Asymptotic directional structure of radiation for fields of algebraic type D

The directional behavior of dominant components of algebraically special spin-s fields near a spacelike, timelike or null conformal infinity is studied. By extending our previous general investigations we concentrate on fields which admit a pair of equivalent algebraically special null directions, such as the Petrov type D gravitational fields or algebraically general electromagnetic fields. We introduce and discuss a canonical choice of the reference tetrad near infinity in all possible situations, and we present the corresponding asymptotic directional structures using the most natural parametrizations.Comment: 20 pages, 6 figure

The 16O(d, 3He)15N reaction at 29 MeV: Reaction mechanism and nuclear structure

The reaction 16O(d, 3He)15N has been investigated using 29 MeV deuterons, and angular distributions were obtained for levels in 15N up to 10 MeV excitation energy. The measured distributions were subjected to distorted-wave (DWBA), compound nucleus (Hauser-Feshbach) and coupled-channel (CCBA) analyses. Only the strong transitions to the - ground state and the - state at 6324 keV exhibit distributions which are well described by DWBA. The spectroscopic factors are in agreement with shell-model estimates. The weak transitions generally show little structure and the spectroscopic factors extracted for these transitions tend to be unreasonably large. Contributions from compound nucleus formation were estimated and found to vary between about 10 % and 100 % of the observed cross sections with an average of the order of 30 %. The CCBA analysis for the transitions to the , + and + states at 5271, 7155 and 7566 keV, respectively, was performed using the spectroscopic amplitudes from weak coupling shell-model wave functions. Inelastic excitations to one-phonon states in the target and residual nuclei were included. The agreement between calculated and experimental distributions is good for both shape and magnitude, a conclusion which is not disturbed by the addition of small compound nucleus contributions. It is evident that spectroscopic factors extracted for the weak transitions on the basis of a direct one-step reaction mechanism alone are unreliable.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21821/1/0000222.pd

Inelastic processes and form factor effects in the 162, 164Dy(3He, d) reactions at 46.5 MeV

The 162, 164Dy(3He, d) reactions at E3He = 46.5 MeV are analyzed using the coupled channels Born approximation (CCBA) and improved form factors derived from a deformed Woods-Saxon potential. The latter are generated using the coupled channels procedure of Rost. The transitions considered populate the -[523], +[411], +[411], -[541] and + orbitals in 163, 165Ho. Indirect processes induced by inelastic scattering are found to have an influence on the cross sections comparable to that deduced for neutron transfer reactions on rare earth nuclei at lower energies. Considered alone, these can alter the cross sections even of strong transitions by a factor of two and of weaker ones by an order of magnitude. For the weaker transitions equally large changes can result when the improved form factors, rather than conventional spherical Woods-Saxon functions, are used in the calculations. In the examples considered these two effects tend to cancel, often, but not always, resulting in predicted cross sections similar in magnitude to the results of conventional DWBA calculations made with spherical Woods-Saxon form factors. The CCBA angular distributions are generally similar in shape to DWBA predictions, which usually give good fits to the experimental angular distributions over the 0-35[deg] range of the data. Compared with DWBA predictions which use (he same optical parameters, but spherical Woods-Saxon form factors, the CCBA with deformed Woods-Saxon form factors is in better overall agreement with the experimental cross-section magnitudes. However there are a number of cases in which the CCBA, although usually predicting larger cross sections than the DWBA, still underestimates the experimental cross sections by nearly factor of two. These cases all occur in the -[541] band or in the strongly Coriolis mixed +[411] and +[411] bands, and include the majority of transitions populating these orbitals. Since both nuclear structure and reaction mechanism effects are interwoven m the calculations, further data would be most useful in probing the origin of the discrepancy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21636/1/0000017.pd

A Global Potential Analysis of the 16^{16}O+28^{28}Si Reaction Using a New Type of Coupling Potential

