Search

Physical review accelerators and beams : PRAB 26(1), 014201 (2023). doi:10.1103/PhysRevAccelBeams.26.014201, Published by American Physical Society, College Park, MD

Physical review accelerators and beams 24(12), 124601 (2021). doi:10.1103/PhysRevAccelBeams.24.124601, We investigated coherent betatron oscillations of a deuteron beam in the storage ring cooler synchrotron and storage ring, excited by a detuned rf Wien filter (WF). The beam oscillations were detected by conventional beam position monitors. With the currently available apparatus, we show that oscillation amplitudes down to 1 ��m can be detected. The interpretation of the response of the stored beam to the detuned rf WF is based on simulations of the beam evolution in the lattice of the ring and realistic time-dependent 3D field maps of the WF. Future measurements of the electric dipole moment of protons will, however, require control of the relative position of counter-propagating beams in the sub-picometer range. Since here the stored beam can be considered as a rarefied gas of uncorrelated particles, we moreover demonstrate that the amplitudes of the zero-point (ground state) betatron oscillations of individual particles are only a factor of about 10 larger than the Heisenberg uncertainty limit. As a consequence of this, we conclude that quantum mechanics does not preclude the control of the beam centroids to sub-picometer accuracy. The smallest Lorentz force exerted on a single particle that we have been able to determine is 10 aN., Published by American Physical Society, College Park, MD

We report on the search for the rare decay η → π 0 e + e − which is of interest to study C violation in the electromagnetic interaction which would indicate contributions from physics beyond the Standard Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Julich, Germany. In total 3 × 10 7 events of the reaction p d → 3 He η have been recorded at an excess energy of Q = 59.8 MeV . Based on this data set the C parity violating decay η → π 0 γ ⁎ → π 0 e + e − via a single-photon intermediate state has been searched for, resulting in new upper limits of Γ ( η → π 0 e + e − ) / Γ ( η → π + π − π 0 ) 3.28 × 10 − 5 and Γ ( η → π 0 e + e − ) / Γ ( η → all ) 7.5 × 10 − 6 ( CL = 90 % ), respectively.

A nonzero electric dipole moment (EDM) of the neutron, proton, deuteron or helion, in fact, of any finite system necessarily involves the breaking of a symmetry, either by the presence of external fields (i.e. electric fields leading to the case of induced EDMs) or explicitly by the breaking of the discrete parity and time-reflection symmetries in the case of permanent EDMs. We discuss two theorems describing these phenomena and report about the cosmological motivation for an existence of CP breaking beyond what is generated by the Kobayashi-Maskawa mechanism in the Standard Model and what this might imply for the permanent electric dipole moments of the nucleon and light nuclei by estimating a window of opportunity for physics beyond what is currently known. Recent - and in the case of the deuteron even unpublished - results for the relevant matrix elements of nuclear EDM operators are presented and the relevance for disentangling underlying New Physics sources are discussed., Comment: 20 pages, chapter for the memorial book "Gerry Brown 90", final version, some typos corrected

The electric dipole moments of the nucleon and light ions are discussed and strategies for disentangling the underlying sources of CP violation beyond the Kobayashi-Maskawa quark-mixing mechanism of the Standard Model are indicated. Contribution to "45 years of nuclear theory at Stony Brook: a tribute to Gerald E. Brown"., Comment: 14 pages, 2 figures, contribution to "45 years of nuclear theory at Stony Brook: a tribute to Gerald E. Brown"

The presently world largest data sample for π 0 → γ e + e − decays studies containing nearly 5 × 10 5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the π 0 → γ U → γ e + e − decay from the p p → p p π 0 reaction was carried out. An upper limit on the square of the U − γ mixing strength parameter ϵ 2 of 5 × 10 − 6 at 90% CL was obtained for the mass range 20 MeV M U 100 MeV . This result together with other recent experimental limits significantly reduces the M U vs. ϵ 2 parameter space which could explain the presently seen deviation between the Standard Model prediction and the direct measurement of the anomalous magnetic moment of the muon.

An exclusive measurement of the decay eta -> pi(+) pi(-) gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd -> He-3 X at a proton be ...

