S-wave Resonances Research Articles

Coupled-channels optical calculation of positron—hydrogen resonances

An application of the coupled-channels optical method is given for the energy-dependent phenomena of positron-hydrogen resonances below the n = 2 excitation threshold. The equivalent local optical potential is used to account for the target polarization and positronium formation. The calculation includes 9 explicitly physical coupled channels. The lowest S-wave resonance energy position and new resonances are found. Angular dependence of the cross section in the resonance region are investigated.

Strongly interacting isotopic Bose-Fermi mixture immersed in a Fermi sea

We have created a triply quantum degenerate mixture of bosonic $^{41}$K and two fermionic species $^{40}$K and $^6$Li. The boson is shown to be an efficient coolant for the two fermions, spurring hopes for the observation of fermionic superfluids with imbalanced masses. We observe multiple heteronuclear Feshbach resonances, in particular a wide s-wave resonance for the combination $^{41}$K-$^{40}$K, opening up studies of strongly interacting {\it isotopic} Bose-Fermi mixtures. For large imbalance, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath.

Borromean three-body heteroatomic resonances

We present an approach to analyze recent experimental evidences of Efimov resonant states in mixtures of ultracold gases, by considering two-species three-body atomic systems bound in a Borromean configuration, where all the two-body interactions are unbound. For such Borromean three-body systems, it is shown that a continuum three-body s-wave resonance emerges from an Efimov state as a scattering length or a three-body scale is moved. The energy and width of the resonant state are determined from a scaling function with arguments given by dimension-less energy ratios relating the two-body virtual state subsystem energies with the shallowest three-body bound state. The peculiar behavior of such resonances is that their peaks are expected to move to lower values of the scattering length, with increasing width, as one raises the temperature. For Borromean systems, two resonant peaks are expected in ultralow-temperature regimes, which will disappear at higher energies. It is shown how a Borromean-Efimov excited bound state turns out to a resonant state by tuning the virtual two-body subsystem energies or scattering lengths, with all energies written in units of the next deeper shallowest Efimov state energy. The resonance position and width for the decay into the continuum are obtained as universal scaling functions (limit cycle) of the dimensionless ratios of the two and three-body scales, which are calculated numerically within a zero-range renormalized three-body model.

S-wave resonances in the positron–helium scattering

We have performed a three-body calculation for investigation of S-wave resonances in the positron–helium scattering in the hyperspherical coordinates. B-splines are used to obtain accurate hyperspherical potential curves and channel functions. An adiabatical hyperspherical approach is used to predict semi-quantitatively the position of the resonances. A multichannel calculation with the stabilization method is also used to extract accurately the positions and widths of resonances. We find four resonances associated with the He+–Ps(n = 2) channel. The resonance parameters for the lowest two resonances are in reasonable agreement with the calculations of Kar and Ho (2004 J. Phys. B: At. Mol. Opt. Phys. 37 3177), and two more new resonances below the Ps(n = 2) threshold are identified.

Low-lying resonances in the 16B nucleus

We investigate the 16B spectrum within a microscopic 15B + n model, involving several channels. The wave functions are defined in the Generator Coordinate Method, associated with the microscopic R-matrix formalism. We suggest that the narrow peak observed in recent experiments above the 15B + n threshold can be assigned to a 0− resonance which decays by d-wave neutron emission. The GCM calculation shows that this state cannot be reproduced in a single-channel B15(3/2−)+n model, and that excited configurations are necessary. Other 1− and 2−s-wave resonances are found at low excitation energies. In particular, we find a 1− state which represents a possible candidate for the 16B ground state. In general, our results show the importance of a multichannel framework. Comparisons with shell-model calculations are briefly discussed.

Determination of the 209Bi( [formula omitted]) 210Bi and 209Bi( [formula omitted]) 210m,gBi reaction cross sections in a cold neutron beam

Gamma-ray transitions following neutron capture in 209Bi have been studied at the cold neutron beam facility of the Budapest Neutron Centre. Measurements have been performed using a coaxial HPGe detector with Compton suppression. Partial capture cross sections at a velocity of 2200 m/s (or a neutron energy of 25.3 meV) have been deduced relatively to the one for the 4055 keV transition after thermal neutron capture in 209Bi. This partial cross section of (8.07 ± 0.14) mb has in turn been obtained from measurements with a bismuth nitrate target relatively to the cross section for the 1884 keV transition after thermal neutron capture in 14N. Shell model calculations have been performed to deduce the half-life of the 3rd excited state at 320 keV and to assign the M1 multipolarity of the 320 keV transition to the ground state. Lower limits of (21.4 ± 0.8) mb and (18.2 ± 0.7) mb for the cross sections to the ground state and to the isomeric state, respectively, have been deduced from the data for the gamma rays feeding these states. The results obtained in this work are in good agreement with experimental data reported in the literature. The thermal total capture cross section has been compared with the contribution due to observed s-wave resonances.

