P-wave Resonance Research Articles

Selection of T-violating amplitude at neutron transmission by oscillating field method

It is proposed in the frame of Ramsey's method to register two-dimensional, depending from neutron phase and neutron energy, spectrum for measurement P,T-violating amplitudes of interaction of the polarized neutrons with the polarized139La nuclei in area of p-wave resonance. The form of phase spectrum and corresponding expressions for the asymmetries are obtained on the basis of formalism of a spin density matrix. It was shown that the ratio of the P,T,-violating to P-violating imaginary amplitudes can be obtained from the measurements of neutron phase spectrum with polarized and unpolarized139La target.

Observation of a threshold enhancement in the plambda invariant-mass spectrum.

An enhancement near the m(p)+M(Lambda) mass threshold is observed in the combined pLambda and pLambda invariant-mass spectrum from J/psi-->pK(-)Lambda;+c.c. decays. It can be fit with an S-wave Breit-Wigner resonance with a mass m=2075+/-12(stat)+/-5(syst) MeV and a width of Gamma=90+/-35(stat)+/-9(syst) MeV; it can also be fit with a P-wave Breit-Wigner resonance. Evidence for a similar enhancement is also observed in psi(')-->pK(-)Lambda;+c.c. decays. The analysis is based on samples of 5.8x10(7)J/psi and 1.4x10(7)psi(') decays accumulated in the BES II detector at the Beijing Electron-Positron Collider.

Three-body analysis of11Li and itsβ-decay to deuteron channel and to halo analog state11Be* (18.3 MeV)

The ground state wave function of11Li obtained in a three-body model proposed earlier (S Kumar and V S Bhasin,Phys. Rev. C65, 034007 (2002)) has been employed to study the probability distributions, momentum distributions and n−n correlation. Complex scaling method has been used to find the energy positions and widths of the three resonant states of11Li above the breakup threshold. The formalism is extended further to study the β-decay of11Li to two channels. One is the β-transition of11Li into a high lying excited state of11Be at 18.3 MeV, i.e.,11Be* and the second is the decay to deuteron +9Li channel. The11Be* state has been considered as a halo analog state identified as a bound three-body (9Li + n + p) system. The n-9Li interaction incorporates both the virtual state and the p-wave resonance observed experimentally. For p-9Li interaction, a Coulomb corrected separable interaction is constructed using charge indepedendence for strong interaction part. The n-p interaction is operative only in3S1 state corresponding to the isotopic spin Th =0. As a result the11Be* state has the same isotopic spin as that of9Li core, i.e.,T = 3/2. Using these realistic parameters as input and without invoking any other free parameter, the model has been used to predict the strength of the Gamow-Teller β-decay of11Li to11Be*, i.e.,Bgt = 1.5 and the value of the branching ratio to9Li + deuteron channel to be 1.3 × 10-4. These results are found to be in rather good agreement with the recent experimental findings.

Nature of parity violation in neutron interaction with lead

The effect of parity violation in the interaction of thermal neutrons with lead was discovered in a number of studies. According to the existing theory, this effect is explained by the mixing of compound states characterized by different parities (s-and p-wave resonances). In view of the absence of a p-wave resonance in the region of thermal neutron energies, it is of importance to reveal a level below the neutron binding energy, a so-called negative resonance. The energy dependence of the cross section for radiative neutron capture on lead was measured in the present study, and it is shown that, for the 207Pb isotope, the results of this measurement deviate from the \({1 \mathord{\left/ {\vphantom {1 {\sqrt E }}} \right. \kern-\nulldelimiterspace} {\sqrt E }}\) law, thereby suggesting the presence of a strong negative resonance. The parameters of this resonance are estimated.

P-wave resonances in positron–lithium scattering

Three P-wave resonances in positron–lithium system below the Ps (n=2) threshold are calculated using the stabilization method. A model potential is used to represent the interaction between the core and the outermost active electron. Hylleraas-type wave functions are used to represent the correlation effects between the outer electron, the positron and the core. Our results are compared with those in the literature.

Reggeized model forηandη′photoproduction

A reggeized model for \eta and \eta' photoproduction on the nucleon is presented. In this model, t-channel vector meson exchanges are described in terms of Regge trajectories to comply with the correct high energy behavior. We compare this reggeized model with an isobar model (\eta-MAID), where the t-channel exchanges are described by \rho and \omega poles. Both models contain the same resonance contributions, and describe current \gamma p \to \eta p data up to E_\gamma^{lab} = 2 GeV quite well, but only the reggeized model can be successfully extended to higher energies. For the \gamma p \to \eta' p reaction, the reggeized model is found to be able to give a satisfactory description. For the differential cross section data from SAPHIR, we find that the observed linear forward rise in cos(theta) near E_\gamma^{lab} = 1.6 GeV can be well described by the interference of an S11 resonance and the Regge trajectory exchanges without any need for an additional P-wave resonance.