We present a new, model-independent method to analyze radiative decays of mesons to a vector, isovector pair of pions of invariant mass square below the first significant ππ threshold in the vector channel. It is based on a combination of chiral perturbation theory and dispersion theory. This allows for a controlled inclusion of resonance physics without the necessity to involve vector meson dominance explicitly. As an example, the method is applied to an analysis of the reactions η → π + π − γ and η ′ → π + π − γ .

In the first production run of the WASA experiment at COSY, the eta decay into three neutral pions was measured in proton-proton interactions at a proton beam kinetic energy of 1.4 GeV. The Dalitz ...

In recent years a two-scale expansion was established to study reactions of the type NN → NN π within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this Letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator.

Isospin-breaking effects in nuclear matter are studied in the framework of a medium-modified Skyrme model. The proposed effective Lagrangian incorporates both the medium influence of the surrounding nuclear environment on the single nucleon properties and an explicit isospin-breaking effect in the mesonic sector. The approach predicts that the neutron-proton mass difference decreases in isospin-symmetric nuclear matter but by a very small amount only., 8 pages, 4 figures, revised version

J. Bsaisou, J. de Vries, C. Hanhart, S. Liebig, Ulf-G. Meisner, D. Minossi, A. Nogga and A. Wirzba Institute for Advanced Simulation, Institut fur Kernphysik, and Julich Center for Hadron Physics, Forschungszentrum Julich, D-52425 Julich, Germany JARA — Forces and Matter Experiments, Forschungszentrum Julich, D-52425 Julich, Germany JARA — High Performance Computing, Forschungszentrum Julich, D-52425 Julich, Germany Helmholtz-Institut fur Strahlenund Kernphysik and Bethe Center for Theoretical Physics, Universitat Bonn, D-53115 Bonn, Germany

We report on how to tackle the problem of establishing a chiral effective field theory in nuclear matter with explicit pion fields and in the presence of external sources (Ann. Phys. 297, 27 (2002)). We have made use of the results of J.A. 011er (Phys. Rev. C 65, 025204 (2002)) where the generating functional for the in-medium chiral SU(2) x SU(2) Lagrangian has been derived. Within this approach we develop the so-called standard power counting rules for the calculation of in-medium pion properties if the residual nucleon energies are of the order of the pion mass. In addition, for the case of vanishing residual nucleon energies, a modified scheme (non-standard counting) is introduced. For both schemes the pertinent scales where the chiral expansions have to break down are established as well. We have performed a systematic analysis of n-point in-medium Green functions up to and including next-to-leading order when the standard rules apply. These include the in-medium contributions to quark condensates, pion propagators, pion masses and couplings of the axial-vector, vector and pseudoscalar currents to pions.

We provide a consistent and complete calculation of the electric dipole moments of the deuteron, helion, and triton in the framework of chiral effective field theory. The CP-conserving and CP-violating interactions are treated on equal footing and we consider CP-violating one-, two-, and three-nucleon operators up to next-to-leading-order in the chiral power counting. In particular, we calculate for the first time EDM contributions induced by the CP-violating three-pion operator. We find that effects of CP-violating nucleon-nucleon contact interactions are larger than those found in previous studies based on phenomenological models for the CP-conserving nucleon-nucleon interactions. Our results which apply to any model of CP violation in the hadronic sector can be used to test various scenarios of CP violation. As examples, we study the implications of our results on the QCD $\theta$-term and the minimal left-right symmetric model., Comment: 23 pages, 3 figures, 4 tables, corresponds to the published version (including erratum - see Appendix C)

We show that the proposed measurements of the electric dipole moments of light nuclei in storage rings would put strong constraints on models of flavor-diagonal CP violation. Our analysis is exemplified by a comparison of the Standard Model including the QCD theta term, the minimal left-right symmetric model, a specific version of the so-called aligned two-Higgs doublet model, and briefly the minimal supersymmetric extension of the Standard Model. By using effective field theory techniques we demonstrate to what extend measurements of the electric dipole moments of the nucleons, the deuteron, and helion could discriminate between these scenarios. We discuss how measurements of electric dipole moments of other systems relate to the light-nuclear measurements., Comment: Published version

QCD superconductors in the color-flavor-locked (CFL) phase support light excitations (generalized pions) in the form of particle–particle or hole–hole excitations. We analyze the generalized process π 0 → γ γ in the weak coupling limit and show that it is related to the recently suggested Wess–Zumino–Witten (WZW) term. In dense QCD, the radiative decay of the generalized pion is constrained by geometry and vanishes at large density.

