arXiv subjects

Wolfram Weise

Publications and source records attributed to Wolfram Weise.

At least 19 recordsLinked to original sources

arXiv2026

Size and Distance Scales in the Nucleon and in Dense Baryonic Matter

This memorial tribute to Mannque Rho follows a line of thoughts and ideas that he continuously inspired and shaped over many decades: from the two-scales picture of low-energy nucleon structure to dense and cold baryonic matter as it is realized in the cores of neutron stars. Early groundbreaking concepts are recalled and updated by recent advanced analyses of the `core' and `cloud' sizes of the nucleon. Implications for dense nuclear matter are then discussed and confronted with empirical information from Bayesian inference analyses of neutron star observables.

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arXiv2026

Liquid-gas phase transition of nuclear matter

A survey is presented summarizing the empirical evidence for and interpretations of a first-order liquid-gas phase transition in nuclear matter. Earlier developments and the present state of knowledge about the extraction of the critical point for such a transition, primarily from the systematics of multifragmention data, are outlined. By analogy with a Van der Waals equation of state, the empirically deduced critical temperature and pressure permit to draw a schematic picture of the underlying nuclear potential. More detailed approaches to the liquid-gas transition using self-consistent nuclear Hartree-Fock and variational calculations are described. Critical exponents are reported. Then chiral effective field theory, as the low-energy realization of QCD, is discussed in the context of nuclear thermodynamics. Its implications for the liquid-gas transition in symmetric nuclear matter as well as in neutron-rich matter are reviewed.

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arXiv2025

Implications of latest NICER data for the neutron star equation of state

As an update to our previously performed Bayesian inference analyses of the neutron star matter equation-of-state and related quantities, the additional impact of the recently published NICER data of PSR J0437-4751 is examined. Including the mass and radius distributions of this pulsar in our data base results in modest shifts from previously inferred median posterior values of radii $R$ and central densities $n_c$ for representative $1.4\,M_\odot$ and $2.1\,M_\odot$ neutron stars: radii are reduced by about $0.2-0.3$ km to values of $R_{1.4} = 12.1\pm 0.5$ km and $R_{2.1} = 11.9^{+0.5}_{-0.6}$ km (at the 68\% level), and central densities increase slightly to values of $n_c(1.4\,M_\odot)/n_0 = 2.8\pm 0.3$ and $n_c(2.1\,M_\odot)/n_0 = 3.8_{-0.7}^{+0.6}$ (in units of equilibrium nuclear matter density, $n_0 = 0.16$ fm$^{-3}$), i.e., they still fall below five times nuclear saturation density at the 68\% level. As a further significant result, the evidence established by analyzing Bayes factors for a negative trace anomaly measure, $Δ= 1/3-P/\varepsilon < 0$, inside heavy neutron stars is raised to strong.

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arXiv2024

Sizes of the Nucleon

Evidences are updated and strengthened for the two-scales picture of low-energy nucleon structure as a compact `hard' valence quark core surrounded by a `soft' cloud of quark-antiquark pairs (the meson cloud). These considerations are quantified by a spectral analysis of the mean-squared radii associated with the isoscalar and isovector electric form factors of the nucleon. Further supporting arguments come from corresponding studies of the axial and mass form factors and their inferred radii. Separating low-mass (mesonic) and high-mass (short-range) contributions in the spectral representations of each of these form factors, we conclude that a central core with an r.m.s. radius of about 1/2 fm results consistently as the common feature in all cases. Implications are discussed for baryonic matter at densities beyond that of equilibrium nuclear matter.

