arXiv subjects

Norbert Kaiser

Publications and source records attributed to Norbert Kaiser.

At least 37 records · Page 2Linked to original sources

arXiv2016

Thermodynamics of isospin-asymmetric nuclear matter from chiral effective field theory

The density and temperature dependence of the nuclear symmetry free energy is investigated using microscopic two- and three-body nuclear potentials constructed from chiral effective field theory. The nuclear force models and many-body methods are benchmarked to properties of isospin-symmetric nuclear matter in the vicinity of the saturation density as well as the virial expansion of the neutron matter equation of state at low fugacities. The free energy per particle of isospin-asymmetric nuclear matter is calculated assuming a quadratic dependence of the interaction contributions on the isospin asymmetry. The spinodal instability at subnuclear densities is examined in detail.

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arXiv2016

Leading three-baryon forces from SU(3) chiral effective field theory

Leading three-baryon forces are derived within SU(3) chiral effective field theory. Three classes of irreducible diagrams contribute: three-baryon contact terms, one-meson exchange and two-meson exchange diagrams. We provide the minimal non-relativistic terms of the chiral Lagrangian, that contribute to these diagrams. SU(3) relations are given for the strangeness S=0 and -1 sectors. In the strangeness-zero sector we recover the well-known three-nucleon forces from chiral effective field theory. Explicit expressions for the lambda-nucleon-nucleon chiral potential in isospin space are presented.

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arXiv2015

Shear Viscosities from Kubo Formalism in a large-$N_{\rm c}$ Nambu--Jona-Lasinio Model

In this work the shear viscosity of strongly interacting matter is calculated within a two-flavor Nambu--Jona-Lasinio model as a function of temperature and chemical potential. The general Kubo formula is applied, incorporating the full Dirac structure of the thermal quark spectral function and avoiding commonly used on-shell approximations. Mesonic fluctuations contributing via Fock diagrams provide the dominant dissipative processes. The resulting ratio $η/s$ (shear viscosity over entropy density) decreases with temperature and chemical potential. Interpolating between our NJL results at low temperatures and hard-thermal-loop results at high temperatures a minimum slightly above the AdS/CFT benchmark $η/s=1/4π$ is obtained.

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arXiv2014

Nuclear thermodynamics from chiral low-momentum interactions

We investigate the thermodynamic equation of state of isospin-symmetric nuclear matter with microscopic nuclear forces derived within the framework of chiral effective field theory. Two- and three-body nuclear interactions constructed at low resolution scales form the basis for a perturbative calculation of the finite-temperature equation of state. The nuclear force models and many-body methods are benchmarked against bulk properties of isospin-symmetric nuclear matter at zero temperature, which are found to be well reproduced when chiral nuclear interactions constructed at the lowest resolution scales are employed. The calculations are then extended to finite temperatures, where we focus on the liquid-gas phase transition and the associated critical point. The Maxwell construction is applied to construct the physical equation of state, and the value of the critical temperature is determined to be T_c =17.2-19.1 MeV, in good agreement with the value extracted from multifragmentation reactions of heavy ions.

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arXiv2014

Alpha-clustered hypernuclei and chiral SU(3) dynamics

Light hypernuclei with an $α$ cluster substructure of the core nucleus are studied using an accurate cluster approach (the Hyper-THSR wave function) in combination with a density-dependent $Λ$ hyperon-nuclear interaction derived from chiral SU(3) effective field theory. This interaction includes important two-pion exchange processes involving $ΣN$ intermediate states and associated three-body mechanisms as well as effective mass and surface terms arising in a derivative expansion of the in-medium $Λ$ self-energy. Applications and calculated results are presented and discussed for $_Λ^9$Be and $^{13}_Λ$C. Furthermore, the result of the lightest $α$ clustered hypernucleus, $^5_Λ$He using realistic $ab initio$ four nucleon density is shown.

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arXiv2013

Single-particle potential from resummed ladder diagrams

A recent work on the resummation of fermionic in-medium ladder diagrams to all orders is extended by calculating the complex single-particle potential $U(p,k_f)+ i\,W(p,k_f)$ for momenta $p k_f$. The on-shell single-particle potential is constructed by means of a complex-valued in-medium loop that includes corrections from a test-particle of momentum $\vec p$ added to the filled Fermi sea. The single-particle potential $U(k_f,k_f)$ at the Fermi surface as obtained from the resummation of the combined particle and hole ladder diagrams is shown to satisfy the Hugenholtz-Van-Hove theorem. The perturbative contributions at various orders $a^n$ in the scattering length are deduced and checked against the known analytical results at order $a^1$ and $a^2$. The limit $a\to\infty$ is studied as a special case and a strong momentum dependence of the real (and imaginary) single-particle potential is found. This indicates an instability against a phase transition to a state with an empty shell inside the Fermi sphere such that the density gets reduced by about 5%. For comparison, the same analysis is performed for the resummed particle-particle ladder diagrams alone. In this truncation an instability for hole-excitations near the Fermi surface is found at strong coupling. For the set of particle-hole ring diagrams the single-particle potential is calculated as well. Furthermore, the resummation of in-medium ladder diagrams to all orders is studied for a two-dimensional Fermi gas with a short-range two-body contact-interaction.