The two-nucleon contributions to the electric dipole moment (EDM) of the deuteron, induced by the QCD $\theta$-term, are calculated in the framework of effective field theory up-to-and-including next-to-next-to-leading order. In particular we find for the difference of the deuteron EDM and the sum of proton and neutron EDM induced by the QCD $\theta$-term a value of $(-5.4\pm 3.9)\,\bar\theta\times 10^{-4}\, e$ fm. The by far dominant uncertainty comes from the CP- and isospin-violating $\pi NN$ coupling constant., Comment: 14 pages, 6 figures, EPJ style, discussion improved, conclusions unchanged

We calculate the branching ratios of the yet unmeasured eta' decays into four pions, based on a combination of chiral perturbation theory and vector-meson dominance. The decays eta' --> 2(pi+ pi-) and eta' --> pi+ pi- 2pi0 are P-wave dominated and can largely be thought to proceed via two rho resonances; we predict branching fractions of (1.0+-0.3)*10^-4 and (2.4+-0.7)*10^-4, respectively, not much lower than the current experimental upper limits. The decays eta' --> 4pi0 and eta --> 4pi0, in contrast, are D-wave driven as long as conservation of CP symmetry is assumed, and are significantly further suppressed; any experimental evidence for the decay eta --> 4pi0 could almost certainly be interpreted as a signal of CP violation. We also calculate the CP-violating amplitudes for eta' --> 4pi0 and eta --> 4pi0 induced by the QCD theta-term.

We determine the $N_c$--dependence of the resonance contributions to the Adler--Weisberger sum rule for the inverse square $1/g_A^2$ of the axial charge coupling constant and show that in the large $N_c$ limit the contributions of the Roper-like excitations scale as $O(1/N_c)$. Consistency with the $1/N_c^2$ scaling of the $1/g_A^2$ term in the sum rule requires these contributions to cancel against each other.

The scattering of a point panicle in two dimensions from two (or three) equally-sized (and spaced) circular hard disks is one of the simplest classically hyperbolic scattering problems. Because of this simplicity such systems are well suited for the study of the semiclassical periodic-orbit approximation in the cycle expansion of the dynamical zeta function applied to a quantum-mechanical scattering problem. Especially the predictions of the semiclassical cycle expansion for the quantum-mechanical resonances can be tested in these n -disk systems. Whereas for high wave numbers the cycle expansion gives quite accurate results, there are systematic deviations for low wave numbers from the exact quantum-mechanical values. The low-lying quantum-mechanical resonance poles of the 2- and 3-disk problem are constructed and compared to the cycle-expansion results. The characteristic determinant of the scattering matrix is expanded in terms of simple traces which in turn are related to the classical periodic orbits and possible creeping contributions. It will be shown that for large separations of the disks the correct resonance-pole positions can be extracted just from the knowledge of the lowest traces whose semiclassical limit are the fundamental periodic orbits. Creeping-orbit corrections are shown to be small.

We show that color can leak from a QCD bag if we allow for pseudoscalar isoscalar singlet ($\eta'$) coupling at the surface. To enforce total confinement of color an additional boundary term is suggested. New relations between the $\eta'$ mass and decay constant and the QCD gluon condensates are derived and compared with the empirical parameters., Comment: 7 pages, LaTeX, Nordita - 92/68 N

Using the Cheshire cat principle, we present simple arguments for describing the parameters of η′ (flavor singlet pseudoscalar) from QCD. New relations relating the η′ mass and decay constant to the QCD gluon condensates are derived and compared with the empirical parameters.

We show that color can leak from a QCD bag if we allow for a pseudoscalar singlet coupling at the surface, much like the baryon number in chiral bag models. To enforce total confinement of color an additional boundary term is suggested.

