimer Relaxation in Cold Atoms with a Large Scattering Length

Efimov physics refers to universal phenomena associated with a discrete scaling symmetry in the 3-body problem with a large scattering length. The first experimental evidence for Efimov physics was the recent observation of a resonant peak in the 3-body recombination rate for 133Cs atoms with large negative scattering length. There can also be resonant peaks in the atom-dimer relaxation rate for large positive scattering length. We calculate the atom-dimer relaxation rate as a function of temperature and show how measurements of the relaxation rate can be used to determine accurately the parameters that govern Efimov physics.

cond-mat.other↗

arXiv2009

Efimov physics in a finite volume

Three bosons with large scattering length show universal properties that do not depend on the details of the interaction at short distances. In the three-boson system, these properties include a geometric spectrum of shallow three-body states called "Efimov states" and log-periodic dependence of scattering observables on the scattering length. We investigate the modification of the Efimov states in a finite cubic box and calculate the dependence of their energies on the box size using effective field theory. We explicitly verify the renormalization of the effective field theory in the finite volume.

nucl-th↗

arXiv2009

Universality in QCD and Halo Nuclei

Effective Field Theory (EFT) provides a powerful framework to exploit a separation of scales in order to perform systematically improvable, model-independent calculations. We apply this method to strongly interacting quantum systems with short-range interactions and large scattering length. Such systems display remarkable universal properties which include a geometric spectrum of shallow three-body states called "Efimov states" and log-periodic dependence of scattering observables on the scattering length. We review the EFT for large scattering length and some of its applications in the physics of cold atoms and nuclear physics. In particular, we discuss the possibility of an infrared limit cycle in QCD and the extension of the EFT to halo nuclei.

nucl-th↗

arXiv2008

A Model Study of Discrete Scale Invariance and Long-Range Interactions

We investigate the modification of discrete scale invariance in the bound state spectrum by long-range interactions. This problem is relevant for effective field theory descriptions of nuclear cluster states and manifestations of the Efimov effect in nuclei. As a model system, we choose a one dimensional inverse square potential supplemented with a long-range Coulomb interaction. We study the renormalization and bound-state spectrum of the system as a function of the Coulomb interaction strength. Our results indicate, that the counterterm required to renormalize the inverse square potential alone is sufficient to renormalize the full problem. However, the breaking of the discrete scale invariance through the Coulomb interaction leads to a modified bound state spectrum. The shallow bound states are strongly influenced by the Coulomb interaction while the deep bound states are dominated by the inverse square potential.

nucl-th↗

arXiv2008

Three-Body Recombination of Identical Bosons with a Large Positive Scattering Length at Nonzero Temperature

For identical bosons with a large scattering length, the dependence of the 3-body recombination rate on the collision energy is determined in the zero-range limit by universal functions of a single scaling variable. There are six scaling functions for angular momentum zero and one scaling function for each higher partial wave. We calculate these universal functions by solving the Skorniakov--Ter-Martirosian equation. The results for the 3-body recombination as a function of the collision energy are in good agreement with previous results from solving the 3-body Schroedinger equation for 4He atoms. The universal scaling functions can be used to calculate the 3-body recombination rate at nonzero temperature. We obtain an excellent fit to the data from the Innsbruck group for 133Cs atoms with a large positive scattering length.

cond-mat.other↗

arXiv2008

alpha-alpha Scattering in Halo Effective Field Theory

We study the two-alpha-particle (alpha-alpha) system in an Effective Field Theory (EFT) for halo-like systems. We propose a power counting that incorporates the subtle interplay of strong and electromagnetic forces leading to a narrow resonance at an energy of about 0.1 MeV. We investigate the EFT expansion in detail, and compare its results with existing low-energy alpha-alpha phase shifts and previously determined effective-range parameters. Good description of the data is obtained with a surprising amount of fine-tuning. This scenario can be viewed as an expansion around the limit where, when electromagnetic interactions are turned off, the Be-8 ground state is at threshold and exhibits conformal invariance. We also discuss possible extensions to systems with more than two alpha particles.

nucl-th↗

arXiv2008

Universal properties and structure of halo nuclei

The universal properties and structure of halo nuclei composed of two neutrons (2n) and a core are investigated within an effective quantum mechanics framework. We construct an effective interaction potential that exploits the separation of scales in halo nuclei and treat the nucleus as an effective three-body system. The uncertainty from higher orders in the expansion is quantified through theoretical error bands. First, we investigate the possibility to observe excited Efimov states in 2n halo nuclei. Based on the experimental data, 20C is the only halo nucleus candidate to possibly have an Efimov excited state, with an energy less than 7 keV below the scattering threshold. Second, we study the structure of 20C and other 2n halo nuclei. In particular, we calculate their matter form factors, radii, and two-neutron opening angles.

nucl-th↗

arXiv2008

A-dependence of phi-meson production in p+A collisions

A systematic analysis of the A-dependence of phi-meson production in proton-nucleus collisions is presented. We apply different formalisms for the evaluation of the phi-meson distortion in nuclei and discuss the theoretical uncertainties of the data analysis. The corresponding results are compared to theoretical predictions. We also discuss the interpretation of the extracted results with respect to different observables and provide relations between frequently used definitions. The perspectives of future experiments are evaluated and estimates based on our systematical study are given.

