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Blobrana


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Title: Strange Electric Form Factor of the Proton
Authors: D. B. Leinweber, S. Boinepalli, A. W. Thomas, P. Wang, A. G. Williams, R. D. Young, J. M. Zanotti, J. B. Zhang

By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low-mass quenched lattice QCD simulations of the individual quark contributions to the electric charge radii of the baryon octet, we obtain an accurate determination of the strange electric charge radius of the proton. While this analysis provides a value for G_E^s(Q^2=0.1 GeV^2) in agreement with the best current data, the theoretical error is comparable with that expected from future HAPPEx results from JLab. Together with the earlier determination of G_M^s, this result considerably constrains the role of hidden flavor in the structure of the nucleon.

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Title: Gluon flux-tube distribution and linear confinement in baryons
Authors: F. Bissey, F-G. Cao, A. R. Kitson, A. I. Signal, D. B. Leinweber, B. G. Lass****, A. G. Williams

Using three-point function techniques, correlations between the vacuum action density and the positions of quarks are used to identify the formation of gluon flux-tubes within baryons. A high-statistics approach based on the translational and rotational symmetry of the four-dimensional lattice volume is adopted to avoid the need for gauge-dependent smoothing techniques. Vacuum field fluctuations are found to be suppressed in the presence of static quarks such that flux tubes represent the expulsion or suppression of gluon-field fluctuations. By considering numerous different link paths in the creation of the static quark sources, we are able to explore the dependence of the observed flux tubes on the source shape. In particular, "T," "L" and "Y" shapes are considered to access a variety of flux-tube topologies including the ground state. T-shape paths are observed to relax towards a Y-shape topology as opposed to a Delta shape. L-shape topologies give rise to a large potential. Upon identifying the precise geometry of the flux tube formation, we are able to perform a quantitative comparison between the flux tube length and the associated static-quark potential. For every source considered we find the flux-tube length and associated potential to provide a universal string tension. With this new knowledge, one can conclude that the flux tube configuration of the ground state potential for large quark separations is that which minimizes the flux tube length. The characteristic flux tube radius of the baryonic ground state potential is found to be 0.38(3) fm with vacuum-field fluctuations suppressed by 7.2(6)%. The node connecting the flux tubes is 25% larger at 0.47(2) fm with a 15(3)% larger suppression of the vacuum action at 8.1(7)%.

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Title: Precision Electromagnetic Structure of Octet Baryons in the Chiral Regime
Authors: S. Boinepalli, D. B. Leinweber, A. G. Williams, J. M. Zanotti, J. B. Zhang

The electromagnetic properties of the baryon octet are calculated in quenched QCD on a 20^3 x 40 lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover (FLIC) fermion action. FLIC fermions enable simulations to be performed efficiently at quark masses as low as 300 MeV. By combining FLIC fermions with an improved-conserved vector current, we ensure that discretisation errors occur only at O(a^2) while maintaining current conservation. Magnetic moments and electric and magnetic radii are extracted from the electric and magnetic form factors for each individual quark sector.
From these, the corresponding baryon properties are constructed. Our results are compared extensively with the predictions of quenched chiral perturbation theory. We detect substantial curvature and environment sensitivity of the quark contributions to electric charge radii and magnetic moments in the low quark mass region. Furthermore, our quenched QCD simulation results are in accord with the leading non-analytic behaviour of quenched chiral perturbation theory, suggesting that the sum of higher-order terms makes only a small contribution to chiral curvature.

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Blobrana


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New clue to Quarks
Particle physicists around the world will be designing their next generation of billion-dollar experiments following new findings from a University of Adelaide-led research team.

The Holy Grail of the world's particle physicists is to discover and describe new particles that make up the Universe's tiniest building blocks.
The University of Adelaide's Associate Professor Derek Leinweber, leading a team of international theoretical physicists, has established a new approach to precision calculations on the properties of subatomic particles.
The proton, one of the three main components of an atom, is known to consist of point-like particles called quarks, bound together by gluons. There are six different types of quarks and the most mysterious of these is the strange quark, which "boils up" inside the proton and then "simmers back out of existence".
The new finding, published recently in the prestigious international journal Physical Review Letters, is a precise calculation of the strange quark's distribution within the proton. The calculation predicts that the short-lived strange quarks display an unanticipated level of symmetry in their journey.