The quantization rules for meson-soliton bound systems are discussed within the framework of the bound state approach to strange and heavy flavored baryons. It is shown that the soliton has to change its quantization according to the number of mesons that are bound to it. As a consequence of this, the quantization rule and the quantum numbers of the baryons in the bound state approach are uniquely determined.

The neutron-proton mass difference in finite nuclei is studied in the framework of a medium-modified Skyrme model. The possible interplay between the effective nucleon mass in finite nuclei and the Nolen-Schiffer anomaly is discussed. In particular, we find that a correct description of the properties of mirror nuclei leads to a stringent restriction of possible modifications of the nucleon's effective mass in nuclei., Comment: 13 pages, 6 figures (some typos corrected and references added)

We investigate small-amplitude fluctuations about B =1 and B =2 soliton solutions of the Skyrme lagrangian on a 3-sphere S 3 ( L ) of radius L . In the B =1 problem we explicitly construct all the eigenvalues and eigenfunctions for the uniform background and all the asymptotic eigenvalues for the localized background. The unstable, zero and conformal modes are discussed in detail and a physical interpretation is given. In the B =2 problem we present the unstable and zero modes.

Isospin-breaking effects in the baryonic sector are studied in the framework of a medium-modified Skyrme model. The neutron-proton mass difference in infinite, asymmetric nuclear matter is discussed. In order to describe the influence of the nuclear environment on the skyrmions, we include energy-dependent charged and neutral pion optical potentials in the s- and p-wave channels. The present approach predicts that the neutron-proton mass difference is mainly dictated by its strong part and that it strongly decreases in neutron matter., 11 pages, 6 figures; some new references added

We consider the spin-isospin-independent central part of the residual nucleon-nucleon potential in finite spherical nuclei taking into account the deformation effects of the nucleons within the surrounding nuclear environment. It is shown that inside the nucleus the short-range repulsive contribution of the potential is increased and the intermediate attraction is decreased. We identify the growth of the radial component of the spin-isospin independent short-range part of the in-medium nucleon-nucleon interaction as the responsible agent that prevents the radial collapse of the nucleus., 9 pages, 3 eps figures

We report on how to tackle the problem of establishing a chiral effective field theory in nuclear matter with explicit pion fields and in the presence of external sources (Ann. Phys. 297. 27 (2002)). We have made use of the results of J.A. Oller (Phys. Rev. C 65, 025204 (2002)) where the generating functional for the in-medium chiral SU (2) x SU (2) Lagrangian has been derived. Within this approach we develop the so-called standard power counting rules for the calculation of in-medium pion properties if the residual nucleon energies are of the order of the pion mass. In addition, for the case of vanishing residual nucleon energies. a modified scheme (non-standard counting) is introduced. For both schemes the pertinent scales where the chiral expansions have to break down are established as well. We have performed a systematic analysis of n-point in-medium Green functions up to and including next-to-leading order when the standard rules apply. These include the in-medium contributions to quark condensates, pion propagators, pion masses and couplings of the axial-vector, vector and pseudoscalar currents to pions.

We investigate the possible modifications of the nucleons' electromagnetic form factors in the framework of a modified Skyrme model allowing for nucleon deformation and using realistic nuclear mass distributions. We show that such effects are small in light nuclei., Comment: 5 pp, 2 figures

The deformation of a nucleon embedded in various finite nuclei is considered by taking into account the distortion of the chiral profile functions under the action of an external field representing the nuclear density. The baryon charge distribution of the nucleon inside light, medium-heavy and heavy nuclei is discussed. The mass of the nucleon decreases as it is placed deeper inside the nucleus and reaches its minimum at the center of the nucleus. We discuss the quantization of non-spherical solitons and its consequences for the mass splitting of the delta states. We show that bound nucleons acquire an intrinsic quadrupole moment due to the deformation effects. These effects are maximal for densities of nuclei about \rho(R)\sim 0.3...0.35 \rho(0). We also point out that scale changes of the electromagnetic radii can not simply be described by an overall swelling factor., Comment: 29 pp, REVTeX, 8 figures, more detailed discussion on quantization and intrinsic quadrupole moments, references added