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arXiv2013

Relativistic SU(3) chiral baryon-baryon Lagrangian up to order q^2

We construct the most general chiral effective Lagrangian for baryon-baryon contact interactions in flavor SU(3) up to order q^2 using a covariant power counting. A subset of these contact terms contributes to the baryon-baryon potential in chiral effective field theory. The Lorentz invariant effective Lagrangian is constructed to fulfill the invariance under charge conjugation, parity transformation, Hermitian conjugation and the local chiral symmetry group SU(3)_L x SU(3)_R. Goldstone bosons and external fields are included as well, thus providing additional four-baryon contact vertices involving e.g. pseudoscalar mesons and/or photons. In order to eliminate the linearly dependent terms, we use the Fierz identities, the equations of motion, and a Cayley-Hamilton relation for SU(3). As an application the baryon-baryon scattering contact potentials in low partial waves are considered.

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arXiv2012

Shear Viscosity of a Hot Pion Gas

The shear viscosity of an interacting pion gas is studied using the Kubo formalism as a microscopic description of thermal systems close to global equilibrium. We implement the skeleton expansion in order to approximate the retarded correlator of the viscous part of the energy-momentum tensor. After exploring this in $gϕ^4$ theory we show how the skeleton expansion can be consistently applied to pions in chiral perturbation theory. The shear viscosity $η$ is determined by the spectral width, or equivalently, the mean free path of pions in the heat bath. We derive a new analytical result for the mean free path which is well-conditioned for numerical evaluation and discuss the temperature and pion-mass dependence of the mean free path and the shear viscosity. The ratio $η/s$ of the interacting pion gas exceeds the lower bound $1/4π$ from AdS/CFT correspondence.

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arXiv2012

Charmonium potentials: Matching perturbative and lattice QCD

Central and spin-spin potentials for charmonium, constructed from Nambu-Bethe-Salpeter amplitudes in lattice simulations of full QCD, are matched with results from perturbative QCD at an appropriate distance scale. This matching is made possible by defining the perturbative potentials through Fourier transforms with a low-momentum cutoff. The central (spin-independent) potential is compared with potentials derived from an expansion in powers of the inverse quark mass. A well-controlled continuation of the charmonium spin-spin potential from lattice QCD to short distances is performed. The mass splittings of the charmonium singlet and triplet states below the open charm threshold, obtained from the matched spin-spin potential, are in good agreement with the experimental values.

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arXiv2012

Nuclear thermodynamics and the in-medium chiral condensate

The temperature dependence of the chiral condensate in isospin-symmetric nuclear matter at varying baryon density is investigated using thermal in-medium chiral effective field theory. This framework provides a realistic approach to the thermodynamics of the correlated nuclear many-body system and permits calculating systematically the pion-mass dependence of the free energy per particle. One- and two-pion exchange processes, $Δ(1232)$-isobar excitations, Pauli blocking corrections and three-body correlations are treated up to and including three loops in the expansion of the free energy density. It is found that nuclear matter remains in the Nambu-Goldstone phase with spontaneously broken chiral symmetry in the temperature range $T\lesssim 100\,$MeV and at baryon densities at least up to about twice the density of normal nuclear matter, $2ρ_0 \simeq 0.3\, $fm$^{-3}$. Effects of the nuclear liquid-gas phase transition on the chiral condensate at low temperatures are also discussed.

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arXiv2012

Chiral thermodynamics of nuclear matter

The free energy and the equation of state of isospin-asymmetric nuclear matter are calculated at finite temperature up to three loop order in the framework of in-medium chiral perturbation theory, systematically incorporating one- and two-pion exchange dynamics to this order. Effects from the two-pion exchange with explicit Δ-isobar excitation are included, as well as three-body forces. We construct the phase diagram of nuclear matter for different proton fractions x_p and investigate the dependence of nuclear matter properties on the isospin-asymmetry. A detailed study of the liquid-gas phase transition is performed. For isospin-symmetric nuclear matter we find a critical temperature of 15.1 MeV; as the isospin-asymmetry is increased, the liquid-gas coexistence region decreases until it disappears at x_p \approx 0.05, while nuclear matter becomes unbound at x_p \approx 0.12. The quadratic expansion of the free energy in the asymmetry parameter δ is a good approximation at low temperature even for large δ. An estimate of chiral four-body correlations in nuclear and neutron matter is also performed and the corrections from such four-body interactions are found to be small.

