arXiv subjects
Publications and source records attributed to Avraham Gal.
Diquark models have been applied with varying degree of success to tetraquark and pentaquark states involving both light and heavy quark degrees of freedom. We discuss the applicability of such models to light quark dibaryons, viewed as three-diquark objects. Highlighting the case of the d*(2380) dibaryon resonance, we demonstrate the inapplicability of diquark models in the light quark sector.
We present an effective field theory (EFT) at leading order to describe light single-$Λ$ hypernuclei. Owing to the weak $Λ$ binding and to the $ΛN$ short interaction range, meson exchange forces are approximated by contact interactions within a pionless EFT where the only degrees of freedom are baryons. At leading order, the $Λ$-nuclear interaction contains two 2-body (singlet and triplet) and three 3-body interaction terms, a total of 5 terms associated with 5 coupling strengths or low energy constants (LECs). We adopt the 2-body LECs from hyperon-nucleon scattering data and interaction models that constrain the $ΛN$ scattering lengths, while the 3-body LECs are adjusted using both 3-body and 4-body hypernuclear binding energies. To calculate the binding energies for A-body systems with A$>$2, we expand the wavefunctions using a correlated Gaussian basis. The stochastic variational method is employed to select the non-linear parameters. The resulting \nopieft~is then applied to calculate the $Λ$ separation energy in $_Λ^5$He, where the adjusted 3-body interactions largely resolve the known overbinding problem of $_Λ^5$He.
We address the $Λ$-hypernuclear `overbinding problem' in light hypernuclei which stands for a 1--3 MeV excessive $Λ$ separation energy calculated in $_Λ^5$He. This problem arises in most few-body calculations that reproduce ground-state $Λ$ separation energies in the lighter $Λ$ hypernuclei within various hyperon-nucleon interaction models. Recent pionless effective field theory nuclear few-body calculations are extended in this work to $Λ$ hypernuclei. At leading order, the $ΛN$ low-energy constants are associated with $ΛN$ scattering lengths and the $ΛNN$ low-energy constants are fitted to $Λ$ separation energies ($B_Λ^{\rm exp}$) for $A\leq 4$. The resulting pionless-EFT interaction reproduces in few-body stochastic variational method calculations the reported value $B_Λ^{\rm exp}(_Λ^5 $He)=3.12$\pm$0.02 MeV within a fraction of MeV over a broad range of momentum-space cut-off parameters. Possible consequences and extensions to heavier hypernuclei and to neutron-star matter are discussed.
A recent suggestion [PLB 774 (2017) 522] that purely-$Λ^{\ast}(1405)$ nuclei provide the absolute minimum energy in charge-neutral baryon matter for baryon-number $A\gtrsim 8$, is tested within RMF calculations. A broad range of $Λ^{\ast}$ interaction strengths, commensurate with $(\bar K \bar K NN)_{I=0}$ binding energy assumed to be of order 100 MeV, is scanned. It is found that the binding energy per $Λ^{\ast}$, $B/A$, saturates for $A\gtrsim 120$ with values of $B/A$ considerably below 100 MeV, implying that $Λ^{\ast}(1405)$ matter is highly unstable against strong decay to $Λ$ and $Σ$ hyperon aggregates. The central density of $Λ^{\ast}$ matter is found to saturate as well, at roughly twice nuclear matter density. Moreover, it is shown that the underlying very strong $\bar K N$ potentials, fitted for isospin $I=0$ to the mass and width values of $Λ^{\ast}(1405)$, fail to reproduce values of single-nucleon absorption fractions deduced across the periodic table from $K^-$ capture-at-rest bubble chamber experiments.
In this contribution, dedicated to the memory of Walter Greiner, we discuss the structure and width of the recently established d*(2380) dibaryon, confronting the consequences of our Pion Assisted Dibaryons hadronic model with those of quark motivated calculations. In particular, its relatively small width of about 70 MeV favors hadronic structure for the d*(2380) dibaryon rather than a six-quark structure.
Recent studies of $η$ nuclear quasibound states by the Jerusalem-Prague Collaboration are reviewed, focusing on stochastic variational method self consistent calculations of $η$ few-nucleon systems. These calculations suggest that a minimum value Re$\,a_{ηN} \approx 1$ fm (0.7 fm) is needed to bind $η\,^3$He ($η\,^4$He).
