arXiv subjects
Publications and source records attributed to Avraham Gal.
The main features of coupled-channel Kbar-N dynamics near threshold and its repercussions in few-body Kbar-nuclear systems are briefly reviewed highlighting the I=1/2 Kbar-N-N system. For heavier nuclei, the extension of mean-field calculations to multi-Kbar nuclear quasibound states is discussed focusing on kaon condensation.
Explicit expressions for the parity-violating s-wave and the parity-conserving p-wave contributions to pi- weak-decay rates of Lambda hypernuclei in the 1p shell are given in the weak-coupling limit, to update previous shell-model calculations and to compare with recent pi- spectra and total decay rates measured by the FINUDA Collaboration. A useful sum rule for the summed strength of Lambda_{1s} --> pi- + proton_{1p} weak decays is derived. Fair agreement between experiment and calculations is reached, using the primary s-wave amplitude and Cohen-Kurath nuclear wavefunctions. The role of the p-wave amplitude is studied in detail for Lambda-15N and found to be secondary. Previous assignments of ground-state spin-parity values are confirmed and a new assignment is made, spin-parity 3/2+ for Lambda-15N, based on the substantial suppression calculated here for spin-parity 1/2+.
Selected topics in Strangeness Nuclear Physics are reviewed: Lambda-hypernuclear spectroscopy and structure, Sigma-hyperon nuclear interaction, multistrangeness, and anti-kaons in nuclei.
Comments are made on Akaishi, Myint and Yamazaki's interpretation of antikaonic quasibound nuclear states as generalized Kapur-Peierls decaying states [arXiv:0805.4382]. We argue that these `intrinsic decaying states' have little to do with low-energy antikaon-nucleon dynamics.
We have searched for bound states in the pion-Lambda-nucleon system by solving the nonrelativistic Faddeev equations, as well as a relativistic version, with input separable pion-nucleon, pion-Lambda, and Lambda-nucleon interactions. A bound-state solution, driven by the Delta(1232) and Sigma(1385) p-wave meson-baryon resonances, was found in the channel (I,JP)=(3/2,2+), provided the Lambda lab momentum at which the Lambda-nucleon S=1,L=0,J=1 phase shift becomes negative is larger than about 750-800 MeV/c. Other strange and charmed pion-baryon-baryon systems that might have bound states of a similar nature are listed.
The major contributions of Richard H. Dalitz to hypernuclear physics, since his first paper in 1955 to his last one in 2005 covering a span of 50 years during which he founded and led the theoretical study of hypernuclei, are reviewed from a personal perspective. Topical remarks on the search for quasi-bound antikaon-nuclear states and on kaon condensation are made.
Experimental evidence for antikaon-nuclear quasibound states is briefly reviewed. Theoretical and phenomenological arguments for and against deep antikaon-nucleus potentials which might allow for narrow quasibound states are reviewed, with recent calculations suggesting widths larger than 100 MeV for binding energy smaller than 100 MeV. Results of RMF calculations that provide a lower limit of 50+/-10 MeV for the width of deeply bound states are discussed.
The major contributions of Richard H. Dalitz to hypernuclear physics, since his first paper in 1955 to his last one in 2005 covering a span of 50 years during which he founded and led the theoretical study of hypernuclei, are reviewed from a personal perspective. Topical remarks on the search for quasi-bound antikaon-nuclear states are made.
Experimental evidence for antikaon-nuclear quasi-bound states is briefly reviewed. Theoretical and phenomenological arguments for and against the existence of such states are considered, based on constructing antikaon-nuclear optical potentials from various sources. Results of RMF calculations that provide a lower limit of Gamma(antikaon) = 50 +/- 10 MeV on the width of antikaon-nuclear quasi-bound states are discussed.
Multiple-scattering approximations to Faddeev calculations of the K^- d scattering length are reviewed and compared with published Kbar-N-N <--> pi-Y-N fully reactive Faddeev calculations. A new multiple-scattering approximation which goes beyond the `fixed-center' assumption for the nucleons is proposed, aiming at accuracies of 5-10%. A precise value of the K^- d scattering length from the measurement of the K^- d 1s atomic level shift and width, planned by the DEAR/SIDDHARTA collaboration, plus a precise value for the K^- p scattering length by improving the K^- p atom measurements, are essential for extracting the K^- n scattering length, for resolving persistent puzzles in low-energy Kbar-N phenomenology and for extrapolating into Kbar-nuclear systems.
