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Search Results: 1 - 10 of 453107 matches for " J. C. Williams "
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Transverse Momentum Structure of Diffractive DIS Models
J. C. Williams
Physics , 1999,
Abstract: The transverse momentum distribution of the diffractive final state provides an interesting test of models of diffractive deep-inelastic scattering at HERA. We present a comparison of several colour-singlet exchange models with thrust transverse momentum data from a recent H1 analysis. We also study the effect of constraints imposed on the kinematically-accessible phase space by data selection cuts and find that, as a result of the pseudo-rapidity cut which is used by H1 to select diffractive events, there is no dijet contribution at low transverse momenta. We are able to fit the large transverse momentum part of the data with a two-gluon dijet model. The results of this analysis are compared with a previous study of large pseudo-rapidity gap structure function data, and we discuss ways in which one might reconcile the results of the two analyses. We conclude that a significant small-$\beta$ 3-jet contribution is probably required to explain the data, and show that the combination of a two-gluon dijet model and an exponentially-decaying thrust transverse momentum distribution provides a good fit over the entire kinematic range of the thrust data.
Trap-imbalanced fermion mixtures
M. Iskin,C. J. Williams
Physics , 2007, DOI: 10.1103/PhysRevA.77.013605
Abstract: We analyze the ground state phases of two-component ($\sigma \equiv \lbrace \uparrow, \downarrow \rbrace$) population- and mass-balanced ($N_\uparrow = N_\downarrow$ and $m_\uparrow = m_\downarrow$) but trap-imbalanced ($\omega_\uparrow \ne \omega_\downarrow$) fermion mixtures as a function of interaction strength from the weak attraction Bardeen-Cooper-Schrieffer (BCS) to the strong attraction Bose-Einstein condensation (BEC) limit. In the BCS limit, we find that the unpolarized superfluid (UPS) fermions exist away from the central core of the trapping potentials, and are surrounded by partially polarized normal (P$\sigma$PN) fermions. As the interactions increase towards unitarity, we find that the central P$\sigma$PN core first transitions to a UPS, and then expands towards the edges until the entire mixture becomes a UPS in the BEC limit.
Trapped p-wave superfluids: a local density approach
M. Iskin,C. J. Williams
Physics , 2008, DOI: 10.1103/PhysRevA.77.041607
Abstract: The local density approximation is used to study the ground state superfluid properties of harmonically trapped p-wave Fermi gases as a function of fermion-fermion attraction strength. While the density distribution is bimodal on the weakly attracting BCS side, it becomes unimodal with increasing attraction and saturates towards the BEC side. This non-monotonic evolution is related to the topological gapless to gapped phase transition, and may be observed via radio-frequency spectroscopy since quasi-particle transfer current requires a finite threshold only on the BEC side.
Trapped fermion mixtures with unequal masses: a Bogoliubov-de Gennes approach
M. Iskin,C. J. Williams
Physics , 2008,
Abstract: We use the Bogoliubov-de Gennes formalism to analyze the ground state phases of harmonically trapped two-species fermion mixtures with unequal masses. In the weakly attracting limit and around unitarity, we find that the superfluid order parameter is spatially modulated around the trap center, and that its global maximum occurs at a finite distance away from the trap center where the mixture is locally unpolarized. As the attraction strength increases towards the molecular limit, the spatial modulations gradually disappear while the Bardeen-Cooper-Schrieffer (BCS) type nonmodulated superfluid region expands until the entire mixture becomes locally unpolarized.
Population imbalanced fermions in harmonically trapped optical lattices
M. Iskin,C. J. Williams
Physics , 2008, DOI: 10.1103/PhysRevA.78.011603
Abstract: The attractive Fermi-Hubbard Hamiltonian is solved via the Bogoliubov-de Gennes formalism to analyze the ground state phases of population imbalanced fermion mixtures in harmonically trapped two-dimensional optical lattices. In the low density limit the superfluid order parameter modulates in the radial direction towards the trap edges to accommodate the unpaired fermions that are pushed away from the trap center with a single peak in their density. However in the high density limit while the order parameter modulates in the radial direction towards the trap center for low imbalance, it also modulates towards the trap edges with increasing imbalance until the superfluid to normal phase transition occurs beyond a critical imbalance. This leads to a single peak in the density of unpaired fermions for low and high imbalance but leads to double peaks for intermediate imbalance.
A Comparison of Genetic Programming with Genetic Algorithms for Wire Antenna Design
P. J. Williams,T. C. A. Molteno
International Journal of Antennas and Propagation , 2008, DOI: 10.1155/2008/197849
Abstract: This work compares the performance of genetic programming (GP) against traditional fixed-length genome GA approaches on the optimization of wire antenna designs. We describe the implementation of a GP electromagnetic optimization system for wire structures. The results are compared with the traditional GA approach. Although the dimensionality of the search space is much higher for GP than GA, we find that the GP approach gives better results than GA for the same computational effort. In addition, we find that a more expressive antenna structure grammar, dramatically, improves the performance of the GP approach.