Charge symmetry breaking (CSB) is particularly strong in the A=4 mirror hypernuclei $_Λ^4$H--$_Λ^4$He. Recent four-body no-core shell model calculations that confront this CSB by introducing $Λ$-$Σ^0$ mixing to leading-order chiral effective field theory hyperon-nucleon potentials are reviewed, and a shell-model approach to CSB in p-shell $Λ$ hypernuclei is outlined.
$K^-$ mesons offer a unique setting where mesic atoms have been studied both experimentally and theoretically, thereby placing constraints on the possible existence and properties of meson-nuclear quasibound states. Here we review progress in this field made recently by the Jerusalem--Prague Collaboration using near-threshold $K^-N$ scattering amplitudes generated in several meson--baryon coupled channels models inspired by a chiral EFT approach. Our own procedure of handling subthreshold kinematics self consistently is used to transform these free-space energy dependent amplitudes to in-medium density dependent amplitudes from which $K^-$ optical potentials are derived. To fit the world data of kaonic atoms, these single-nucleon optical potentials are augmented by multi-nucleon terms. It is found that only two of the studied models reproduce also the single-nucleon absorption fractions available from old bubble chamber experiments. These two models are then checked for possible $K^-$ nuclear quasibound states, despite realizing that $K^-$ optical potentials are not constrained by kaonic atom data at densities exceeding half nuclear-matter density. We find that when such states exist, their widths are invariably above 100 MeV, forbiddingly large to allow observation. Multi-nucleon absorption is found to be substantial in this respect. This suggests that observable strongly bound $K^-$ mesons are limited to the very light systems, such as $K^-pp$.
Attempts to reproduce theoretically the relatively small width, $Γ_{d^{\ast}}=80\pm 10$ MeV, of the $I(J^P)=0(3^+)$ $d^{\ast}$(2380) dibaryon resonance established by the WASA-at-COSY Collaboration are discussed. The validity of associating the $d^{\ast}$(2380) in quark-based models primarily with a tightly bound $ΔΔ$ configuration is questioned. The $d^{\ast}$(2380) width and decay branching ratios into $NNππ$, $NNπ$ and $NN$ final states are studied within the Gal-Garcilazo hadronic model in which the $d^{\ast}$(2380) is a $πNΔ$ resonance embedded in the $NNππ$ continuum some 80 MeV below the $ΔΔ$ threshold. In particular, predictions are made for the branching ratios of the unobserved yet $d^{\ast}(2380)\to NNπ$ decays which are suppressed in a purely-$ΔΔ$ dibaryon model. Comments are also made on a possible connection of the ABC effect observed in the $pn\to d^{\ast}\to dπ^0π^0$ resonance reaction to the $d^{\ast}$(2380) dibaryon.
Several topics presented and discussed at MESON2016 are highlighted, including pentaquarks, dibaryons and meson-nuclear bound states.
Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].
Ongoing progress in understanding and evaluating charge symmetry breaking in $Λ$ hypernuclei is discussed in connection to recent measurements of the $_Λ^{4}{\rm H}(0^+_{\rm g.s.})$ binding energy at MAMI [A1 Collaboration: PRL 114 (2015) 232501] and of the $_Λ^{4}{\rm He}(1^+_{\rm exc})$ excitation energy at J-PARC [E13 Collaboration: PRL 115 (2015) 222501].
Charge symmetry breaking (CSB) in the $Λ$-nucleon strong interaction generates a charge dependence of $Λ$ separation energies in mirror hypernuclei, which in the case of the $A=4$ mirror hypernuclei $0^+$ ground states is sizable, $ΔB^{J=0}_Λ\equiv B^{J=0}_Λ (_Λ^4{\rm He}) -B^{J=0}_Λ(_Λ^4{\rm H})=230\pm 90$ keV, and of opposite sign to that induced by the Coulomb repulsion in light hypernuclei. Recent ab-initio calculations of the ($_Λ^4{\rm H},{_Λ^4{\rm He}}$) mirror hypernuclei $0^+_{\rm g.s.}$ and $1^+_{\rm exc}$ levels have demonstrated that a $Λ- Σ^0$ mixing CSB model due to Dalitz and von Hippel (1964) is capable of reproducing this large value of $ΔB^{J=0}_Λ$. These calculations are discussed here with emphasis placed on the leading-order chiral EFT hyperon-nucleon strong-interaction Bonn-Juelich potential model used and the no-core shell-model calculational scheme applied. The role of one-pion exchange in producing sizable CSB level splittings in the $A=4$ mirror hypernuclei is discussed.
