Carl B. Dover

The physics of nucleon-antinucleon annihilation

Abstract The nucleon-antinucleon (NN) annihilation process provides a fertile testing ground for microscopic hadron exchange and/or quark models. We review a variety of such approaches here, including models which treat the underlying quark-gluon degrees of freedom explicitly. A first principles calculation of NN annihilation in the framework of non-perturbative quantum chromodynamics (QCD) is still beyond our reach, so we adopt a more phenomenological approach, in which we try various annihilation topologies combined with different prescriptions for the dependence of the effective quark-antiquark (QQ) creation/destruction operator on spin, flavor and color. The NN system offers a rich ensemble of annihilation channels, whose relative branching ratios BR provide strong constraints on dynamical models. Recent experiments at LEAR show that values of BR display a significant dependence on L, the NN relative orbital angular momentum, spin S and isospin I. These dynamical selection rules (DSR), i.e. the suppression of transitions allowed a priori by conservation of the quantum numbers {J,π,C,G}, provide key signatures of the annihilation mechanism, and suggest dynamical content beyond simple statistical or SU(2)/SU(3) flavor symmetry models. We investigate to what extent the observed DSR enable us to unravel the quark dynamics of the annihilation process.

HYPERNUCLEAR SPECTROSCOPY IN THE P SHELL

Abstract A comprehensive shell-model approach to Λ-hypernuclear spectroscopy in the p shell is developed. The available data on the spectra of 9ΛBe, 12ΛC, 13ΛC, 14ΛN and 16ΛO are interpreted in this framework, leading to constraints on the residual ΛN interaction and the one-body Λ-nucleus potential. The mechanism for the formation of Λ hypernuclei via the (K−, π−) reaction is treated in the relativistic distorted wave approximation, with careful attention paid to Fermi-averaging of the elementary K−n → π−Λ amplitude and recoil corrections. Departures from the simple weak coupling picture, arising from configuration mixing, are emphasized. This leads to approximate dynamical symmetries in hypernuclei which are forbidden in ordinary nuclei by the Pauli principle. Further experiments in the p shell are suggested which may reveal other aspects of ΛN interactions.

The interaction of nucleons with antinucleons I. General features of the NN spectrum in potential models

We discuss the general features of the spectrum of antinucleon-nucleon (NN) bound states and resonances, using real potentials obtained via G-parity transformation from several realistic NN models. The main properties of the NN level ordering emerge from a model independent study of the coherences of meson exchange forces, which are complementary to those characterizing the NN system. However, the absolute energies of NN states, and the level density near threshold are very model dependent.

The nucleon-nucleus potential in the Hartree-Fock approximation with Skyrme's interaction

Abstract We investigate the connection between the self-consistent nuclear field in the Hartree-Fock (HF) approximation with the Skyrme interaction and the real part of the phenomenological nucleon-nucleus optical potential. We show that a simple local transformation u(r) = ( m ∗ (r) m ) 1 2 u L (r) of the radial wave function u(r) leads to an ordinary Schrodinger equation for uL(r). The energy-dependent potential VL(r, E) which appears in this equation is identified with the real part of the nucleon optical potential. We decompose VL(r, E) into a central part Vav, a symmetry part Vsym and a spin-orbit part Vs.o.. The radial form factors which characterize Vav, Vsym and Vs.o. are expressed in closed form in terms of the total nuclear density ϱ(r) and the neutron excess density Δϱ(r). Using force parameters obtained by fitting bound-state properties of closed-shell nuclei, we calculate VL numerically and compare to empirical potentials. We find that Vav strongly resembles a Woods-Saxon form, with depth V0 ≈ − 43 MeV, radius R 0 = 1.10A 1 3 + 0.75 fm and diffuseness a ≈ 0.55 fm ; V av also contains an energy-dependent part of depth 0.43 E and radius R0E = R0 − 0.25 fm. It is shown that Vsym is proportional to Δϱ(r) times a smooth function of the density. The calculated symmetry potential changes from a surface form for medium nuclei (Ca) to a surface plus volume for heavier nuclei (Zr, Sn, Pb). The form factor for Vs.o. is close to a Thomas form, but peaks at a radius inside that of the central potential.

On the production and spectroscopy of Σ hypernuclei

Abstract The production mechanisms and the spectroscopy of Σ hypernuclei are critically examined, with a view towards understanding the data on the interactions of the Σ with nucleons (N) and nuclei in a consistent framework. Data from Σ− atoms and quasi-free Σ production in the (K−, π±) reactions are used to derive information on the Σ-nucleus well depth. Starting from several parametrization of the ΣN effective interaction V ΣN in the nucleus, we perform a series of shell model calculations of the energy spectrum of the p-shell Σ hypernuclei. The models for VΣN are motivated by successful one-boson exchange (OBE) descriptions of the free space ΣN scattering data. Certain levels of the Σ hypernucleaus reflect the coherent energy shifts due to the strong spin and isopin dependence of VΣN. We isolate optimum experimental cases for the study of such coherent effects as well as other components of VΣN, for instance the spin-orbit part. If the single-particle Σ-nucleus potential is indeed deep enough to support a spectrum of quasi-stable states, which is not clear experimentally, their width in some cases depends sensitively on the isospin purity and the degree of configuration mixing induced by VΣN. The production cross sections of selected Σ hypernuclear states in the (K−, π±) reactions are estimated in distorted wave Born approximation (DWBA); these are also sensitive to deviations of the wave functions from pure isospin and/or j-j coupling limits. On the basis of our results, we suggest several experiments which may resolve some of the open questions concerning the ΣN and Σ-nucleus interactions.