We investigate the relevance of the instanton-induced determinantal 't Hooft interaction to the eta-nucleon coupling within the framework of a three-flavor linear sigma model in the OZI-rule-respecting basis. Instantons, in combination with the spontaneous breaking of chiral symmetry, are shown to provide the major mechanism for the ideal mixing between pseudoscalar strange and non-strange quarkonia. As long as 't Hooft's interaction captures most of the basic features of the axial QCD gluon anomaly, we identify the anomaly as the main culprit for the appearance of octet flavor symmetry in the anomalous sectors of the pseudoscalar (and axial vector) mesons. Within this context, unitary spin is shown to be an accidental symmetry due to anomalous gluon dynamics rather than a fundamental symmetry in its own right. Though we find the eta-nucleon coupling constant to obey a Goldberger-Treiman like relation, the latter does not take its origin from a pole dominance of the induced pseudoscalar form factor of the octet axial current, but from a subtle flavor-mixing mechanism that is traced back to instanton dynamics. The model presented allows for possible generalizations to non-ideal mixing angles and different values of the meson decay constants in the strange and non-strange sectors, respectively. Finally, we discuss the issue as to what extent the eta meson may be considered as a Goldstone boson under the constraints of the anomaly-produced unitary spin., Comment: 28 pages, LaTeX, 4 figures.ps, uses elsart.cls and graphicx

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the $^3$He(e,e'$\pi^+)^3$H reaction have been measured in the excitation region of the $\Delta$ resonance to determine the longitudinal ($L$), transverse ($T$), and the $LT$ interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and $\Delta$-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the $\pi^+$ mass of $\Delta m_{\pi^+} = (-1.7^{+ 1.7}_{- 2.1})$ MeV/c$^2$ in the neutron-rich nuclear medium at a density of $\rho = (0.057^{+ 0.085}_{- 0.057})$ fm$^{-3}$. Our data are consistent with the $\Delta$ self energy determined from measurements of $\pi^0$ photoproduction from $^4$He and heavier nuclei., Comment: Elsart, 12 pages and 4 figures, Correspondent: Professor Dr. Dr. h.c. mult. Achim Richter, richter@ikp.tu-darmstadt.de, submitted to Phys. Rev. Lett

We discuss the Overhauser effect (particle-hole pairing) versus the BCS effect (particle-particle or hole-hole pairing) in QCD at large quark density. In weak coupling and to leading logarithm accuracy, the pairing energies can be estimated exactly. For a small number of colors, the BCS effect overtakes the Overhauser effect, while for a large number of colors the opposite takes place, in agreement with a recent renormalization group argument. In strong coupling with large pairing energies, the Overhauser effect may be dominant for any number of colors, suggesting that QCD may crystallize into an insulator at a few times nuclear matter density, a situation reminiscent of dense Skyrmions. The Overhauser effect is dominant in QCD in 1+1 dimensions, although susceptible to quantum effects. It is sensitive to temperature in all dimensions., 20 pages, 1 figure

QCD superconductors in the color-flavor-locked (CFL) phase support excitations (generalized mesons) that can be described as pairs of particles or holes (rather than particle-hole) around a gapped Fermi surface. In weak coupling and to leading logarithm accuracy the scalar and pseudoscalar excitations are massless and the vector and axial-vector excitations are massive and degenerate. The massless scalar excitations are combined with the longitudinal gluons leading to the Meissner effect in the CFL phase. The mass of the composite vector excitations is close to twice the gap in weak coupling, but goes asymptotically to zero with increasing coupling thereby realizing Georgi's vector limit in cold and dense matter. We discuss the possible mixing of the composite scalar and vector excitations with the gluons, their possible coupling to the modified photons and their decay into light pseudoscalars in the CFL phase. The issue of hidden gauge-symmetry in the QCD superconductor is critically examined. The physical implications of our results on soft dilepton and neutrino emission in cold and dense matter are briefly discussed., Comment: 43 pages, 4 figures, further discussions and references added

QCD superconductors in the color-flavor-locked (CFL) phase sustain light Goldstone modes (that will be referred to as generalized pions) that can be described as pairs of particle and/or hole excitations around a gapped Fermi surface. In weak coupling and to leading logarithm accuracy, their form factor, mass and decay constant can be evaluated exactly. These modes are found to satisfy an axial-Ward-identity, constraining the mass of the Goldstone modes in the CFL phase., Comment: 13 pages, 4 figures, version to be published in PLB