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arXiv2024

Constraints on Phase Transitions in Neutron Star Matter

Recent inference results of the sound velocity in the cores of neutron stars are summarized. Implications for the equation of state and the phase structure of highly compressed baryonic matter are discussed. In view of the strong constraints imposed by the heaviest known pulsars, the equation of state must be very stiff in order to ensure the stability of these extreme objects. This required stiffness limits the possible appearance of phase transitions in neutron star cores. For example, a Bayes factor analysis quantifies strong evidence for squared sound velocities $c_s^2 > 0.1$ in the cores of 2.1 solar-mass and lighter neutron stars. Only weak first-order phase transitions with a small phase coexistence density range $Δρ/ρ< 0.2$ (at the 68\% level) in a Maxwell construction still turn out to be possible within neutron stars. The central baryon densities in even the heaviest neutron stars do not exceed five times the density of normal nuclear matter. In view of these data-based constraints, much discussed issues such as the quest for a phase transition towards restored chiral symmetry, and the active degrees of freedom in cold and dense baryonic matter, are reexamined.

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arXiv2023

Evidence against a strong first-order phase transition in neutron star cores: impact of new data

With the aim of exploring the evidence for or against phase transitions in cold and dense baryonic matter, the inference of the sound speed and equation-of-state for dense matter in neutron stars is extended in view of recent new observational data. The impact of the heavy (2.35 $M_\odot$) black widow pulsar PSR J0952-0607 and of the unusually light supernova remnant HESS J1731-347 is inspected. In addition a detailed re-analysis is performed of the low-density constraint based on chiral effective field theory and of the perturbative QCD constraint at asymptotically high densities, in order to clarify the influence of these constraints on the inference procedure. The trace anomaly measure, $Δ= 1/3 - P/\varepsilon$, is also computed and discussed. A systematic Bayes factor assessment quantifies the evidence (or non-evidence) of low averaged sound speeds $(c_s^2 \leq 0.1)$, a prerequisite for a phase transition, within the range of densities realized in the core of neutron stars. One of the consequences of including PSR J0952-0607 in the data base is a further stiffening of the equation-of-state, resulting for a 2.1 solar-mass neutron star in a reduced central density of less than five times the equilibrium density of normal nuclear matter at the 68\% level. The evidence against small sound speeds in neutron star cores is further strengthened. Within the inferred 68\% posterior credible bands, only a weak first-order phase transition with a coexistence density interval $Δn/n \lesssim 0.2$ would be compatible with the observed data.

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arXiv2023

Sound velocity, equation of state and strangeness in neutron star matter

This presentation in two parts starts with a brief review on the speed of sound in neutron star matter as inferred from observational data. It is pointed out that, in view the strong constraints imposed by observed properties of the heaviest neutron stars, the equation of state must be very stiff. Stringent limits can be set on the possible appearance of phase transitions in neutron star cores. The second part discusses issues concerning the occurance of strangeness degrees of freedom, in particular of hyperons, in neutron stars. Here a focus is on the role of repulsive hyperon-nuclear three-body forces, potentially resolving the so-called hyperon puzzle.

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arXiv2022

Inference of the sound speed and related properties of neutron stars

Information on the phase structure of strongly interacting matter at high baryon densities can be gained from observations of neutron stars and their detailed analysis. In the present work Bayesian inference methods are used to set constraints on the speed of sound in the interior of neutron stars, based on recent multi-messenger data in combination with limiting conditions from nuclear physics at low densities. Two general parametric representations are introduced for the sound speed $c_s$ in order to examine the independence with respect to choices for the parametrisation of Priors. Credible regions for neutron star properties are analysed, in particular with reference to the quest for possible phase transitions in cold dense matter. The evaluation of Bayes factors implies extreme evidence for a violation of the conformal bound, $c_s^2 \leq 1/3$, inside neutron stars. Given the presently existing data base, it can be concluded that the occurrence of a first-order phase transition in the core of even a two-solar-mass neutron star is unlikely, while a continuous crossover cannot be ruled out. At the same time it is pointed out that the discovery of a superheavy neutron star with a mass $ M \sim 2.3 - 2.4\, M_\odot$ would strengthen evidence for a phase change in the deep interior of the star.

