First Measurement of Hard Exclusive π

The polarized cross-section ratio σ_{LT^{'}}/σ_{0} from hard exclusive π^{-}Δ^{++} electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2 GeV/10.6 GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from 1.5 GeV^{2} up to 7 GeV^{2}. The reaction provides a novel access to the d-quark content of the nucleon and to p→Δ^{++} transition generalized parton distributions. A comparison to existing results for hard exclusive π^{+}n and π^{0}p electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry.

First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions.

We report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (z) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high z and an enhancement at low z. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high z. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

Observation of Correlations between Spin and Transverse Momenta in Back-to-Back Dihadron Production at CLAS12.

We report the first measurements of deep inelastic scattering spin-dependent azimuthal asymmetries in back-to-back dihadron electroproduction in the deep inelastic scattering process. In this reaction, two hadrons are produced in opposite hemispheres along the z axis in the virtual photon-target nucleon center-of-mass frame, with the first hadron produced in the current-fragmentation region and the second in the target-fragmentation region. The data were taken with longitudinally polarized electron beams of 10.2 and 10.6 GeV incident on an unpolarized liquid-hydrogen target using the CLAS12 spectrometer at Jefferson Lab. Observed nonzero sinΔϕ modulations in ep→e^{'}pπ^{+}X events, where Δϕ is the difference of the azimuthal angles of the proton and pion in the virtual photon and target nucleon center-of-mass frame, indicate that correlations between the spin and transverse momenta of hadrons produced in the target- and current-fragmentation regions may be significant. The measured beam-spin asymmetries provide a first access in dihadron production to a previously unexplored leading-twist spin- and transverse-momentum-dependent fracture function. The fracture functions describe the hadronization of the target remnant after the hard scattering of a virtual photon off a quark in the target particle and provide a new avenue for studying nucleonic structure and hadronization.

Observation of Azimuth-Dependent Suppression of Hadron Pairs in Electron Scattering off Nuclei.

We present the first measurement of dihadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the gibuu model, which suggests that hadrons form near the nuclear surface and undergo multiple scattering in nuclei.These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nuclei.

First Measurement of Timelike Compton Scattering.

We present the first measurement of the timelike Compton scattering process, γp→p^{'}γ^{*}(γ^{*}→e^{+}e^{-}), obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25

Improved Λp Elastic Scattering Cross Sections between 0.9 and 2.0 GeV/c as a Main Ingredient of the Neutron Star Equation of State.

Strange matter is believed to exist in the cores of neutron stars based on simple kinematics. If this is true, then hyperon-nucleon interactions will play a significant part in the neutron star equation of state. Yet, compared to other elastic scattering processes, there is very little data on Λ-N scattering. This experiment utilized the CEBAF Large Acceptance Spectrometer (CLAS) detector to study the Λp→Λp elastic scattering cross section in the incident Λ momentum range 0.9-2.0 GeV/c. These are the first data on this reaction since the 1970s. The new cross sections have significantly better accuracy and precision than the existing world data, and the techniques developed here can also be used in future experiments.

[Third World cities: points of accumulation, centers of distribution]. / Les villes du Tiers Monde: theatres d'accumulation, centres de diffusion.

PIP: Attention was called over 3 decades ago to the very rapid growth of Third World cities and the significance of the differences between their patterns of urbanization and those of industrialized countries. Their demographic growth occurred much faster and depended much more heavily on high fertility, their economies were geared more to export of raw materials than to manufacturing and were unable to create massive numbers of jobs to absorb the growing labor force except in the unproductive tertiary sector, and it appeared unlikely that they would be able to produce entrepreneurial classes of their own. Several economic developments during the 1970s affected the world economy and the patterns of urbanization of the Third World: the decline of the principal capitalist economies and the multiple increases in the price of oil, the floating exchange rate, the considerable increase in consumer goods, and the increasing costs of labor in industrialized countries, among others, created new conditions. World economic interdependence, international control of investment and exchange, and volume and mobility of capital increased at a time of rapid economic growth in some Third World countries, especially those whose governments took an aggressive role in promoting growth and investment. Some Third World cities now seem to be developing according to a more western model, but the same cannot be said of all Third World countries, and international economic evolution appears to have led to increasing polarization between countries as well as within them. The 1 domain where a certain convergence has occurred is consumption, beginning with the privileged classes and filtering to the lower income groups. Consumption of collective and individual consumer goods, which is concentrated in the largest cities, increases dependence on imports, technology, knowledge, and usually debt. The modern productive sector and its distribution activities become implanted in the cities to such a degree that it becomes more and more difficult for the consumption needs of regional cities and rural areas to be satisfied except through manufactured products from the capitalist sector of the principal city or through imports from industrial countries. Despite the fact that some Third World cities will be enormous by the year 2000 and that their social structures and labor forces will not closely resemble those of European cities, the thesis of "pseudourbanization" appears invalid for several reasons: the model of sectorial changes in the European labor force was not followed by the industrializing countries of North America; some Third World countries (excluding India and China) appear able to absorb most of their surplus rural population into the modern sector, and Third World cities appear less and less to be merely centers of culture. New research during the 1970s on Third World urbanization contributed several crucial elements to the analysis: recognition that insertion of developing countries into the international economic order has been a major influence on their urbanization patterns, appreciation of the role of migration in urbanization, realization of the potential role of the state in mitigating spatial and structural inequalities created by the urbanization process, and recognition of the need for more detailed microeconomic studies and construction of more elaborate models of Third World economies.^ieng

Intraoral leukoplakia, abrasion, periodontal breakdown, and tooth loss in a snuff dipper.

Dentists should be aware that snuff dipping or chewing is increasing in southern states and perhaps in other sections of the United States. These habits can lead to clinical leukoplakia, gingival recession, tooth abrasion, and periodontal bone destruction. The possibility also exists that a malignant transformation of leukoplakia can develop in persons who use snuff and other forms of tobacco.