The interaction of nucleons with antinucleons II. Narrow mesons near threshold—Experiments and theory

Abstract We compare and contrast several theoretical approaches which could account for a new class of narrow mesons recently observed in the mass region 1.5 – 3 GeV/c2, particularly those clustered near the antinucleon-nucleon ( N N ) threshold. We examine dual and quark models as well as N N potential models, focusing attention on differences between the quantum numbers of mesons predicted to lie near threshold. A preliminary comparison with data does not yet yield a strong preference for a particular model, although some crucial experiments are suggested. The main emphasis of this paper is on the meson spectrum predicted by N N potential models. In a previous work, referred to as I, we have discussed the level ordering of the spectrum for a number of realistic meson exchange models. In this sequel, we consider in more detail the candidates for narrow structures close to threshold. Estimates of the mixing of N N with other baryon-antibaryon configurations are given, as well as approximate values for the elastic and annihilation widths of these structures. We discuss criteria for identifying relatively pure N N states, as distinct from other multiquark complexes. We inquire to what extent useful constraints on the medium- and long-range nuclear forces could result from identification of an N N meson. Some speculations on the quantum numbers of the “new” mesons are presented.

Strange cluster formation in relativistic heavy ion collisions

Using the cascade code ARC to simulate relativistic heavy ion collisions at Brookhaven AGS energies (11.7--14.6 GeV/c), the authors have estimated the production rate of strange clusters ranging from a hypothetical doubly strange (S={minus}2) bound ({Lambda}{Lambda}){sub b} dibaryon to the hypernuclei {sub {Lambda}{Lambda}}{sup 6}He and {sub {Xi}{sup 0}{Lambda}{Lambda}}{sup 7}He. For the formation of multi-strange bound systems, high energy heavy ion collisions offer the only feasible method, since one can take advantage of the hyperons which are copiously produced in such collisions (typically 20 {Lambda}`s in a Au + Au central collision at the AGS) to form the composite object by coalescence.

On the production and decay of strangeness S = −2 hypernuclei

Abstract We examine the production and decay mechanisms of doubly strange S = −2 hypernuclei, focusing on reactions induced by kaons. The production of and ∗ (1530) hyperons in (K − , K + ) reactions on nuclear targets is considered, including the possibility of observably narrow single-particle bound states. We present estimates of the baryon (n, p, Λ) decay widths of states, based on calculated N → ΛΛ mixing amplitudes, which indicate that such excitations could be considerably narrower in light nuclei than the estimated decay width of 5–10 MeV in nuclear matter.

Formation of Lambda Lambda hypernuclei by Xi - capture in light nuclei.

We discuss the production of doubly strange {Lambda}{Lambda} hypernuclei in {Xi}{sup {minus}} atomic capture. The two-body {Xi}{sup {minus}}+{sup 6}Li{r arrow}{sub {Lambda}{Lambda}}{sup 6}He+{ital n} reaction is a relatively favorable case, with a yield of order 3%. For heavier {ital p}-shell targets, such as {sup 14}N, the (1{ital s}{sub {Lambda}}){sup 2} ground-state yields in the {Xi}{sup {minus}}+{sup {ital A}Z}{r arrow}{sub {Lambda}{Lambda}}{sup {ital A}}({ital Z}{minus}1)+{ital n} reaction are suppressed, and states of (1{ital s}{sub {Lambda}}1{ital p}{sub {Lambda}}) structure, coupled to an excited nuclear core, are preferentially populated.

A density expansion of the pion-nucleus optical potential. II. Second order terms☆

Abstract The second order terms in a density expansion of the pion optical potential Vopt are evaluated quantitatively. The coefficients of these terms are proportional to various combinations of on- and off-shell nucleon-nucleon T-matrices, averaged over the distribution of two nucleon relative momenta in the Fermi sea. The on-shell contributions can be obtained directly from experimental phase shifts, but the calculation of the off-shell-parts requires a model for the nucleon-nucleon potential. We consider a number of realistic local and nonlocal, separable potentials which fit nucleon-nucleon phase shifts, in order to study the variations in Vopt which arise from differences in the off-shell behavior of T. We find absorptive (imaginary) and dispersive (real) contributions to Vopt which are of comparable magnitude. The dispersive part, which leads to a real energy shift associated with the two nucleon absorptive process, has not been previously estimated quantitatively. We compare our results to empirical potentials obtained by fitting energy level shifts and widths in pi-mesic atoms, as well as theoretical estimates based on threshold cross sections for the processes π + N + N ⇄ N + N.