Observation of a near-threshold enhancement in the pp mass spectrum from radiative J/psi-->gammapp decays.

We observe a narrow enhancement near 2m(p) in the invariant mass spectrum of pp pairs from radiative J/psi-->gammapp decays. No similar structure is seen in J/psi-->pi(0)pp decays. The results are based on an analysis of a 58 x 10(6) event sample of J/psi decays accumulated with the BESII detector at the Beijing electron-positron collider. The enhancement can be fit with either an S- or P-wave Breit-Wigner resonance function. In the case of the S-wave fit, the peak mass is below 2m(p) at M=1859(+3)(-10) (stat)+5-25(syst) MeV/c(2) and the total width is Gamma<30 MeV/c(2) at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle.

Parity nonconservation in nuclear physics

We present an overview of the parity-nonconservation effects in nuclear physics. In the processes of polarized neutron scattering by nuclei, apart from the ordinary dynamical enhancement, we also consider the additional resonant enhancement in the entrance channel due to the proximity of the compound-nucleus p-wave resonance. We discuss the problem of extracting information on the electroweak interaction of nucleons from nuclear data.

Measurement of parity-nonconserving rotation of neutron spin in the 0.734-eV p-wave resonance of 139La

The parity nonconserving spin rotation of neutrons in the 0.734-eV p-wave resonance of 139La was measured with the neutron transmission method. Two optically polarized 3He cells were used before and behind a 5-cm long 139La target, each as a polarizer and an analyzer for neutron spin. The rotation angle was carefully measured by flipping the direction of 3He polarization in the polarizer in sequence. The peak-to-peak value of the spin rotation was determined to be (7.4±1.1)×10−3 rad/cm. The result is found to be consistent with the previous experiments, but the result was statistically improved. The s–p mixing model gives the weak matrix element as xW=(1.71±0.25) meV. The value agrees well with the one deduced from the parity-nonconserving longitudinal asymmetry in the same resonance.

Parity violation in partial neutron capture reactions

We point out that, in the standard resonance theory of parity violation in the compound nucleus, the longitudinal parity violating asymmetry in partial neutron capture cross sections is essentially independent of the partial gamma widths involved. Thus, the same asymmetry is expected for each partial cross section involving the same p-wave resonance. The asymmetries are expected to be enhanced $(\ensuremath{\sim}10%),$ and asymmetry measurements for several partial capture cross sections from a given p-wave resonance would provide a very strong test of the theory. In particular, such a measurement could shed light on the origin of the sign problem observed in ${}^{232}\mathrm{Th}$ and could provide more accurate determinations of the parity violating matrix elements between compound resonances. We propose a class of experiments and examine their feasibility.

The17O(n,α)14Creaction from subthermal up to approximately 350 keV neutron energy

We measured the O-17(n,alpha) C-14 reaction cross section from subthermal up to about 350 keV neutron energy. Two distinct resonances were observed at 178 and 253 keV, respectively, and a bump in our data near 2 keV could be explained by the presence of a negative-energy p-wave resonance. In the lower energy region, the cross section was shown to have a 1/v shape. The O-17(n, alpha) C-14 reaction is of importance in two different applications: it is one of the major reactions responsible for the C-14 pollution in nuclear reactors. Also, this reaction plays a role in primordial and nucleosynthesis in various astrophysical scenarios. For this application, the reaction rate at astrophysically relevant temperatures was calculated.

Positronium–positronium interaction: resonance, scattering length, and Bose–Einstein condensation

The low-energy scattering of ortho positronium (Ps) by ortho Ps has been studied in a full quantum mechanical coupled-channel approach. In the singlet channel (total spin s T =0) we find S- and P-wave resonances at 3.35 eV (width 0.02 eV) and 5.05 eV (width 0.04 eV), respectively, and a binding of 0.43 eV of Ps 2. The scattering length for s T =0 is 3.95 Å and for s T =2 is 0.83 Å. The small s T =2 scattering length makes the spin-polarized ortho Ps atoms an almost noninteracting ideal gas which may undergo Bose–Einstein condensation.

Neutron resonance spectroscopy of104Pd,105Pd,and110Pd

We have measured neutron resonances in the palladium isotopes 104, 105, and 110 for neutron energies from 1 to 2100 eV. Many new p-wave resonances have been observed. Their neutron widths and, in several cases, the radiative widths were measured. The average level spacings and the s-wave and p-wave neutron strength functions were determined. The time-of-flight method was used for both neutron total cross section measurements and total $(n,\ensuremath{\gamma})$ reaction yield measurements at the pulsed spallation neutron source of Los Alamos Neutron Science Center. Well established resonance spectroscopy for these isotopes is essential for the analysis of parity violation data that were recently measured in palladium.

