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A. Gal

Publications and source records attributed to A. Gal.

At least 37 records · Page 2Linked to original sources
arXiv2017

Onset of $η$-nuclear binding in a pionless EFT approach

$ηNNN$ and $ηNNNN$ bound states are explored in stochastic variational method (SVM) calculations within a pionless effective field theory (EFT) approach at leading order. The theoretical input consists of regulated $NN$ and $NNN$ contact terms, and a regulated energy dependent $ηN$ contact term derived from coupled-channel models of the $N^{\ast}(1535)$ nucleon resonance plus a regulated $ηNN$ contact term. A self consistency procedure is applied to deal with the energy dependence of the $ηN$ subthreshold input, resulting in a weak dependence of the calculated $η$-nuclear binding energies on the EFT regulator. It is found, in terms of the $ηN$ scattering length $a_{ηN}$, that the onset of binding $η\,^3$He requires a minimal value of Re$\,a_{ηN}$ close to 1 fm, yielding then a few MeV $η$ binding in $η\,^4$He. The onset of binding $η\,^4$He requires a lower value of Re$\,a_{ηN}$, but exceeding 0.7 fm.

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arXiv2017

Onset of $η$-meson binding in the He isotopes

The onset of binding $η$(548) mesons in nuclei is studied in the He isotopes by doing precise $ηNNN$ and $ηNNNN$ few-body stochastic variational method calculations for two semi-realistic $NN$ potentials and two energy dependent $ηN$ potentials derived from coupled-channel models of the $N^{\ast}(1535)$ nucleon resonance. The energy dependence of the $ηN$ subthreshold input is treated self consistently. It is found that a minimal value of the real part of the $ηN$ scattering length $a_{ηN}$ close to 1 fm is required to bind $η$ mesons in $^3$He, yielding then a few MeV $η$ binding in $^4$He. The onset of $η$-meson binding in $^4$He requires that Re$\,a_{ηN}$ exceeds 0.7 fm approximately. These results compare well with results of recent $ηNNN$ and $ηNNNN$ pionless effective field theory calculations. Related optical-model calculations are also discussed.

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arXiv2016

$K^- N$ amplitudes below threshold constrained by multinucleon absorption

Six widely different subthreshold $K^- N$ scattering amplitudes obtained in SU(3) chiral-model EFT approaches by fitting to low-energy and threshold data are employed in optical-potential studies of kaonic atoms. Phenomenological terms representing $K^-$ multinucleon interactions are added to the EFT inspired single-nucleon part of the $K^-$-nucleus optical potential in order to obtain good fits to kaonic-atom strong-interaction level shifts and widths across the periodic table. Introducing as a further constraint the fractions of single-nucleon $K^-$ absorption at rest from old bubble-chamber experiments, it is found that only two of the models considered here reproduce these absorption fractions. Within these two models, the interplay between single-nucleon and multinucleon $K^-$ interactions explains features observed previously with fully phenomenological optical potentials. Radial sensitivities of kaonic atom observables are also re-examined, and remarks are made on the role of `subthreshold kinematics' in absorption at rest calculations.

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arXiv2016

Strangeness in nuclear physics

Extensions of nuclear physics to the strange sector are reviewed, covering data and models of Lambda and other hypernuclei, multi-strange matter, and anti-kaon bound states and condensation. Past achievements are highlighted, present unresolved problems discussed, and future directions outlined.

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arXiv2015

Antinucleon-nucleus interaction near threshold from the Paris $\bar NN$ potential

A general algorithm for handling the energy dependence of hadron-nucleon amplitudes in the nuclear medium, consistently with their density dependence, has been recently applied to antikaons, eta mesons and pions interacting with nuclei. Here we apply this approach to antiprotons below threshold, analyzing experimental results for antiprotonic atoms across the periodic table. It is also applied to antiproton and antineutron interaction with nuclei up to 400 MeV/c, comparing with elastic scattering and annihilation cross sections. The underlying $\bar pN$ scattering amplitudes are derived from the Paris $\bar NN$ potential, including modifications in the medium. Emphasis is placed on the role of the $P$-wave amplitudes with respect to the repulsive $S$-wave amplitudes.