"Technically the strange quark contribution to the proton's charge distribution has proven elusive. At the University of Adelaide, working with physicists at the University of Edinburgh and the Thomas Jefferson National Accelerator Facility in the US, we've been able to calculate the strange contribution with unprecedented accuracy by applying a unique combination of cutting-edge numerical and analytical approaches. We have combined expertise in fundamental Lattice Simulations on supercomputers together with breakthrough techniques in Effective Field Theory calculations. These are two separate areas of physics which have been used together in a way that no-one else has thought of. It gives particular strength to the University of Adelaide's research in this area. There is a huge industry in particle physics with groups of researchers around the world making new measurements that could reveal physics beyond the standard model of the universe. Our result presents a huge challenge to experimental physicists in planning the next generation of experiments. Billions of dollars are going to be spent, based on this result" - Dr Leinweber, Deputy Director of the University's Special Research Centre for the Subatomic Structure of Matter.

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Blobrana


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Physics researchers working at the High Energy Accelerator Research Organization (KEK) Laboratory in Japan have observed a new type of interaction amongst quarks.
The scientists reported at the Symposium on Lepton-Photon Interactions at High Energies, June 30-July 5 in Uppsala, Sweden, that they had produced first evidence of a beauty quark converting to the lightest of quarks, the down quark.



"Observation of this very rare phenomenon allows us to study if this occurs only through the so-called penguin process (a two-step transition wherein the beauty quark momentarily transforms into the top quark that subsequently transforms into the down quark) as predicted by the standard theory, or through some hitherto unobserved way" - Leo Piilonen, Virginia Tech physics professor and a member of the Belle experiment's research team.

Virginia Tech is a founding institution in the Belle experiment that studies the properties of the beauty (b) quark at the KEK Laboratory.
The Belle experiment studies the decay patterns of the b-quark to search for clues on how the universe is constructed.

Using a the electron-positron colliding beam accelerator, the so-called B Factory has made many important contributions to our understanding of nature at its most fundamental level, including those just announced by Professor Kazuo Abe of the KEK Laboratory in his plenary talk in Sweden.

Virginia Tech physics graduate student Debabrata Mohapatra worked on the analysis that led to the results. He is at the KEK laboratory this summer and will return to Blacksburg to complete his dissertation in August.
Quarks are the most fundamental constituents of material, and it is widely known that six types exist in nature.

Piilonen explains that the purpose of the B Factory is to produce an enormous number of the second heaviest, called the b quark, and clarify the fundamental laws of the universe by detailed studies of its behaviour.
Previous findings include the discovery of many new particle states, discovery of the violation of particle-antiparticle symmetry in b quark decays, and the experimental confirmation of the Kobayashi-Maskawa theory, which characterizes the properties of quarks and predicted this symmetry violation.

In the last year, the performance of the KEKB accelerator has continued to improve, reaching the world's highest luminosity of 1.5x1034 /cm2/sec, and delivering 390 million pairs of B and anti-B mesons to the Belle experiment. (These mesons contain the b quark or its antiparticle.)
The Belle group has investigated the decays of these B mesons in detail, and has observed a new type of interaction wherein the b quark turns into one of the lightest quarks, the d quark.

This phenomenon was confirmed by finding 35 events where the B meson decays into either a rho or an omega meson with an accompanying photon, and 30 events where the B meson disintegrates into two K mesons.

"Observation of this very rare phenomenon allows us to study if this occurs only through the penguin process as predicted by the standard theory, or through some hitherto unobserved way. This opens an exciting new window into our understanding of elementary particle physics: for example, new physics models that incorporate so-called supersymmetry also predict b to d quark transitions, and our announced observation will provide valuable feedback to these models. On the other hand, if we assume that the standard model holds, then our observation will provide a novel measurement of one of the coarsely measured parameters in the Kobayashi-Maskawa theory (Vtd), and lead to a more complete understanding of the standard model " - Leo Piilonen.

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