We report on ab-initio NCSM calculations of the A=4 mirror Lambda hypernuclei Lambda-4H and Lambda-4He, using the Bonn-Juelich LO chiral EFT YN potentials plus a CSB Lambda0--Sigma0 mixing vertex. In addition to reproducing rather well the 0+ (g.s.) and 1+ (exc.) binding energies, these four-body calculations demonstrate for the first time that the observed CSB splitting of mirror levels, reaching hundreds of keV for 0+ (g.s.), can be reproduced using realistic theoretical interaction models, although with a non-negligible momentum cutoff dependence. Our results are discussed in relation to recent measurements of the Lambda-4H (0+ g.s.) binding energy [MAMI A1 Collaboration, Phys. Rev. Lett. 114, 232501 (2015)] and the Lambda-4He (1+ exc.) excitation energy [J-PARC E13 Collaboration, Phys. Rev. Lett. 115, 222501 (2015)].
Professor Richard H. Dalitz passed away on January 13, 2006. He was almost 81 years old and his outstanding contributions are intimately connected to some of the major breakthroughs of the 20th century in particle and nuclear physics. These outstanding contributions go beyond the Dalitz Plot, Dalitz Pair and CDD poles that bear his name. He pioneered the theoretical study of strange baryon resonances, of baryon spectroscopy in the quark model, and of hypernuclei, to all of which he made lasting contributions. His formulation of the "$θ-τ$ puzzle" led to the discovery that parity is not a symmetry of the weak interactions. A brief scientific evaluation of Dalitz's major contributions to particle and nuclear physics is hereby presented, followed by the first comprehensive list of his scientific publications, as assembled from several sources. The list is divided into two categories: the first, main part comprises Dalitz's research papers and reviews, including topics in the history of particle physics, biographies and reminiscences; the second part lists book reviews, public lectures and obituaries authored by Dalitz, and books edited by him. This provides the first necessary step towards a more systematic research of the Dalitz heritage in modern physics. The present 2016 edition updates the original 2006 edition, published in Nucl. Phys. A 771 (2006) 2-7, doi:10.1016/j.nuclphysa.2006.03.007, and 8-25, doi:10.1016/j.nuclphysa.2006.03.008, by including for the first time a dozen or so of publications, found recently in a list submitted to the Royal Society by Dalitz in 2004, that escaped our attention in the original version.
Neutrino signals in electron-capture decays of hydrogen-like parent ions P in storage-ring experiments at GSI are reconsidered, with special emphasis placed on the storage-ring quasi-circular motion of the daughter ions D in two-body decays P --> D + e-neutrino. It is argued that to the extent that daughter ions are detected, these detection rates might exhibit modulations with periods of order seconds, similar to those reported in the GSI storage-ring experiments for two-body decay rates [1,2]. New dedicated experiments in storage rings, or using traps, could explore these modulations.
We discuss a new type of L=0 positive-parity dibaryons, pion-B-B', where the dominant binding mechanism is provided by resonating p-wave pion-baryon interactions. Recent calculations of such pion assisted dibaryons are reviewed with special emphasis placed on the non-strange I(JP)=1(2+) N-Delta dibaryon D_{12}(2150) studied recently at JLab, and on the 0(3+) Delta-Delta dibaryon D_{03}(2380) discovered recently by the WASA-at-COSY Collaboration. We review recent searches by the HADES Collaboration at GSI and by the E15 and E27 Experiments at J-PARC for a strangeness S=-1 I(JP)=1/2(0-) K-pp dibaryon and perhaps also for a strange I(JP)=3/2(2+) N-Sigma(1385) pion assisted dibaryon Y_{3/2(2+)}(2270). Charm C=+1 dibaryons, predicted with these same I(JP) values, are also briefly reviewed.
A recent lattice QCD (LQCD) calculation of $Σ$-$Λ$ mixing by the QCDSF-UKQCD Collaboration [Phys. Rev. D 91, 074512 (2015)] finds a mixing angle about half of that found from the Dalitz-von Hippel (DvH) flavor SU(3) mass formula which relates the $Σ$-$Λ$ mixing matrix element to known octet baryon mass differences and which has been used widely to evaluate charge symmetry breaking effects in $Λ$ hypernuclei. We show that the LQCD-calculated $Σ$-$Λ$ mixing matrix element and octet baryon masses satisfy the DvH mass formula, concluding thereby that a good LQCD evaluation of $Σ$-$Λ$ mixing requires an equally good reproduction of octet baryon mass differences which is yet to be demonstrated.