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arXiv2015

Few-body calculations of $η$-nuclear quasibound states

We report on precise hyperspherical-basis calculations of $ηNN$ and $ηNNN$ quasibound states, using energy dependent $ηN$ interaction potentials derived from coupled-channel models of the $S_{11}$ $N^{\ast}(1535)$ nucleon resonance. The $ηN$ attraction generated in these models is too weak to generate a two-body bound state. No $ηNN$ bound-state solution was found in our calculations in models where Re $a_{ηN}\lesssim 1$ fm, with $a_{ηN}$ the $ηN$ scattering length, covering thereby the majority of $N^{\ast}(1535)$ resonance models. A near-threshold $ηNNN$ bound-state solution, with $η$ separation energy of less than 1 MeV and width of about 15 MeV, was obtained in the 2005 Green-Wycech model where Re $a_{ηN}\approx 1$ fm. The role of handling self consistently the subthreshold $ηN$ interaction is carefully studied.

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arXiv2014

Pion-assisted charmed dibaryon candidate

The Lambda_c(2286)-N system is studied in a chiral constituent quark model and the resulting s-wave interaction is used in separable form within three-body models of the pi-Lambda_c-N system with quantum numbers (C,I,JP)=(+1,3/2,2+). Separable interactions are also used for the dominant p-wave pion-baryon channels dominated by the Delta(1232) and Sigma_c(2520) resonances. Faddeev equations with relativistic kinematics are solved on the real axis to search for bound states and in the complex plane to search for three-body resonances. Some of the models considered generate a very narrow bound state, requiring isospin violation for its strong decay. Other models lead to a narrow resonance (Gamma < 0.4 MeV) for resonance mass below the Sigma_c(2455)-N threshold. This would be the lowest-lying C=+1 dibaryon, with mass estimated approximately as 3370+/-15 MeV.

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arXiv2014

Testing in-medium $πN$ dynamics on pionic atoms

A general algorithm for handling the energy dependence of meson-nucleon amplitudes in the nuclear medium has been recently applied to antikaons and to eta mesons. Here we test this approach on $πN$ amplitudes in pionic atoms where direct comparison can be made with ample experimental results. Applying this algorithm to a large-scale fit of 100 pionic-atom data points across the periodic table, which also include the `deeply-bound' states in Sn and Pb, reaffirms earlier conclusions on the density-dependent renormalization of the $πN$ threshold isovector amplitude $b_1$, or equivalently the renormalization of the pion decay constant $f_π$ in the nuclear medium.

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arXiv2014

In-medium Eta-Nucleon interactions and Eta nuclear bound states

The in-medium Eta-N interaction near and below threshold is constructed from a free-space chirally-inspired meson-baryon coupled-channel model that captures the physics of the N(1535) baryon resonance. Nucleon Pauli blocking and hadron self-energies are accounted for. The resulting energy dependent in-medium interaction is used in self-consistent dynamical calculations of Eta nuclear bound states. Narrow states of width about or less than 2 MeV are found across the periodic table, beginning with A=10, for this in-medium coupled-channel interaction model. The binding energy of the 1s-Eta state increases with A, reaching a value of B(1s-Eta) about 15 MeV. The implications of our self-consistency procedure are discussed with respect to procedures used in other works.

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arXiv2013

Baryon Number Violation

This report, prepared for the Community Planning Study - Snowmass 2013 - summarizes the theoretical motivations and the experimental efforts to search for baryon number violation, focussing on nucleon decay and neutron-antineutron oscillations. Present and future nucleon decay search experiments using large underground detectors, as well as planned neutron-antineutron oscillation search experiments with free neutron beams are highlighted.

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arXiv2013

Eta-nuclear bound states revisited

The strong energy dependence of the s-wave eta-N scattering amplitude at and below threshold, as evident in coupled-channels K-matrix fits and chiral models that incorporate the S11 N*(1535) resonance, is included self-consistently in eta-nuclear bound-state calculations. This approach, applied recently in calculations of kaonic atoms and Kbar-nuclear bound states, is found to impose stronger constraints than ever on the onset of eta-nuclear binding, with a minimum value of Re a_{eta N} approximately 0.9 fm required to accommodate an eta-4He bound state. Binding energies and widths of eta-nuclear states are calculated within several underlying eta-N models for nuclei across the periodic table, including eta-25Mg for which some evidence was proposed in a recent COSY experiment.