Two-point functions related to the pion weak decay constant f_\pi are calculated from the generating functional of chiral perturbation theory in the mean-field approximation and the heavy-baryon limit. The aim is to demonstrate that Lorentz invariance is violated in the presence of background matter. This fact manifests itself in the splitting of both f_\pi and the pion mass into uncorrelated time- and spacelike parts. We emphasize the different in-medium renormalizations of the correlation functions, show the inequivalence between the in-medium values of f_\pi deduced from Walecka-type models, on the one hand, and QCD sum rules, on the other hand, and elaborate on the importance for some nuclear physics observables., Comment: 14 pages, RevTex, no figures, to appear in Nucl.Phys. A

The generating functional of heavy baryon chiral perturbation theory at order O(Q^2) in the mean field approximation (with a pseudoscalar source coupling which is consistent with the PCAC-Ward identities on the current quark level) has been exploited to derive Migdal's in--medium pion propagator. It is shown that the prediction for the density dependence of the quark condensate obtained on the composite hadron level by embedding PCAC within the framework of Migdal's approach to finite Fermi systems is identical to that resulting from QCD., replaced with published version; 16 pages, LaTeX{revtex}, no figures

Chiral effective lagrangians may differ in their prediction of meson-nucleon scattering amplitudes off-meson-mass-shell, but must yield identical S-matrix elements. We argue that the effective meson mass in nuclear matter obtained from chiral effective lagrangians is also unique. Off-mass-shell amplitudes obtained using the PCAC choice of pion field must therefore not be viewed as fundamental constraints on the dynamics, the determination of the effective meson mass in nuclear matter or the possible existence of meson condensates in the ground state of nuclear matter. This hypothesis is borne out by a calculation of the effective mass in two commonly employed formulations of chiral perturbation theory which yield different meson-nucleon scattering amplitudes off-meson-mass-shell., Comment: 23 pages, LaTeX, 2 Postscript figures (fig1.ps, fig2.ps)

Dibaryons configurations are studied in the framework of the bound state soliton model. A generalized axially symmetric ansatz is used to determine the soliton background. We show that once the constraints imposed by the symmetries of the lowest energy torus configuration are satisfied all spurious states are removed from the dibaryon spectrum. In particular, we show that the lowest allowed state in the $S=-2$ channel carries the quantum numbers of the H particle. We find that, within our approximations, this particle is slightly bound in the model. We discuss, however, that vacuum effects neglected in the present calculation are very likely to unbind the H., 24 pages, LaTeX, TAN-FNT-93-12 (it replaces old version which was truncated)

We consider baryon-number-two solutions to the Skyrme effective lagrangian on the surface of a hypersphere, S 3 ( L ). At high densities we find solutions with “radially” uniform energy and baryon densities analogous to solutions previously found for the baryon-number-one problem on S 3 ( L ) and for the dense matter problem in R 3 . The density for this phase transition is essentially the same in the three cases. An interpretation of this phase transition as a simulation of “deconfinement” is offered.

The 18O(p, α)15N reaction has been investigated in the energy range Ep = 72–935 keV. The three known resonances above Ep = 620 keV have been confirmed and four new resonances have been found below Ep = 340 keV. All observed resonances correspond to known compound states in 19F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 109 K. The energy averaged astrophysical reaction rates are compared with predictions.

Angular distributions of neutron polarization from the 14 C(p, n) 14 N and 11 B(α, n) 14 N reactions have been studied for the particle energies E p = 1.788, 2.025, 2.272 and 2.450 MeV, and E α = 2.049 MeV. The polarization was derived from the left-right asymmetry induced by elastic scattering from 4 He. Together with existing measurements of angular distributions and total cross sections for several reaction channels leading to 15 N with an excitation energy between 11.5 and 12.5 MeV, these data were used to deduce from R -matrix analysis a set of resonance parameters for the 15 N levels in this energy range.