Study of Light Exotic Nuclei with a Stochastic Variational Method

The stochastic variational method on correlated Gaussian basis was applied to the study of the structure of light exotic nuclei. The halo structure of {sup 11}Li was described in a {sup 9}Li+n+n picture by including the distortion of the {sup 9}Li core as well as the correlation among the core and the neutrons. The treatment was fully microscopic and successfully reproduced virtually all the properties of {sup 11}Li consistently with {sup 9}Li. Two p-wave resonances of {sup 10}Li were predicted around 300-500 keV above the {sup 9}Li+n threshold.

Resonance neutron capture in 60Ni below 450 keV

High-resolution neutron capture cross-section measurements on 60Ni have been performed at the Geel Linear Accelerator in the energy range from 1 to 450 keV. An experimentally determined weighting function, obtained by a total energy detection set-up, has been applied to the measured capture spectra. The parameters of 275 resonances have been determined in a recent reanalysis using the FANAC R-matrix shape fitting code. Accurate values of the maxwellian-averaged capture cross section for stellar temperatures ranging from kT=5 to 100 keV, corresponding to different scenarios of s-process stellar nucleosynthesis, have been calculated. The distributions of partial radiative widths for s- and p-wave resonances have been derived. A correlation of 0.64 between capture and reduced neutron widths is compatible with the presence of nonstatistical effects in the capture of 60Ni.

Parity violation in neutron resonances of117Sn

Parity nonconservation (PNC) was studied in p-wave neutron resonances of {sup 103}Rh in the neutron energy range 30 to 490 eV. The helicity dependence of the neutron total cross section of rhodium was determined by capture measurements with the time-of-flight method at the Manuel Lujan Neutron Scattering Center at the Los Alamos National Laboratory. A total of 32 p-wave resonances were studied and statistically significant longitudinal asymmetries were observed for resonances at E{sub n}=44.5, 110.8, 321.6, and 432.9 eV. A statistical analysis treating the PNC matrix elements as random variables yields a weak spreading width {Gamma}{sub w}=(1.42{sub {minus}0.59}{sup +1.21}){times}10{sup {minus}7}heV. {copyright} {ital 1999} {ital The American Physical Society}

Resonances in positronium–rubidium and positronium–cesium scattering

Scattering of ortho positronium (Ps) by cesium and rubidium atoms has been investigated employing a three-Ps-state coupled-channel model with Ps(1s,2s,2p) states using a time-reversal-symmetric regularized electron-exchange model potential. We find a narrow S-wave singlet resonance at 5.057 eV of width 0.003 eV in the Ps–Rb system and at 5.067 eV of width 0.003 eV in the Ps–Cs system. Singlet P-wave resonances in both systems are found at 5.3 eV of width 0.4 eV. Singlet D-wave structures are found at 5.4 eV in both systems. The pronounced P- and D-wave resonances in these systems lead to easily detectable local minima in the low-energy elastic cross sections. We also report results for elastic and Ps-excitation cross sections for Ps scattering by Rb and Cs.

Differential cross sections for elastic and inelastic positronium–hydrogen-atom scattering

We report results of differential cross sections for elastic scattering, target-elastic Ps excitations and target-inelastic excitation of hydrogen in a five-state coupled-channel model allowing for Ps(1s)H(2s,2p) and Ps(2s,2p)H(1s) excitations using a recently proposed time-reversal-symmetric regularized electron-exchange model potential. The present model yields a singlet Ps-H S-wave resonance at 4.01 eV of width 0.15 eV and a P-wave resonance at 5.08 eV of width 0.004 eV. We also study the effect of the inclusion of the excited Ps and H states on the convergence of the coupled-channel scheme.

S-, P- and D-wave resonancesin positronium-sodium and positronium-potassium scattering

Scattering of positronium (Ps) by sodium and potassium atomshas been investigated employing a three-Ps-state coupled-channelmodel with Ps(1s, 2s, 2p) states using a time-reversal-symmetricregularized electron-exchange model potential fitted toreproduce accurate theoretical results for PsNa and PsK bindingenergies. We find a narrow S-wave singlet resonance at 4.58 eVof width 0.002 eV in the Ps-Na system and at 4.77 eV of width0.003 eV in the Ps-K system. Singlet P-wave resonances in bothsystems are found at 5.07 eV of width 0.3 eV. Singlet D-wavestructures are found at 5.3 eV in both systems. We alsoreport results for elastic and Ps-excitation cross sections forPs scattering by Na and K.

Parity violation at neutron resonances and related neutron spectroscopy experiments

This paper gives a brief overview of parity violation effects at neutron p-wave resonances observed by the TRIPLE collaboration at Los Alamos together with some information about neutron resonance capture gamma spectroscopy experiments carried out by the Geel collaboration in relation to these parity violation experiments.