945 research outputs found
Baryons and a Nucleon–Nucleon potential in a colour–dielectric quark model with plain gluons
No full textThe quark model contains many-gluon degrees of freedom in the form of a colour-dielectric field χ and a colour interaction between quarks in the form of gluons without self-interaction (plain gluons). The confinement is due to the coupling of the many-gluon field x to the quark's mass. The quark mass is small inside a quark bag and gets infinitely large outside the bag. This leads to absolute quark confinement. We develop a non-relativistic version of the model and fit the mass and the root-mean-square radius of the nucleon. In addition we calculate an adiabatic nucleon-nucleon potential. It shows a short-range repulsion and an intermediate-range attraction between nucleons and is comparable to a Paris potential
Restrictions on the chiral angle in a soliton quark model
No full textThe authors present results of calculations for baryon properties in a quark model that incorporates new developments in lattice QCD as well as chiral invariance. The quarks are confined by a nonlinear chi field, which is motivated by averaging over the gluonic degrees of freedom in lattice QCD. Chiral invariance is achieved by meson fields ( pi , sigma ), which also lead to an interaction between the valence quarks and the Dirac sea. Due to the nonlinearity of the chi field, the authors find very special restrictions for the boundary conditions of the meson fields
Differential Flatness of Quadrotor Dynamics Subject to Rotor Drag for Accurate Tracking of High-Speed Trajectories
No full textIn this letter, we prove that the dynamical model of a quadrotor subject to linear rotor drag effects is differentially flat in its position and heading. We use this property to compute feedforward control terms directly from a reference trajectory to be tracked. The obtained feedforward terms are then used in a cascaded, nonlinear feedback control law that enables accurate agile flight with quadrotors. Compared to the state-of-the-art control methods, which treat the rotor drag as an unknown disturbance, our method reduces the trajectory tracking error significantly. Finally, we present a method based on a gradient-free optimization to identify the rotor drag coefficients, which are required to compute the feedforward control terms. The new theoretical results are thoroughly validated trough extensive comparative experiments
Heavy ions optical potential at finite temperature calculated using a complex effective interaction
No full textRadiative decays of decuplet hyperons
No full textWe calculate the radiative decay widths of decuplet hyperons in a chiral constituent quark model including electromagnetic exchange currents between quarks. Exchange currents contribute significantly to the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is found to be weakened by exchange currents. Differences and similarities between our results and other recent model predictions are discussed.Wagner, Georg; Buchmann, A. J.; Faessler, Aman
Rho-omega mixing, vector meson dominance and the pion form-factor
No full textWe review the current status of ϱ-ω mixing and discuss its implication for our understanding of charge-symmetry breaking. In order to place this work in context we also review the photon-hadron coupling within the framework of vector meson dominance. This leads naturally to a discussion of the electromagnetic form-factor of the pion and of nuclear shadowing.H.B. O'Connell, B.C. Pearce, A.W. Thomas and A.G. William
The spin of the proton
No full textThe twenty years since the announcement of the proton spin crisis by the European Muon Collaboration has seen tremendous progress in our knowledge of the distribution of spin within the proton. The problem is reviewed, beginning with the original data and the suggestion that polarized gluons may play a crucial role in resolving the problem through the U(1) axial anomaly. The discussion continues to the present day where not only have strong limits have been placed on the amount of polarized glue in the proton but the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturabtive structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data.A.W. Thoma
Nucleon and hadron structure changes in the nuclear medium and impact on observables
No full textWe review the effect of hadron structure changes in a nuclear medium using the quark-meson coupling (QMC) model, which is based on a mean field description of non-overlapping nucleon (or baryon) bags bound by the self-consistent exchange of scalar and vector mesons. This approach leads to simple scaling relations for the changes of hadron masses in a nuclear medium. It can also be extended to describe finite nuclei, as well as the properties of hypernuclei and meson-nucleus deeply bound states. It is of great interest that the model predicts a variation of the nucleon form factors in nuclear matter. We also study the empirically observed, Bloom-Gilman (quark-hadron) duality. Other applications of the model include subthreshold kaon production in heavy ion collisions, D and D-bar meson production in antiproton-nucleus collisions, and J/Psi suppression. In particular, the modification of the D and D-bar meson properties in nuclear medium can lead to a large J/Psi absorption cross section, which explains the observed J/Psi suppression in relativistic heavy ion collisions.K. Saito, K. Tsushima and A.W. Thoma
Convergence of chiral effective field theory
No full textCopyright © 2003 Published by Elsevier Science B.V.We formulate the expansion for the mass of the nucleon as a function of pion mass within chiral perturbation theory using a number of different ultra-violet regularisation schemes; including dimensional regularisation and various finite-ranged regulators. Leading and next-to-leading order non-analytic contributions are included through the standard one-loop Feynman graphs. In addition to the physical nucleon mass, the expansion is constrained by recent, extremely accurate, lattice QCD data obtained with two flavors of dynamical quarks. The extent to which different regulators can describe the chiral expansion is examined, while varying the range of quark mass over which the expansions are matched. Renormalised chiral expansion parameters are recovered from each regularisation prescription and compared. We find that the finite-range regulators produce consistent, model-independent results over a wide range of quark mass sufficient to solve the chiral extrapolation problem in lattice QCD.R. D. Young, D. B. Leinweber and A. W. Thomashttp://www.elsevier.com/wps/find/journaldescription.cws_home/419/description#descriptio
The validity of charge symmetry for parton distributions
No full textRecent measurements of the Gottfried Sum Rule have focused attention on the possibility of substantial breaking of flavor symmetry in sea quark distributions of the proton. This has been confirmed by by pp and pD Drell-Yan processes measured at FNAL. The theoretical models used to infer flavor symmetry breaking rely on the assumption that parton distributions are charge symmetric; it is conceivable that current tests of flavor symmetry could be affected by substantial charge symmetry violation. Since all phenomenological parton distributions assume the validity of charge symmetry, in this paper we examine the possibility that charge symmetry is violated [CSV]. We first list definitions for structure functions which do not make the usual assumption that parton distributions obey charge symmetry. We then give some simple model estimates of CSV for both valence and sea quark distributions. Next, we list a set of relations which must hold if charge symmetry is valid, and we review the current experimental limits on charge symmetry violation in parton distributions. We then propose a series of possible experimental tests of charge symmetry. The proposed experiments could either detect charge symmetry violation in parton distributions, or they could provide more stringent upper limits on CSV. We discuss CSV contributions to sum rules, and we propose new sum rules which could differentiate between flavor symmetry, and charge symmetry, violation in nuclear systems.J. T. Londergan and A. W. Thoma
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