Analysis of SEC9 Suppression Reveals a Relationship of SNARE Function to Cell Physiology
Daniel C. Williams, Peter J. Novick
PLOS ONE , 2009, DOI: 10.1371/journal.pone.0005449
Abstract: Background Growth and division of Saccharomyces cerevisiae is dependent on the action of SNARE proteins that are required for membrane fusion. SNAREs are regulated, through a poorly understood mechanism, to ensure membrane fusion at the correct time and place within a cell. Although fusion of secretory vesicles with the plasma membrane is important for yeast cell growth, the relationship between exocytic SNAREs and cell physiology has not been established. Methodology/Principal Findings Using genetic analysis, we identified several influences on the function of exocytic SNAREs. Genetic disruption of the V-ATPase, but not vacuolar proteolysis, can suppress two different temperature-sensitive mutations in SEC9. Suppression is unlikely due to increased SNARE complex formation because increasing SNARE complex formation, through overexpression of SRO7, does not result in suppression. We also observed suppression of sec9 mutations by growth on alkaline media or on a non-fermentable carbon source, conditions associated with a reduced growth rate of wild-type cells and decreased SNARE complex formation. Conclusions/Significance Three main conclusions arise from our results. First, there is a genetic interaction between SEC9 and the V-ATPase, although it is unlikely that this interaction has functional significance with respect to membrane fusion or SNAREs. Second, Sro7p acts to promote SNARE complex formation. Finally, Sec9p function and SNARE complex formation are tightly coupled to the physiological state of the cell.
The subtle effects of sea water acidification on the amphipod Gammarus locusta
C. Hauton, T. Tyrrell,J. Williams
Biogeosciences (BG) & Discussions (BGD) , 2009,
Abstract: We report an investigation of the effects of increases in pCO2 on the survival, growth and molecular physiology of the neritic amphipod Gammarus locusta which has a cosmopolitan distribution in estuaries. Amphipods were reared from juvenile to mature adult in laboratory microcosms at three different levels of pH in nominal range 8.1–7.6. Growth rate was estimated from weekly measures of body length. At sexual maturity the amphipods were sacrificed and assayed for changes in the expression of genes coding for a heat shock protein (hsp70 gene) and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh gene). The data show that the growth and survival of this species is not significantly impacted by a decrease in sea water pH of up to 0.5 units. Quantitative real-time PCR analysis indicated that there was no significant effect of growth in acidified sea water on the sustained expression of the hsp70 gene. There was a consistent and significant increase in the expression of the gapdh gene at a pH of ~7.5 which, when combined with observations from other workers, suggests that metabolic changes may occur in response to acidification. It is concluded that sensitive assays of tissue physiology and molecular biology should be routinely employed in future studies of the impacts of sea water acidification as subtle effects on the physiology and metabolism of coastal marine species may be overlooked in conventional gross "end-point" studies of organism growth or mortality.
The subtle effects of sea water acidification on the amphipod Gammarus locusta
C. Hauton,T. Tyrrell,J. Williams
Biogeosciences Discussions , 2009,
Abstract: We report an investigation of the effects of increases in pCO2 on the growth and molecular physiology of the neritic amphipod Gammarus locusta, which has a cosmopolitan distribution in estuaries. Amphipods were reared from juvenile to mature adult in laboratory microcosms at three different levels of pH in nominal range 8.1–7.6. Growth rate was estimated from weekly measures of body length. At sexual maturity the amphipods were sacrificed and assayed for changes in the expression of genes coding for a heat shock protein (hsp70 gene) and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh gene). The data show that the growth and survival rate of this species is not significantly impacted by a decrease in sea water pH of up to 0.5 units. Quantitative real-time PCR analysis indicated that there was no significant effect of growth in acidified sea water on the expression of the hsp70 gene. However, there was a consistent and significant increase in the expression of the gapdh gene at a pH of ~7.5 which indicated a possible disruption to oxidative metabolic processes. It was concluded that future predicted changes in sea water pH may have subtle effects on the physiology and metabolism of coastal and marine species which may be overlooked in studies of whole organism response.
Effects of Inhomogeneity on the Spectrum of the Mott-Insulator State
G. Pupillo,E. Tiesinga,C. J. Williams
Physics , 2003, DOI: 10.1103/PhysRevA.68.063604
Abstract: We investigate the existence of quantum {\it quasi} phase transitions for an ensemble of ultracold bosons in a one-dimensional optical lattice, performing exact diagonalizations of the Bose-Hubbard Hamiltonian. When an external parabolic potential is added to the system {\it quasi} phase transitions are induced by the competition of on-site mean-field energy, hopping energy, and energy offset among lattice sites due to the external potential and lead to the coexistence of regions of particle localization and delocalization in the lattice. We clarify the microscopic mechanisms responsible for these {\it quasi} phase transitions as a function of the depth of the external potential when the on-site mean-field energy is large compared to the hopping energy. In particular, we show that a model Hamiltonian involving a few Fock states can describe the behavior of energy gap, mean particle numbers per site, and number fluctuations per site almost quantitatively. The role of symmetry on the gap as a function of the depth of the external trapping potential is elucidated. We discuss possible experimental signatures of {\it quasi} phase transitions studying the single particle density matrix and explain microscopically the occurrence of local maxima in the momentum distribution. The role of a thermal population of the excited states on the momentum distribution is discussed.
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