nucl-th↗

arXiv2007

Model-independent extraction of two-photon effects in elastic electron-proton scattering

We address the discrepancy between the Rosenbluth and polarization transfer data for the electromagnetic form factors of the nucleon. Assuming that the effect of two-photon corrections on the polarization transfer data is negligible, we obtain a model-independent estimate of the two-photon correction Delta^(2γ). We analyze the polarization transfer data and the cross section data separately using dispersion relations. A central value as well as an error estimate for Delta^(2γ) is then obtained from a comparison of the two analyses. The resulting values for Delta^(2γ) are in good agreement with direct calculations available in the literature.

hep-ph↗

arXiv2007

Pion-mass dependence of three-nucleon observables

We use an effective field theory (EFT) which contains only short-range interactions to study the dependence of a variety of three-nucleon observables on the pion mass. The pion-mass dependence of input quantities in our ``pionless'' EFT is obtained from a recent chiral EFT calculation. To the order we work at, these quantities are the 1S0 scattering length and effective range, the deuteron binding energy, the 3S1 effective range, and the binding energy of one three-nucleon bound state. The chiral EFT input we use has the inverse 3S1 and 1S0 scattering lengths vanishing at mpi_c=197.8577 MeV. At this ``critical'' pion mass, the triton has infinitely many excited states with an accumulation point at the three-nucleon threshold. We compute the binding energies of these states up to next-to-next-to-leading order in the pionless EFT and study the convergence pattern of the EFT in the vicinity of the critical pion mass. Furthermore, we use the pionless EFT to predict how doublet and quartet nd scattering lengths depend on mpi in the region between the physical pion mass and mpi=mpi_c.

nucl-th↗

arXiv2007

Effective Range Corrections to Three-Body Recombination for Atoms with Large Scattering Length

Few-body systems with large scattering length a have universal properties that do not depend on the details of their interactions at short distances. The rate constant for three-body recombination of bosonic atoms of mass m into a shallow dimer scales as \hbar a^4/m times a log-periodic function of the scattering length. We calculate the leading and subleading corrections to the rate constant which are due to the effective range of the atoms and study the correlation between the rate constant and the atom-dimer scattering length. Our results are applied to 4He atoms as a test case.

cond-mat.other↗

arXiv2007

Universal Properties of the Four-Body System with Large Scattering Length

Few-body systems with large scattering length have universal properties that do not depend on the details of their interactions at short distances. We study the universal bound state properties of the four-boson system with large scattering length in an effective quantum mechanics approach. We compute the four-body binding energies using the Yakubovsky equations for positive and negative scattering length. Moreover, we study the correlation between three- and four-body energies and present a generalized Efimov plot for the four-body system. These results are useful for understanding the cluster structure of nuclei and for the creation of weakly-bound tetramers with cold atoms close to a Feshbach resonance.

nucl-th↗

arXiv2007

Effective Field Theory for Dilute Fermions with Pairing

Effective field theory (EFT) methods for a uniform system of fermions with short-range, natural interactions are extended to include pairing correlations, as part of a program to develop a systematic Kohn-Sham density functional theory (DFT) for medium and heavy nuclei. An effective action formalism for local composite operators leads to a free-energy functional that includes pairing by applying an inversion method order by order in the EFT expansion. A consistent renormalization scheme is demonstrated for the uniform system through next-to-leading order, which includes induced-interaction corrections to pairing.

nucl-th↗

arXiv2007

The pp -> K^+Sigma^+n cross section from missing mass spectra

We utilize existing inclusive data on K^+ meson momentum spectra of the reaction pp -> K^+X at T_p = 2.3 - 2.85 GeV to deduce total cross sections for pp -> K^+Σ^+n. The method used to extract those cross sections is explained and discussed in detail. Our result for T_p = 2.85 GeV is consistent with the data point from a direct measurement at the same beam energy. The cross section obtained for T_p = 2.3 GeV is with 13.7\pm2.3 μb considerably smaller than the value found in a recent experiment by the COSY-11 Collaboration at a somewhat lower beam energy, indicating that the pp -> K^+Σ^+n reaction cross section could exhibit a rather unusual energy dependence.

hep-ph↗

arXiv2006

Efimov Physics in Cold Atoms

Atoms with a large scattering length have universal low-energy properties that do not depend on the details of their structure or their interactions at short distances. In the 2-atom sector, the universal properties are familiar and depend only on the scattering length. In the 3-atom sector for identical bosons, the universal properties include the existence of a sequence of shallow triatomic molecules called Efimov trimers and log-periodic dependence of scattering observables on the energy and the scattering length. In this review, we summarize the universal results that are currently known. We also summarize the experimental information that is currently available with an emphasis on 3-atom loss processes.

cond-mat.other↗

arXiv2006

Few-Body Effects in Cold Atoms and Limit Cycles

Physical systems with a large scattering length have universal properties independent of the details of the interaction at short distances. Such systems can be realized in experiments with cold atoms close to a Feshbach resonance. They also occur in many other areas of physics such as nuclear and particle physics. The universal properties include a geometric spectrum of three-body bound states (so-called Efimov states) and log-periodic dependence of low-energy observables on the physical parameters of the system. This behavior is characteristic of a renormalization group limit cycle. We discuss universality in the three- and four-body sectors and give an overview of applications in cold atoms.

cond-mat.other↗