arXiv subjects

A. Gal

Publications and source records attributed to A. Gal.

At least 73 records · Page 4Linked to original sources

arXiv2007

In-medium nuclear interactions of low-energy hadrons

Experimental and theoretical developments of the last decade in the study of exotic atoms and some related low-energy reactions are reviewed, in order to provide information on the in-medium hadron-nucleon t matrix over a wide range of densities up to central nuclear densities. In particular, we review pionic deeply bound atomic states and related evidence for partial restoration of chiral symmetry in dense nuclear matter. The case for relatively narrow deeply bound atomic states for antikaons and antiprotons is made, based on the physics of strong nuclear absorption. Recent experimental suggestions for signals of antikaon-nuclear deeply bound states are reviewed, and dynamical models for calculating binding energies, widths and densities of antikaon nuclear states are discussed. Specific features of low-energy in-medium interactions of kaons, antiprotons and of Sigma hyperons are discussed, and suggestions to study experimentally Cascade atoms are reviewed.

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arXiv2007

Faddeev calculation of a $K^- p p$ quasi-bound state

We report on the first genuinely three-body ${\bar K}NN - πΣN$ coupled-channel Faddeev calculation in search for quasi-bound states in the $K^- p p$ system. The main absorptivity in the $K^- p$ subsystem is accounted for by fitting to $K^- p$ data near threshold. Our calculation yields one such quasi-bound state, with $I=1/2$, $J^π=0^-$, bound in the range $B \sim 55-70$ MeV, with a width of $Γ\sim 95-110$ MeV. These results differ substantially from previous estimates, and are at odds with the $K^- p p \to Λp$ signal observed by the FINUDA collaboration.

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arXiv2006

$\bar K$ nuclear bound states in a dynamical model

A comprehensive data base of K- atom level shifts and widths is re-analyzed in order to study the density dependence of the Kbar-nuclear optical potential. Significant departure from a t*rho form is found only for nuclear densities about and less than 20% of nuclear-matter density, and extrapolation to nuclear-matter density yields an attractive potential, about 170 MeV deep. Partial restoration of chiral symmetry compatible with pionic atoms and low-energy pion-nuclear data plays no role at the relevant low-density regime, but this effect is not ruled out at high densities. Kbar-nuclear bound states are generated across the periodic table self consistently, using a relativistic mean-field model Lagrangian which couples the Kbar to the scalar and vector meson fields mediating the nuclear interactions. The reduced phase space available for Kbar absorption from these bound states is taken into account by adding an energy-dependent imaginary term which underlies the corresponding Kbar-nuclear level widths, with a strength required by fits to the atomic data. Substantial polarization of the core nucleus is found for light nuclei, and the binding energies and widths calculated in this dynamical model differ appreciably from those calculated for a static nucleus. A wide range of binding energies is spanned by varying the Kbar couplings to the meson fields. Our calculations provide a lower limit of Gamma(Kbar) = 50 +/- 10 MeV on the width of nuclear bound states for Kbar binding energy in the range B(Kbar) = 100 - 200 MeV. Comments are made on the interpretation of the FINUDA experiment at DAFNE, Frascati, which claimed evidence for deeply bound (K- pp) states in light nuclei.

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arXiv2005

Antiproton-nucleus potentials from global fits to antiprotonic X-rays and radiochemical data

We report on global fits of optical-model parameters to 90 data points for $\bar p$ X-rays and 17 data points of radiochemical data put together. With the help of separate fits to the two kinds of data it is possible to determine phenomenologically the radial region where the absorption of antiprotons takes place and to obtain neutron densities which represent the average behaviour over the periodic table. A finite-range attractive and absorptive $\bar p$-nuclear isoscalar potential fits the data well. Self-consistent dynamical calculations within the RMF model demonstrate that the polarization of the nucleus by the {\it atomic} antiproton is negligible.

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arXiv2005

Traces of Theta^+ pentaquark in K^+ nucleus dynamics

Long-standing anomalies in K^+ nucleus integral cross sections could be resolved by extending the impulse-approximation t*rho optical-potential framework to incorporate K^+ absorption on pairs of nucleons. Substantially improved fits to the data at p(lab)=500-700 MeV/c are obtained. An upper bound of 3.5 mb for the absorption cross section of K^+ per nucleon is derived. We conjecture that the underlying microscopic absorption process is K^+ n N --> Theta^+ N, where Theta^+(1540) is the newly discovered exotic Y=2, I=0, Z=1 pentaquark baryon, and estimate that the cross section for K^+ d --> Theta^+ p is a fraction of millibarn. Comments are made on Theta^+ production reactions on nuclei.

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arXiv2004

Widths of $\bar K$-nuclear deeply bound states in a dynamical model

The relativistic mean field (RMF) model is applied to a system of nucleons and a $\bar K$ meson, interacting via scalar and vector boson fields. The model incorporates the standard RMF phenomenology for bound nucleons and, for the $\bar K$ meson, it relates to low-energy ${\bar K}N$ and $K^-$ atom phenomenology. Deeply bound $\bar K$ nuclear states are generated dynamically across the periodic table and are exhibited for $^{12}$C and $^{16}$O over a wide range of binding energies. Substantial polarization of the core nucleus is found for these light nuclei. Absorption modes are also included dynamically, considering explicitly the reduced phase space for $\bar K$ absorption from deeply bound states. The behavior of the calculated width as function of the $\bar K$ binding energy is studied in order to explore limits on the possible existence of narrow $\bar K$ nuclear states.