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arXiv2022

Equation of state and strangeness in neutron stars

A brief survey is presented of our present understanding of the equation-of-state of cold, dense matter and the speed of sound in the interior of neutron stars, based on the constraints inferred from observational data. The second part focuses on strangeness in baryonic matter and the role of hyperon-nuclear two- and three-body forces, with reference to the "hyperon puzzle" in neutron stars and possible scenarios for its solution.

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arXiv2022

Theory of kaon-nuclear systems

The strong interaction between an antikaon and a nucleon is at the origin of various interesting phenomena in kaon-nuclear systems. In particular, the interaction in the isospin $I=0$ channel is sufficiently attractive to generate a quasi-bound state, the $Λ(1405)$ resonance, below the $\bar{K}N$ threshold. Based on this picture, it may be expected that the $\bar{K}N$ interaction also generates quasi-bound states in kaon-nuclear systems, sometimes called kaonic nuclei. At the same time, the $\bar{K}N$ quasi-bound picture of the $Λ(1405)$ is also related to the discussion of hadronic molecules in hadron spectroscopy. Here an overview is presented of the theoretical studies developed for kaon-nucleon and kaon-nuclear systems. We start from the modern understanding of the $Λ(1405)$ resonance. We then discuss the $\bar{K}N$ interaction and various aspects of few-body kaonic nuclei. Heavier kaon-nuclear systems are examined from the viewpoint of nuclear many-body physics, with focus on the properties of antikaons in nuclear matter. Related topics, such as the $K^{-}p$ momentum correlation functions in high-energy collisions and the studies of kaonic atoms, are also discussed.

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arXiv2021

Fluctuations and phases in baryonic matter

The phase structure of baryonic matter is investigated with focus on the role of fluctuations beyond the mean-field approximation. The prototype test case studied is the chiral nucleon-meson model, with added comments on the chiral quark-meson model. Applications to the liquid-gas phase transition in nuclear matter and extensions to dense matter are performed. The role of vacuum fluctuations and thermal excitations is systematically explored. It is pointed out that such fluctuations tend to stabilise the hadronic phase characterised by spontaneously broken chiral symmetry, shifting the chiral restoration transition to very high densities. This stabilisation effect is shown to be further enhanced by additional dynamical fluctuations treated with functional renormalisation group methods.

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arXiv2020

Hard-core deconfinement and soft-surface delocalization from nuclear to quark matter

We propose a conceptual distinction between hard and soft realizations of deconfinement from nuclear to quark matter. In the high density region of Hard Deconfinement the repulsive hard cores of baryons overlap each other and bulk thermodynamics is dominated by the core properties that can be experimentally accessed in high-energy scattering experiments. We find that the equation of state estimated from a single baryon core is fairly consistent with those empirically known from neutron star phenomenology. We next discuss a novel concept of Soft Deconfinement, characterized by quantum percolation of quark wave-functions, at densities lower than the threshold for Hard Deconfinement. We make a brief review of quantum percolation in the context of nuclear and quark matter and illustrate a possible scenario of quark deconfinement at high baryon densities.

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arXiv2020

Hyperon-nucleon three-body forces and strangeness in neutron stars

Three-body forces acting on a $Λ$ hyperon in a nuclear medium are investigated, with special focus on the so-called hyperon puzzle in neutron stars. The hyperon-nucleon two-body interaction deduced from SU(3) chiral effective field theory is employed at next-to-leading order. Hyperon-nucleon three-body forces are approximated using saturation by decuplet baryons and are transcribed to density-dependent effective two-body interactions. These together are taken as input in a Brueckner-Bethe-Goldstone equation with explicit treatment of the $ΛN\leftrightarrowΣN$ and $ΛNN\leftrightarrowΣNN$ coupled channels. Single-particle potentials of a $Λ$ hyperon in symmetric nuclear matter and neutron matter are calculated. With parameters of the $ΛNN$ three-body force constrained by hypernuclear phenomenology, extrapolations to high baryon density are performed. By comparison of the $Λ$ and neutron chemical potentials at densities characteristic of the core of neutron stars it is found that the combined repulsive effects of two- and three-body correlations makes the appearance of $Λ$ hyperons in neutron stars energetically unfavourable, thus offering a possible solution to a longstanding question.