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arXiv2013

Kaonic atoms and in-medium K-N amplitudes II: interplay between theory and phenomenology

A microscopic kaonic-atom optical potential $V^{(1)}_{K^-}$ is constructed, using the Ikeda-Hyodo-Weise NLO chiral $K^-N$ subthreshold scattering amplitudes constrained by the kaonic hydrogen SIDDHARTA measurement, and incorporating Pauli correlations within the Waas-Rho-Weise generalization of the Ericson-Ericson multiple-scattering approach. Good fits to kaonic atom data over the entire periodic table require additionally sizable $K^-NN$--motivated absorptive and dispersive phenomenological terms, in agreement with our former analysis based on a post-SIDDHARTA in-medium chirally-inspired NLO separable model by Cieplý and Smejkal. Such terms are included by introducing a phenomenological potential $V^{(2)}_{K^-}$ and coupling it self consistently to $V^{(1)}_{K^-}$. Properties of resulting kaonic atom potentials are discussed with special attention paid to the role of $K^-$-nuclear absorption and to the extraction of density-dependent amplitudes representing $K^-$ multi-nucleon processes.

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arXiv2012

Search for the neutron-rich hypernucleus 9ΛHe

Search for the neutron-rich hypernucleus 9LHe is reported by the FINUDA experiment at DAFNE, INFN-LNF, studying (pi+, pi-) pairs in coincidence from the K-stop + 9Be --> 9LHe + pi+ production reaction followed by 9LHe --> 9Li + pi- weak decay. An upper limit of the production rate of 9LHe undergoing this two-body pi- decay is determined to be (2.3 +/- 1.9) 10-6/K-stop at 90% confidence level.

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arXiv2012

First observation of the hyper superheavy hydrogen 6ΛH

Three candidate events of the neutron-rich hypernucleus 6ΛH were uniquely identified in the FINUDA experiment at DAΦNE, Frascati, by observing π+ mesons from the (K-stop,π+) production reaction on 6Li targets, in coincidence with π-mesons from 6ΛH \rightarrow 6He+π- weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9\pm2.0)\cdot10-6/K- stop for the 6ΛH production rate times the two-body π- weak decay branching ratio. The 6ΛH binding energy with respect to 5H + Λ was determined jointly from production and decay to be BΛ = (4.0 \pm 1.1) MeV, assuming that 5H is unbound with respect to 3H + 2n by 1.7 MeV. The binding energy determined from production is higher, in each one of the three events, than that determined from decay, with a difference of (0.98 \pm 0.74) MeV here assigned to the 0+g.s. \rightarrow 1+ excitation. The consequences of this assignment to Λ hypernuclear dynamics are briefly discussed.

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arXiv2012

Realistic calculations of Kbar-N-N, Kbar-N-N-N, and Kbar-Kbar-N-N quasibound states

Binding energies and widths of three-body KbarNN, and of four-body KbarNNN and KbarKbarNN nuclear quasibound states are calculated in the hyperspherical basis, using realistic NN potentials and subthreshold energy dependent chiral KbarN interactions. Results of previous K^-pp calculations are reproduced and an upper bound is placed on the binding energy of a K^-d quasibound state. A self consistent handling of energy dependence is found to restrain binding, keeping the calculated four-body ground-state binding energies to relatively low values of about 30 MeV. The lightest strangeness -2 particle-stable Kbar nuclear cluster is most probably KbarKbarNN. The calculated Kbar N -> pi Y conversion widths range from approximately 30 MeV for the KbarNNN ground state to approximately 80 MeV for the KbarKbarNN ground state.

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arXiv2012

Kaonic atoms and in-medium $K^-N$ amplitudes

Recent work on the connection between in-medium subthreshold $K^-N$ amplitudes and kaonic atom potentials is updated by using a next to leading order chirally motivated coupled channel separable interaction model that reproduces $\bar KN$ observables at low energies, including the very recent SIDDHARTA results for the atomic $K^-$-hydrogen $1s$ level shift and width. The corresponding $K^-$-nucleus potential is evaluated self-consistently within a single-nucleon approach and is critically reviewed with respect to empirical features of phenomenological optical potentials. The need to supplement the single-nucleon based approach with multi-nucleon interactions is demonstrated by showing that additional empirical absorptive and dispersive terms, beyond the reach of chirally motivated $K^-$-nucleus potentials, are required in order to achieve good agreement with the bulk of the data on kaonic atoms.

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