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arXiv2004

Cluster models of Lambda-Lambda-6He and Lambda-9Be hypernuclei

Configuration space Faddeev calculations are performed for the binding energy of Lambda-Lambda-6He and Lambda-9Be bound states, here considered as alpha-Lambda-Lambda and alpha-alpha-Lambda clusters respectively, in order to study the dependence of the calculated binding energy on the alpha-Lambda potential input. For Lambda-Lambda-6He, using realistic interactions, the uncertainty in extracting the Lambda-Lambda S=L=0 interaction strength does not exceed 0.1 MeV, which is a fraction of the order of magnitude derived for other theoretical uncertainties. For Lambda-9Be, the dependence of the calculated binding energy on the alpha-Lambda potential is considerably larger, of order 1 MeV. Our results for Lambda-9Be suggest that the odd-state alpha-Lambda interaction is substantially reduced with respect to the even-state component.

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arXiv2003

Testing Chiral Dynamics in Pionic Atoms

The energy dependence of chirally expanded pi N isoscalar and isovector amplitudes b_0(E) and b_1(E) respectively, for zero-momentum off shell pions near threshold, is used to impose the minimal substitution requirement E -> E - V_c on the properly constructed pion optical potential within a large-scale fit to 100 pionic-atom data across the periodic table which also include the recently established `deeply bound' pionic atoms of Pb and Sn. This fit cannot be reconciled with the well known free-space values of the pi N threshold amplitudes. In contrast, introducing the empirically known energy dependence for on-shell pions leads to a better fit and to satisfactory values for the pi N threshold amplitudes. The difference between these two approaches is briefly discussed.

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arXiv2003

Antiprotonic Potentials from Global Fits to the PS209 Data

The experimental results for strong interaction effects in antiprotonic atoms by the PS209 collaboration consist of high quality data for several sequences of isotopes along the periodic table. Global analysis of these data in terms of a $\bar p$-nucleus optical potential achieves good description of the data using a s-wave finite-range \bar p N interaction. Equally good fits are also obtained with a poorly-defined zero-range potential containing a p-wave term.

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arXiv2003

Renormalization of the isovector $πN$ amplitude in pionic atoms

The extraction of the isovector s-wave pi N amplitude from pionic atoms is studied with special emphasis on uncertainties and their dependence on the assumptions made regarding the neutron density distributions in nuclei and on the size of the data base used . Only `global' analyses of pionic-atom data reveal a discrepancy between the extracted isovector s-wave pi N amplitude b_1=-0.108\pm0.007 m_π^{-1} and its free pi N counterpart b_1^{free}=-0.0885^{+0.0010}_{-0.0021} m_π^{-1}, where the uncertainty in the neutron densities is included in the error analysis. The role of `deeply bound' pionic atom states is discussed and the reason for failure of these states to provide new information is explained.

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arXiv2003

Faddeev calculations for the A=5,6 Lambda-Lambda hypernuclei

Faddev calculations are reported for Lambda-Lambda-5H, Lambda-Lambda-5He and Lambda-Lambda-6He in terms of two Lambda hyperons plus the respective nuclear clusters, using Lambda-Lambda central potentials considered in past non-Faddeev calculations of Lambda-Lambda-6He. The convergence with respect to the partial-wave expansion is studied, and comparison is made with some of these Lambda-Lambda hypernuclear calculations. The Lambda-Lambda <--> Xi-N mixing effect is briefly discussed.

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arXiv2002

The s-wave repulsion and deeply bound pionic atoms: fact and fancy

Fits to a large data set of pionic atoms show that the `missing' s-wave repulsion is accounted for when a density dependence suggested recently by Weise is included in the isovector term of the s-wave pion optical potential. The importance of using large data sets is demonstrated and the role of deeply bound pionic atom states is discussed.

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arXiv2002

To bind or not to bind: Lambda-Lambda hypernuclei and Xi hyperons

Four-body Faddeev-Yakubovsky calculations for Lambda-Lambda-p-n do not produce a bound state for Lambda-Lambda hydrogen 4, although suitably defined three-body Faddeev calculations for Lambda-Lambda-d produce a 1+ bound state for Lambda-Lambda interactions fitted to the Lambda-Lambda helium 6 emulsion event from KEK E373. This is opposite to the normal situation where a four-body Faddeev-Yakubovsky calculation yields stronger binding than that due to a suitably defined three-body Faddeev calculation. For stranger systems, Faddeev calculations using Lambda-Xi interactions which simulate the Nijmegen soft-core model NSC97 (which is close to reproducing the KEK event) suggest that Lambda-Xi helium 6 marks the onset of nuclear stability for Xi hyperons.