Cross section for the subbarrier fission of 244Cm

The cross section for 244Cm fission induced by neutrons of energy in the range between 0.07 eV and 20 keV was measured by using the lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow). The parameters of the resonance areas were determined for the lowest eight s-wave neutron resonances, and the respective fission widths were evaluated. Also, the parameters of the intermediate structure in the cross section for the subbarrier fission of 244Cm nuclei were evaluated. The results were compared with available data and recommendations based on evaluations.

Above threshold s-wave resonances illustrated by the [formula omitted] states in 9Be and 9B

We solve the persistent problem of the structure of the lowest 1/2+ resonance in 9Be which is important to bridge the A=8 gap in nucleosynthesis in stars. We show that the state is a genuine three-body resonance even though it decays entirely into neutron–8Be relative s-waves. The necessary barrier is created by “dynamical” evolution of the wave function as the short-distance α-5He structure is changed into the large-distance n-8Be structure. This decay mechanism leads to a width about two times smaller than table values. The previous interpretations as a virtual state or a two-body resonance are incorrect. The isobaric analog 1/2+ state in 9B is found to have energy and width in the vicinity of 2.0 MeV and 1.5 MeV, respectively. We also predict another 1/2+ resonance in 9B with similar energy and width.

Computation of resonance energies and widths in e ± H collisions using the stabilization method

We report a calculation of S-wave resonances (energies and widths) in positron-hydrogen scattering below the N=2 hydrogen and the N=2 positronium excitation thresholds with the stabilization method using Hylleraastype wavefunctions. Three resonances have been extracted out of which one is associated with the hydrogen n=2 threshold and the other two are found lying below the Ps(n=2) formation threshold. The present results agree with the most accurate calculations obtained by other methods.

Effect of screened Coulomb potentials on the resonance states of two-electron highly stripped atoms using the stabilization method

Resonance states of C4+, O6+, Ne8+ ions interacting with screened Coulomb potentials have been investigated using the stabilization method. We employ highly correlated exponential basis functions for S-, P-state calculations and employ CI-type basis functions for D-state calculations. We report a total of seven S-wave resonances of which four belong to 2sns (n = 2, 3, 4, 5) series and three belong to the 2pnp (n = 2, 3, 4) series. In addition, a total of eight P-wave resonances of which three belong to 2snp+ (n = 2, 3, 4), three belong to the 2snp− (n = 3, 4, 5) series, two belong to the 2pnd (n = 3, 4) and the lowest D-wave resonance for each systems and each screening parameters below the n = 2 thresholds of the respective subsystems are reported. In the unscreened case, we have found some resonance states that have not been reported before. In the screened case our reported results are new and show some interesting behaviour with and without electron–electron screening.

Low-lying non-normal parity states in 8B measured by proton elastic scattering on 7Be

A new measurement of proton resonance scattering on 7Be was performed up to the center-of-mass energy of 6.7 MeV using the low-energy RI beam facility CRIB (CNS Radioactive Ion Beam separator) at the Center for Nuclear Study of the University of Tokyo. The excitation function of Be7+p elastic scattering above 3.5 MeV was measured successfully for the first time, providing important information about the resonance structure of the 8B nucleus. The resonances are related to the reaction rate of 7Be(p,γ)8B, which is the key reaction in solar 8B neutrino production. Evidence for the presence of two negative parity states is presented. One of them is a 2− state observed as a broad s-wave resonance, the existence of which had been questionable. Its possible effects on the determination of the astrophysical S-factor of 7Be(p,γ)8B at solar energy are discussed. The other state had not been observed in previous measurements, and its spin and parity were determined as 1−.

Experimental Approach to Explosive Nucloeosynthesis with RI Beams

Experimental e orts to investigate stellar reactions under high-temperature and high density conditions have been made as a major program using the RI beams from the Center for Nuclear Study (CNS) low-energy inight RI beam separator (CRIB) at University of Tokyo in order to understand the evolution of the universe, as well as various stellar phenomena. Speci cally, two subjects of hydrogen burning are discussed here. One is a reaction study of the pp-chain and the second is of the explosive hydrogen burning, the rp-process. Some s-wave resonances have been identi ed by the thick target method in the crucial reaction processes in the hydrogen burning. The resonant scattering with the thick target method also succeeded in identifying inelastic resonant scattering, giving proton widths for the rst excited state of the target nucleus. This provided very e ciently the reaction rate estimate for the process under high-temperature equilibrium conditions. Possibilities of the CRIB facility in near future are also brie y discussed.

S-Wave Resonances in Positron Scattering from Li and Na

We have calculated S-wave resonances in the e+-Li and e+-Na systems using the stabilization method in the framework of hyperspherical coordinates. We have located all the previously known S-wave resonances predicted by Ho [1,2] and observed relatively good agreement for the resonance positions. In addition, the resonances associated with the Ps (n=2) threshold for the e+-Na and e+-Li systems are reported for the first time.

Bound states and resonance states of the plasma-embedded tdμ and ddμ molecular ions

We have investigated the bound states and resonance states of plasma-embedded tdμ and ddμ molecular ions using accurate correlated basis functions. The plasma effect has been taken care of by considering the Debye shielding approach of plasma modeling which admits a variety of plasma conditions. The density of resonance states are calculated using the stabilization method. The ground and excited states energies, and the S-wave resonance energies of tdμ and ddμ molecular ions immersed in plasmas are reported for various shielding parameters, along with the 1S and 2S threshold energies of the tμ and dμ atoms.