Following the prediction by Akaishi and Yamazaki of relatively narrow $\bar K$-nuclear states, deeply bound by over 100 MeV where the main decay channel $\bar K N \to πΣ$ is closed, several experimental signals in stopped $K^-$ reactions on light nuclei have been interpreted recently as due to such states. In this talk I review (i) the evidence from $K^-$-atom data for a {\it deep} $\bar K$-nucleus potential, as attractive as $V_{\bar K}(ρ_0) \sim -(150 - 200)$ MeV at nuclear matter density, that could support such states; and (ii) the theoretical arguments for a {\it shallow} potential, $V_{\bar K}(ρ_0) \sim -(40 - 60)$ MeV. I then review a recent work by Mareš, Friedman and Gal in which $\bar K$-nuclear bound states are generated dynamically across the periodic table, using a RMF Lagrangian that couples the $\bar K$ to the scalar and vector meson fields mediating the nuclear interactions. Substantial polarization of the core nucleus is found for light nuclei, with central nuclear densities enhanced by almost a factor of two. The binding energies and widths calculated in this dynamical model differ appreciably from those calculated for a static nucleus. These calculations provide a lower limit of $Γ_{\bar K} \sim 50 \pm 10$ MeV on the width of nuclear bound states for $\bar K$ binding energy in the range $B_{\bar K} = 100 - 200$ MeV.
Recent experiments on production of Lambda-Lambda hypernuclei have renewed interest in extracting the Lambda-Lambda interaction from the few events identified since the inception of this field forty years ago. Few-body calculations relating to this issue are reviewed, particularly with respect to the possibility that A=4 marks the onset of Lambda-Lambda binding in nuclei. The Nijmegen soft-core model potentials NSC97 qualitatively agree with the strength of the Lambda-Lambda interaction deduced from the newly determined binding energy of Lambda-Lambda-6He. Applying the extended NSC97 model to stranger nuclear systems suggests that A=6 marks the onset of Xi binding, with a particle stable Lambda-Xi-6He, and that strange hadronic matter is robustly bound.
Using to a minimum extent special relativity input, and relying on the Lorentz-force expression for the force acting on a charged particle in motion under the influence of electric (E) and magnetic (B) fields, the Maxwell curl equations are shown to follow from the covariance of the two Maxwell divergence equations ${÷\cdot E} = 4πρ$ and ${÷\cdot B} = 0$.
Antiproton-nucleus optical potentials fitted to $\bar p$-atom level shifts and widths are used to calculate the recently reported very low energy ($p_{L}<100$ MeV/c) $\bar p$ cross sections for annihilation on light nuclei. The apparent suppression of annihilation upon increasing the atomic charge $Z$ and mass number $A$ is resolved as due to the strong effective repulsion produced by the very absorptive optical potential which keeps the $\bar p$-nucleus wavefunction substantially outside the nuclear surface, so that the resulting reaction cross section saturates as function of the strength of Im $V_{\rm opt}$. This feature, for $E >0$, parallels the recent prediction, for $E < 0$, that the level widths of $\bar p$ atoms saturate and, hence, that $\bar p$ deeply bound atomic states are relatively narrow. Predictions are made for $\bar p$ annihilation cross sections over the entire periodic table at these very low energies and the systematics of the calculated cross sections as function of $A$, $Z$ and $E$ are discussed and explained in terms of a Coulomb-modified strong-absorption model. Finally, optical potentials which fit simultaneously low-energy $\bar p - ^4$He observables for $E < 0$ as well as for $E > 0$ are used to assess the reliability of extracting Coulomb modified $\bar p$ nuclear scattering lengths directly from the data.
Recent optical-potential studies of the phenomenology of kaonic atoms are reviewed. It is shown that the data can be fitted by a complex optical potential with either a relatively shallow attractive component (about -50 MeV at nuclear-matter density), as suggested by a self consistent application of chirally motivated coupled-channels models, or a relatively deep attractive component (about -180 MeV at nuclear-matter density) as suggested by a mean-field extrapolation of a phenomenological low-density expansion. Both classes of these optical potentials, due to their strongly absorptive component, predict similarly structured relatively narrow `deeply bound' kaonic-atom states with widths saturating at about 2 MeV. Possible formation reactions are briefly discussed. The more speculative case for relatively narrow deeply bound kaonic nuclear states is briefly mentioned. Recent works on other strange hadronic atom systems, for the negatively charged Sigma and Cascade hyperons, are also discussed.
An effective equation of state (EoS) which generalizes the Lattimer-Swesty equation for nuclear matter is presented for matter at supernuclear densities including strange baryons. It contains an adjustable baryon potential energy density, based on models of local potentials for the baryon-baryon interactions. The features of the equation rely on the properties of nuclei for the nucleon-nucleon interactions, and mainly on experimental data from hypernuclei for the hyperon-nucleon and hyperon-hyperon interactions. The EoS is used to calculate equilibrium compositions and thermodynamic properties of high density matter with strangeness in two astrophysical contexts: neutron star matter (transparent to neutrinos) and proto-neutron star matter (opaque to neutrinos). The effective EoS reproduces typical properties of high density matter found in theoretical microscopic models. Of these, the main result is that hyperons appear in both types of matter at about twice the nuclear saturation density, and that their appearance significantly softens the EoS. The range of maximal masses of neutron stars found in a comprehensive parameter survey is 1.4-1.7 $M_\odot$. Another typical result is that the maximal mass of a proto-neutron star with strange baryons is higher than that of an evolved neutron star (opposite to the case of nuclear matter), setting the stage for a "delayed collapse" scenario.