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arXiv2002

Faddeev-Yakubovsky search for Lambda-Lambda hydrogen-4

Evidence for particle stability of Lambda-Lambda hydrogen-4 (4LLH) has been suggested by the BNL-AGS E906 experiment. We report on Faddeev-Yakubovsky calculations for the four-body Lambda-Lambda-p-n system using Lambda-N interactions which reproduce the observed binding energy of Lambda hydrogen-3 (3LH) within a Faddeev calculation for the Lambda-p-n subsystem. No 4LLH bound state is found over a wide range of Lambda-Lambda interaction strengths, although the Faddeev equations for a three-body Lambda-Lambda-d model of 4LLH admit a 1+ bound state for as weak a Lambda-Lambda interaction strength as required to reproduce the binding energy of Lambda-Lambda Helium-6 (6LLHe).

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arXiv2002

Faddeev-Yakubovsky calculations for light Lambda-Lambda hypernuclei

New Faddeev-Yakubovsky calculations are reported for Lambda-Lambda-6He and Lambda-Lambda-10Be in terms of clusters of alpha's and Lambda's, using Lambda-Lambda s-wave potentials motivated by several of the Nijmegen model interactions. The self consistency of the Lambda-Lambda hypernuclear world data for these species is discussed. The newly reported Lambda-Lambda-6He event is found to be compatible with Lambda-Lambda interaction strengths provided by the Nijmegen soft-core one-boson-exchange model NSC97. Faddeev calculations for Lambda-Lambda-5H and Lambda-Lambda-5He suggest that these Lambda-Lambda hypernuclei are stable against emitting Lambda's for any (essentially attractive) Lambda-Lambda interaction, whereas calcualtions for Lambda-Lambda-4H do not allow a clear-cut conclusion whether or not it is particle stable.

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arXiv2002

Light Lambda-Lambda Hypernuclei and the Onset of Stability for Lambda-Xi Hypernuclei

New Faddeev-Yakubovsky calculations for light Lambda-Lambda hypernuclei are presented in order to assess the self consistency of the Lambda-Lambda hypernuclear binding-energy world data and the implied strength of the Lambda-Lambda interaction, in the wake of recent experimental reports on Lambda-Lambda-4H and Lambda-Lambda-6He. Using Gaussian soft-core simulations of Nijmegen one-boson-exchange model interactions, the Nijmegen soft-core model NSC97 simulations are found close to reproducing the recently reported binding energy of Lambda-Lambda-6He, but not those of other species. For stranger systems, Faddeev calculations of light Lambda-Xi hypernuclei, using a simulation of the strongly attractive Lambda-Xi interactions due to the same model, suggest that Lambda-Xi-6He marks the onset of nuclear stability for Xi hyperons.

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arXiv2001

Study of chirally motivated low-energy $K^-$ optical potentials

The $K^-$ optical potential in the nuclear medium is evaluated self consistently from a free-space $K^-N$ $t$ matrix constructed within a coupled-channel chiral approach to the low-energy $\bar K N$ data. The chiral-model parameters are fitted to a select subset of the low-energy data {\it plus} the $K^-$ atomic data throughout the periodic table. The resulting attractive $K^-$ optical potentials are relatively `shallow', with central depth of the real part about 55 MeV, for a fairly reasonable reproduction of the atomic data with $χ^2 / N \approx 2.2$. Relatively `deep' attractive potentials of depth about 180 MeV, which result in other phenomenological approaches with $χ^2 / N \approx 1.5$, are ruled out within chirally motivated models. Different physical data input is required to distinguish between shallow and deep $K^-$ optical potentials. The ($K^{-}_{\rm stop},π$) reaction could provide such a test, with exclusive rates differing by over a factor of three for the two classes of potentials. Finally, forward ($K^-,p$) differential cross sections for the production of relatively narrow deeply bound $K^-$ {\it nuclear} states are evaluated for deep $K^-$ optical potentials, yielding values considerably lower than those estimated before.

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arXiv2000

Unified optical-model approach to low-energy antiproton annihilation on nuclei and to antiprotonic atoms

A successful unified description of $\bar p$ nuclear interactions near E=0 is achieved using a $\bar p$ optical potential within a folding model, $V_{\rm opt} \sim \bar v * ρ$, where a $\bar p p$ potential $\bar v$ is folded with the nuclear density $ρ$. The potential $\bar v$ fits very well the measured $\bar p p$ annihilation cross sections at low energies ($p_L < 200$ MeV/c) and the $1s$ and $2p$ spin-averaged level shifts and widths for the $\bar p$H atom. The density-folded optical potential $V_{\rm opt}$ reproduces satisfactorily the strong-interaction level shifts and widths over the entire periodic table, for $A > 10$, as well as the few low energy $\bar p$ annihilation cross sections measured on Ne. Both $\bar v$ and $V_{\rm opt}$ are found to be highly absorptive, which leads to a saturation of reaction cross sections in hydrogen and on nuclei. 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 $E$ is 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. The relationship between different kinds of scattering lengths is discussed and previously published systematics of the $\bar p$ nuclear scattering lengths is updated.

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