Resonance states of three self-gravitating bosons and fermions below the N = 3 threshold of the two-body subsystem

The resonance states of systems consisting of three identical bosons and three fermions interacting with an attractive 1/r potential are investigated using highly correlated exponential basis functions. The method of complex-coordinate rotation is used to extract energies and widths for the resonances lying below the N = 3 excited state threshold of the respective two-body subsystem. We have obtained a total of 24 S-wave resonances for the three-boson system and a total of 19 S-wave resonances in total spin-3/2 (the 4S state) and a total of 15 S-wave resonances in total spin-1/2 (the 2S state) below the N = 3 threshold. For the resonances associated with the N = 3 threshold, we have obtained two, three and four series of Rydberg states for the three-boson system, three-fermion system for 2S states and three-fermion system for 4S states, respectively. The resonance energies and widths of these states for the three-body self-gravitating bosons and fermions are reported.

S-wave resonances in positron-sodium scattering

We have calculated S-wave resonances in the positron-sodium system using the stabilization method in the framework of hyperspherical coordinates. Resonances below the Ps(n=2) threshold have been investigated. We have located all the previously known S-wave resonances by Kar and Ho [Eur. Phys. J. D 35, 453 (2005)] and Ward [J. Phys. B 22, 3763 (1989)] and observed relatively good agreement for the resonance positions. In addition, two new higher-lying resonances associated with the Ps(n=2) threshold are found to be at energies of -0.066 56 and -0.063 53 a.u.

Meson-baryon s-wave resonances with strangeness -3

Starting from a consistent SU(6) extension of the Weinberg-Tomozawa (WT) meson-baryon chiral Lagrangian (Phys. Rev. D 74, 034025 (2006)), we study the s-wave meson-baryon resonances in the strangeness S = −3 and negative-parity sectors. Those resonances are generated by solving the Bethe-Salpeter equation with the WT interaction used as kernel. The considered mesons are those of the 35-SU(6)-plet, which includes the pseudoscalar (PS) octet of pions and the vector (V) nonet of the rho-meson. For baryons we consider the 56-SU(6)-plet, made of the 1/2+ octet of the nucleon and the 3/2+ decuplet of the Delta. Quantum numbers I(J P ) = 0(3/2−) are suggested for the experimental resonances ω*(2250)− and ω*(2380)−. Among other, resonances with I = 1 are found, which minimal quark content is sss \( \bar l \) l′, being s the strange quark and l, l′ any of the the light up or down quarks. A clear signal for such a pentaquark would be a baryonic resonance with strangeness −3 and electric charge −2 or 0, in proton charge units. We suggest looking for K − Ξ − resonances with masses around 2100 and 2240 MeV in the sector 1(1/2−), and for π ± ω − and K − Ξ*− resonances with masses around 2260 MeV in the sector 1(3/2−).

Resonantly paired fermionic superfluids

We present a theory of a degenerate atomic Fermi gas, interacting through a narrow Feshbach resonance, whose position and therefore strength can be tuned experimentally, as demonstrated recently in ultracold trapped atomic gases. The distinguishing feature of the theory is that its accuracy is controlled by a dimensionless parameter proportional to the ratio of the width of the resonance to Fermi energy. The theory is therefore quantitatively accurate for a narrow Feshbach resonance. In the case of a narrow s-wave resonance, our analysis leads to a quantitative description of the crossover between a weakly paired BCS superconductor of overlapping Cooper pairs and a strongly paired molecular Bose–Einstein condensate of diatomic molecules. In the case of pairing via a p-wave resonance, that we show is always narrow for a sufficiently low density, we predict a detuning-temperature phase diagram, that in the course of a BCS–BEC crossover can exhibit a host of thermodynamically distinct phases separated by quantum and classical phase transitions. For an intermediate strength of the dipolar anisotropy, the system exhibits a p x + i p y paired superfluidity that undergoes a topological phase transition between a weakly coupled gapless ground state at large positive detuning and a strongly paired fully gapped molecular superfluid for a negative detuning. In two dimensions the former state is characterized by a Pfaffian ground state exhibiting topological order and non-Abelian vortex excitations familiar from fractional quantum Hall systems.

H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

SUMMARY Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura’s technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification factor.

Global microscopic nuclear level densities within the HFB plus combinatorial method for practical applications

New energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial model are proposed for practical applications. The combinatorial model includes a detailed microscopic calculation of the intrinsic state density and of the rotational enhancement factor, but a phenomenological treatment of the vibrational effect. The calculations make a coherent use of nuclear structure properties determined within the deformed Skyrme–Hartree–Fock–Bogolyubov framework. The present model predicts the experimental s-wave and p-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives reliable extrapolations to low energies where experimental data on the cumulative number of levels can be extracted. Level densities for more than 8500 nuclei are made available in a table format for practical applications.