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arXiv2020

$K^-p$ Correlation Function from High-Energy Nuclear Collisions and Chiral SU(3) Dynamics

The two-particle momentum correlation function of a K^-p pair from high-energy nuclear collisions is evaluated in the KbarN-piSigma-piLambda coupled-channels framework. The effects of all coupled channels together with the Coulomb potential and the threshold energy difference between K^-p and Kbar0 n are treated completely for the first time. Realistic potentials based on the chiral SU(3) dynamics are used which fit the available scattering data. The recently measured correlation function is found to be well reproduced by allowing variations of the source size and the relative weight of the source function of piSigma with respect to that of KbarN. The predicted K^-p correlation function from larger systems, which is less affected by the piSigma source function, indicates that the investigation of its source size dependence is useful in providing further constraints in the study of the KbarN interaction.

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arXiv2020

Hyperon-nuclear interactions from SU(3) chiral effective field theory

The interaction between hyperons and nucleons has a wide range of applications in strangeness nuclear physics and is a topic of continuing great interest. These interactions are not only important for hyperon-nucleon scattering but also essential as basic input to studies of hyperon-nuclear few- and many-body systems including hypernuclei and neutron star matter. We review the systematic derivation and construction of such baryonic forces from the symmetries of quantum chromodynamics within non-relativistic SU(3) chiral effective field theory. Several applications of the resulting potentials are presented for topics of current interest in strangeness nuclear physics.

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arXiv2020

Continuity from neutron matter to two-flavor quark matter with $^1 S_0$ and $^3 P_2$ superfluidity

This study is performed with the aim of gaining insights into the possible applicability of the quark-hadron continuity concept, not only in the idealized case of three-flavor symmetric quark matter, but also for the transition from neutron matter to two-flavor quark matter. A key issue is the continuity between neutron superfluidity and a corresponding superfluid quark phase produced by $d$-quark pairing. Symmetry arguments are developed and relevant dynamical mechanisms are analyzed. It is pointed out that the $^3P_2$ superfluidity in dense neutron matter has a direct analogue in the $^3P_2$ pairing of $d$-quarks in two-flavor quark matter. This observation supports the idea that the quark-hadron continuity hypothesis may be valid for such systems. Possible implications for neutron stars are briefly discussed.

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arXiv2019

Neutron Star Matter as a Relativistic Fermi Liquid

The equation of state (EoS) of neutron star matter, constrained by the existence of two-solar-mass stars and gravitational wave signals from neutron star mergers, is analysed using the Landau theory of relativistic Fermi liquids. While the phase diagram of dense and cold QCD matter is still open for scenarios ranging from hadronic to quark matter in the center of neutron stars, a Fermi-liquid treatment is motivated by a microscopic approach starting from a chiral nucleon-meson field theory combined with nonperturbative functional renormalization group methods. In this scheme effects of multipionic fluctuations and repulsive nuclear many-body correlations suggest that the transition to chiral symmetry restoration is shifted to densities above those typically encountered in the neutron star core. Under such conditions a Fermi-liquid description in terms of nucleon quasiparticles appears to be justified. The leading Landau parameters are derived and discussed. Our results are contrasted with a well-known Fermi liquid, namely liquid $^3$He.

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arXiv2019

Dense Baryonic Matter and Strangeness in Neutron Stars

Recent developments of chiral effective field theory (ChEFT) applications to nuclear and neutron matter are summarized, with special emphasis on a (non-perturbative) extension using functional renormalisation group methods. Topics include: nuclear thermodynamics, extrapolations to dense baryonic matter and constraints from neutron star observables. Hyperon-nuclear interactions derived from SU(3) will be discussed with reference to the "hyperon puzzle" in neutron star matter.

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