A new approach has been used to explain the experimental data for the $^{16}$O+$^{28}$Si system over a wide energy range in the laboratory system from 29.0 to 142.5 MeV. A number of serious problems has continued to plague the study of this system for a couple of decades. The explanation of anomalous large angle scattering data; the reproduction of the oscillatory structure near the Coulomb barrier; the out-of-phase problem between theoretical predictions and experimental data; the consistent description of angular distributions together with excitation functions data are just some of these problems. These are long standing problems that have persisted over the years and do represent a challenge calling for a consistent framework to resolve these difficulties within a unified approach. Traditional frameworks have failed to describe these phenomena within a single model and have so far only offered different approaches where these difficulties are investigated separately from one another. The present work offers a plausible framework where all these difficulties are investigated and answered. Not only it improves the simultaneous fits to the data of these diverse observables, achieving this within a unified approach over a wide energy range, but it departs for its coupling potential from the standard formulation. This new feature is shown to improve consistently the agreement with the experimental data and has made major improvement on all the previous coupled-channels calculations for this system.Comment: 21 pages with 12 figure

Magnetic moments of antidecuplet pentaquarks

We analyze the magnetic moment of the exotic pentaquarks of the flavor antidecuplet in the constituent quark model for the case in which the ground state is in an orbital L(p)=0(+) or a L(p)=1(-) state. We derive sum rules for the magnetic moments. The magnetic moment of the Theta(1540) is found to be 0.38, 0.09 and 1.05 mu_N for J(p)=1/2(-), 1/2(+) and 3/2(+), respectively, which is compared with the results obtained in other approaches.Comment: 15 pages, 1 figure, 3 tables. Revised version, extended introduction and discussion, accepted for publication in Physics Letters

Probing superfast quarks in nuclei through dijet production at the LHC

We investigate dijet production from proton-nucleus collisions at the Large Hadron Collider (LHC) as a means for observing superfast quarks in nuclei with Bjorken $x>1$. Kinematically, superfast quarks can be identified through directly measurable jet kinematics. Dynamically, their description requires understanding several elusive properties of nuclear QCD, such as nuclear forces at very short distances, as well as medium modification of parton distributions in nuclei. In the present work, we develop a model for nuclear parton distributions at large $x$ in which the nuclear dynamics at short distance scales are described by two- and three-nucleon short range correlations (SRCs). Nuclear modifications are accounted for using the color screening model, and an improved description of the EMC effect is reached by using a structure function parametrization that includes higher-twist contributions. We apply QCD evolution at the leading order to obtain nuclear parton distributions in the kinematic regime of the LHC, and based on the obtained distributions calculate the cross section for dijet production. We find not only that superfast quarks can be observed at the LHC, but also that they provide sensitivity to the practically unexplored three-nucleon SRCs in nuclei. Additionally, the LHC can extend our knowledge of the EMC effect to large $Q^2$ where higher-twist effects are negligible.Comment: 44 pages, 17 figures, final version to be published in EJP

Heavy Quark Analogues of the Theta and Their Excitations

Predictions for the low-lying excitation spectrum of positive parity pentaquark systems containing one c or b antiquark and four light u,d quarks are obtained in the quark model picture for models with spin-dependent interactions given either by effective color magnetic (CM) exchange or effective Goldstone boson (GB) exchange. For the GB model, 4 excited states are predicted to lie within roughly the N-Delta splitting of the J=1/2 positive parity ground state while, for the CM model, 10 states are expected in the same range. Both the lowest excitation energy and the relative splittings are much smaller in the CM case. These predictions are on the same footing as those for the analogous splittings in the non-exotic baryon sector and, as such, provide a means of not only testing the models, but potentially ruling out either one, or both.Comment: 11 pages, RevTeX

Measurement of tibial nerve excursion during ankle joint dorsiflexion in a weight-bearing position with ultrasound imaging

The ability of peripheral nerves to stretch and slide is thought to be of paramount importance to maintain ideal neural function. Excursion in peripheral nerves such as the tibial can be measured by analysis of ultrasound images. The aim of this study was to assess the degree of longitudinal tibial nerve excursion as the ankle moved from plantar flexion to dorsiflexion in a standardised weight-bearing position. The reliability of ultrasound imaging to measure tibial nerve excursion was also quantified