Applications of the Skyrme model to dense baryonic matter reveal a transition from a low-density phase of localized skyrmions to a high-density phase possessing an additional half-skyrmion symmetry. We suggest that this transition should be regarded as the restoration of chiral symmetry at high densities. This is based on (i) the vanishing of the matrix element for pion decay and (ii) the appearance of parity doublets in both the strange and non-strange sectors at high density and (iii) the appearance of a triplet of massless “Goldstone bosons” at low density.

Double- Δ (1232) formation following pion absorption in and above the 3.3 resonance region is shown to provide an important absorption mechanism for heavy nuclei, if the Δ N → ΔΔ interaction behaves according to SU (4) quark model rules. The ΔΔ configuration is formed preferentially in T = 2 states and decays predominantly by emission of four nucleons, unlike the standard π NN → Δ N → NN two-nucleon process. We show that this mechanism is able to explain some of the striking features observed in proton emission spectra following π ± -nucleus absorption in the Δ region.

The electric quadrupole to magnetic dipole transition ratio E2/M1 for the reaction γN → Δ(1232) is calculated in a modified Skyrme model, which includes stabilizing fourth- and sixth-order terms. The coefficients of these terms are determined from vector meson couplings in the static limit; the other parameters are given by their physical values. We obtain for E2/M1 a negative sign and a magnitude which is consistent with the empirical value.

We examine the model dependence and nuclear-structure sensitivity of several {Delta}{sub 33}-dominated processes contributing to pion double charge exchange on nuclei in the region of the {Delta}{sub 33} resonance. These processes include the {Delta}{sub 33}-nucleon interaction {ital V}{sub {ital N}{Delta}} and sequential scattering, in which the pion undergoes single charge exchange on two different nucleons. In all cases, the scattering takes place through the exchange of an intermediate {pi} and {rho} meson. Sequential-mediated double charge exchange is found to be only moderately sensitive to short-range correlations, meson-nucleon form factors, and the rho meson, whereas {ital V}{sub {ital N}{Delta}}-mediated double charge exchange is very sensitive to all these effects. Results are given for double charge exchange on {sup 18}O (double isobaric analog transitions) and {sup 16}O (nonanalog transitions). Sequential double charge exchange is shown to favor non-spin-flip matrix elements of the transition operator whereas {ital V}{sub {ital N}{Delta}}-mediated double charge exchange favors spin-flip matrix elements. The energy dependence of the zero-degree cross sections for {ital V}{sub {ital N}{Delta}} and sequential scattering are also different: Sequential tends to increase monotonically from 100 to 300 MeV, whereas {ital V}{sub {ital N}{Delta}} peaks at about 150 MeV. The delta-nucleon interaction is found likely tomore » dominate over sequential scattering in nonanalog double charge exchange. The {ital V}{sub {ital N}{Delta}} is also large in analog double charge exchange, but it does not enable us to explain the anomalous behavior of the {sup 18}O differential cross sections.« less

The reaction12C+12C has been studied in the energy rangeE cm=2.45–6.15 MeV byγ-ray spectroscopy. Gamma-ray transitions from a large number of excited states in20Ne,23Na and23Mg were observed, which show strong and rapid yield variations. When the influence of the Coulomb barrier is removed, these structures appear superimposed on a flat reaction yield, which does not show a strong increase at low energies, in contrast to previous work. These results obviate the need for the hypothesis of absorption under the barrier at least down toE cm=2.45 MeV. The nuclear and astrophysical aspects of the data are discussed.

The sensitivity of pion double charge exchange to nuclear structure in the shell model is examined. We consider the nonanalog ground-state to ground-state (T = 0 to T = 2) transitions on a variety of nuclear targets. The reaction model, which assumes that double charge exchange is driven by a ..pi..+rho-meson--mediated, ..delta../sub 33/-nucleon interaction, reproduces the energy dependence and shape of the empirical cross section for all the (..pi../sup +/,..pi../sup -/) transitions examined. The magnitude of the cross section is particularly sensitive to collectivity in the nuclear wave functions. All parameters of the model, with the exception of the range of the meson-..delta../sub 33/ form factor, are fixed by theoretical arguments. The form factor is then determined by normalizing to the /sup 16/O(..pi../sup +/,..pi../sup -/) /sup 16/ Ne cross section. Predictions are made for a variety of other low-lying 0/sup +/ transitions.