Search Results (13 total)

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

Topological chaos relies on the periodic motion of obstacles in a two-dimensional flow in order to form nontrivial braids. This motion generates exponential stretching of material lines, and hence efficient mixing. Boyland, Aref, and Stremler [J. Fluid Mech. 403, 277 (2000)] have studied a specific periodic motion of rods that exhibits topological chaos in a viscous fluid. We show that it is possible to extend their work to cases where the motion of the stirring rods is topologically trivial by considering the dynamics of special periodic points that we call “ghost rods”, because they play a similar role to stirring rods. The ghost rods framework provides a new technique for quantifying chaos and gives insight into the mechanisms that produce chaos and mixing. Numerical simulations for Stokes flow support our results.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

We report measurements of cross sections for the reaction ¹H(e,e^′K⁺)Y, for both the Λ and Σ⁰ hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q²<2 (GeV/c)². Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ⁰ results have not been previously reported.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

The large-time asymptotic behavior of a two-stage reaction (A+B→R,  B+R→S) with initially segregated reactants is described. The concentration of the reactants is found to be significantly less than the initial concentrations in a depletion zone of width proportional to t^1/2, where t is time; the reaction takes place in a thinner zone of width proportional to t^1/6. Similarity solutions for the chemical concentration profiles in the reaction zone are calculated, and are compared with numerical simulations of the full partial differential reaction-diffusion equations. The large-time asymptotic scalings reported here are the same as in the absence of the secondary reaction, but we find that the location of the reaction zone is significantly shifted due to the secondary reaction. The reaction zone may behave in an exotic fashion at large time, moving first one way, then reversing its direction.

The ¹H(e,e^′K⁺)Λ reaction was studied as a function of the squared four-momentum transfer, Q², and the virtual photon polarization, ɛ. For each of four Q² settings, 0.52, 0.75, 1.00, and 2.00 (GeV/c)², the longitudinal and transverse virtual photon cross sections were extracted in measurements at three virtual photon polarizations. The Q² dependence of the σ_L/σ_T ratio differs significantly from current theoretical predictions. This, combined with the precision of the measurement, implies a need for revision of existing calculations.

Photons spontaneously emitted from single atoms are necessarily antibunched in time. The statistics of the photon time record, however, can show sub-Poissonian or even super-Poissonian statistics and depend strongly on the time interval used for analysis. In this work we have analyzed photon arrival times from resonantly excited barium atoms in an atomic beam observed under single-atom conditions. The time-interval dependence of photon statistics is presented, showing the connection between antibunching and sub-Poissonian statistics (at times comparable to the spontaneous-decay lifetime of the atom) as well as the evolution of the statistics in the long-time limit.

Two manifestations of the quantum nature of coherent, resonant light scattering from atoms are photon antibunching and sub-Poissonian statistics. The existence of both effects has been demonstrated previously; in the present work, using individual barium atoms, measurements of the dependence of the latter effect on the frequency of the radiation field are presented. The distinction between the two phenomena is demonstrated by data that exhibit antibunching, yet have a photon counting distribution with variance equal to the Poisson value.

An experimental technique which permits the study of single three-level atoms prior to quantum jumps to a shelving state has been used to study the second-order photon statistics of laser-excited barium atoms. Measured values of Mandel’s Q parameter agree with the theory derived for two-level atoms. This result illustrates the extent to which the presence of additional levels can be ignored in certain systems: Although the third level in this system is responsible for the quantum jumps, prior to each jump the system’s behavior is within experimental errors, exactly like that of a two-level atom. The negative values of Q obtained for a range of laser intensities demonstrate the sub-Poisson character of the emission.

A single three-level atom with the ground state strongly coupled to an excited state and weakly coupled to a third intermediate state exhibits a telegraph signal of fluorescent emission when driven by laser light at the strong coupling frequency. The ‘‘on’’ periods correspond to normal resonance fluorescence, whereas the ‘‘off’’ periods correspond to the atom being ‘‘shelved’’ in the intermediate state. Such a system has interesting statistical properties. The photon statistics during the ‘‘on’’ periods correspond to two-level behavior and are close to Poisson in nature. However, the photon number distribution of a succession of ‘‘on’’ periods exhibits Bose-Einstein statistics. Thus the three-level system can exhibit both ‘‘quiet’’ and ‘‘noisy’’ behavior. The mathematical connection between the two cases is established, and an atomic beam experiment is described which records a mixture of the two statistical extremes. The composition of the mixture is varied, and the data are shown to be well represented by an analysis based upon the model developed.

The resonance fluorescence from the ¹³⁸Ba I 5535-Å 6s²¹S₀-6s6p ¹P₁ transition has been measured as a function of incident intensity using a Doppler-free atomic beam geometry and a single-mode cw dye laser. The upper ¹P₁ level has a weak branch to intermediate metastable D states. A long laser-atom interaction region was used, which ensured that essentially all atoms could be pumped into the D states during their transit through the laser beam. By analyzing the shape of the saturation curve a value for the branching ratio to the 6s5d ¹,3D levels of (3.57±0.38)×10^-3 has been obtained.

We measured the analyzing power for the ¹⁶O(p→,n)¹⁶F (4^-,6.37 MeV) reaction at 134.0 MeV and the differential cross section for the same reaction at 135.2 MeV. The shape of the cross section for the transition to this unnatural parity stretched state is described well by a distorted-wave impulse-approximation calculation using a (πd_{5 / 2},νp_{3 / 2}^-1)_4^- configuration and the effective interaction derived by Love and Franey from nucleon-nucleon phase shifts. The analyzing power from this calculation reproduces all of the qualitative features of the data and supports the use of the impulse approximation as an excellent starting point for describing the reaction mechanism. Quantitative agreement between the experimental and theoretical analyzing power can be improved by eliminating the imaginary tensor term of this interaction and taking the real part to be that derived by Love from the Sussex matrix elements. The sensitivity of the calculations to the choice of optical potentials and the importance of spin-orbit distortion is explored.

NUCLEAR REACTIONS ¹⁶O(p→,n)¹⁶F, E=134 MeV; measured neutron spectra at 12 angles between θ=0° and 62.9°; extracted σ(θ) and A(θ) to J^π=4^-,6.37 MeV state of ¹⁶F. Compared angular distributions of σ(θ) and A(θ) with calculations based on a nucleon-nucleon effective interaction.