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arXiv2011

Chiral nuclear thermodynamics

We calculate the equation of state of nuclear matter for arbitrary isospin-asymmetry up to three loop order in the free energy density in the framework of in-medium chiral perturbation theory. In our approach 1π- and 2π-exchange dynamics with the inclusion of the Δ-isobar excitation as an explicit degree of freedom, corresponding to the long- and intermediate-range correlations, are treated explicitly. Few contact terms fixed to reproduce selected known properties of nuclear matter encode the short-distance physics. Two-body as well as three-body forces are systematically included. We find a critical temperature of about 15 MeV for symmetric nuclear matter. We investigate the dependence of the liquid-gas first-order phase transition on isospin-asymmetry. In the same chiral framework we calculate the chiral condensate of isospin-symmetric nuclear matter at finite temperatures. The contribution of the Δ-isobar excitation is essential for stabilizing the condensate. As a result, we find no indication of a chiral phase transition for densities ρ\lesssim 2 ρ_0 and at temperatures T \lesssim 100 MeV.

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arXiv2011

Mass dependence of the heavy quark potential and its effects on quarkonium states

The heavy quark-antiquark potential is accessible in perturbative QCD and in lattice simulations. The perturbative short-distance part of the potential is contructed via a restricted Fourier transform, covering the momentum region where perturbative QCD is applicable. We show that for the leading order static term as well as for the mass dependent corrections, the perturbative part can be matched at intermediate distances with results from lattice QCD. From these matched potentials, quarkonium spectra with a single free parameter (the heavy quark mass) are derived and compared with empirical spectra. Furthermore, charm and bottom quark masses are deduced.

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arXiv2011

Quark-antiquark potential to order 1/m and heavy quark masses

An updated heavy quark-antiquark potential is constructed by matching the short-distance perturbative part to long-distance lattice QCD results at an intermediate r scale. The static potential and the order 1/m potential are both analyzed in this way. Effects if order 1/m in charmonium and bottomonium spectra are discussed in comparison. Charm and bottom quark masses are deduced from the spectra and related to the quark masses of other schemes.

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arXiv2009

The heavy quark-antiquark potential from lattice and perturbative QCD

The heavy quark-antiquark potential in perturbative QCD is subject to ambiguities. We show how to derive a well-defined and stable short-distance potential that can be matched to results from lattice QCD simulations at intermediate distances. The static potential as well as the order 1/m potential are discussed.

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arXiv2004

Nuclear density functional constrained by low-energy QCD

We have developed a relativistic point-coupling model of nuclear many-body dynamics constrained by the low-energy sector of QCD. The effective Lagrangian is characterized by density-dependent coupling strengths determined by chiral one- and two-pion exchange (with single and double delta isobar excitations) and by large isoscalar background fields that arise through changes of the quark condensate and the quark density at finite baryon density. The model has been tested in the analysis of nuclear ground-state properties along different isotope chains of medium and heavy nuclei. The agreement with experimental data is comparable with purely phenomenological predictions. The built-in QCD constraints and the explicit treatment of pion exchange restrict the freedom in adjusting parameters and functional forms of density-dependent couplings. It is shown that chiral pionic fluctuations play an important role for nuclear binding and saturation mechanism, whereas background fields of about equal magnitude and opposite sign generate the effective spin-orbit potential in nuclei.

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arXiv2001

Aspects of near threshold neutral pion photoproduction off protons

We investigate near threshold neutral pion photoproduction off protons to fourth order in heavy baryon chiral perturbation theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary π^+ n threshold is in agreement with expectations from the final state theorem. We also analyze the fourth order corrections to the P-wave low-energy theorems and show that potentially large Delta-isobar contributions are cancelled by sizeable pion loop effects. This solidifies the parameter free third order predictions, which are in excellent agreement with the data.

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arXiv2001

Chiral 3$π$-exchange NN-potentials: Results for dominant next-to-leading order contributions

We calculate in (two-loop) chiral perturbation theory the local NN-potentials generated by the three-pion exchange diagrams with one insertion from the second order chiral effective pion-nucleon Lagrangian proportional to the low-energy constants $c_{1,2,3,4}$. The resulting isoscalar central potential vanishes identically. In most cases these $3π$-exchange potentials are larger than the ones generated by the diagrams involving only leading order vertices due to the large values of $c_{3,4}$ (which mainly represent virtual $Δ$-excitation). A similar feature has been observed for the chiral $2π$-exchange. We also give suitable (double-integral) representations for the spin-spin and tensor potentials generated by the leading-order diagrams proportional to $g_A^6$ involving four nucleon propagators. In these cases the Cutkosky rule cannot be used to calculate the spectral-functions in the infinite nucleon mass limit since the corresponding mass-spectra start with a non-vanishing value at the $3π$-threshold. Altogether, one finds that chiral $3π$-exchange leads to small corrections in the region $r\geq 1.4$ fm where $1π$- and chiral $2π$-exchange alone provide a very good strong NN-force as shown in a recent analysis of the low-energy pp-scattering data-base.

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