108,456 research outputs found
Safe domain and elementary geometry
A classical problem of mechanics involves a projectile fired from a given
point with a given velocity whose direction is varied. This results in a family
of trajectories whose envelope defines the border of a 'safe' domain. In the
simple cases of a constant force, harmonic potential and Kepler or Coulomb
motion, the trajectories are conic curves whose envelope in a plane is another
conic section which can be derived either by simple calculus or by geometrical
considerations. The case of harmonic forces reveals a subtle property of the
maximal sum of distances within an ellipse.Comment: Also available from http://democrite.in2p3.fr/democrite-0002317
Speculations in hadron spectroscopy
A selected survey is presented of the recent progress in hadron spectroscopy.
This includes spin-singlet charmonium states, excitations of charmonium and
open-charm mesons, double-charm baryons, and pentaquark candidates. Models
proposing exotic bound states or resonances are reviewed. The sector of exotic
mesons with two heavy quarks appears as particularly promising
Elusive multiquark spectroscopy
A review is presented of past and recent attempts to build multiquark states
within current models already describing ordinary mesons and baryons. This
includes: coherence in the chromomagnetic interaction, tetraquarks with two
heavy quarks, Steiner-tree models of confinement, and hadronic molecules, in
particular in the hidden-charm sector. Some emphasis is put on the difficulties
encountered when extrapolating toward higher configurations the dynamics of
confining forces, starting from the simple case of a quark and an antiquark, or
three quarks in a colour singlet
Historical Survey of the Quasi-Nuclear Baryonium
We review ideas and speculations concerning possible bound states or
resonances of the nucleon--antinucleon system.Comment: 12 pages, Talk at the Workshop on Hadron Spectroscopy, Frascati,
Italy, march 199
Recent issues in hadron spectroscopy
A brief survey is presented of recently discovered hadrons, some of them
presumably demonstrating a new kind of internal structure. This includes :
spin-singlet quarkonium, mesons with unexpected mass or width, baryons with two
heavy quarks, and pentaquark candidates. Flavour configurations with a
combination of light and heavy quarks appear as particularly promising.Comment: Based on an invited talk at the session on Subnucleon degrees of
freedo
Stability of multiquarks in a simple string model
A simple string model inspired by the strong-coupling regime of Quantum
ChromoDynamics is used as a potential for studying the spectrum of multiquark
systems with two quarks and two antiquarks, with a careful treatment of the
four-body problem. It is found that the ground state is stable, lying below the
threshold for dissociation into two isolated mesons.Comment: 4 pages, 2 figures, Refs. added, typos corrected, discussion
improved, to appear in Phys. Rev.
Positivity domains for pairs of triples of spin observables
Positivity restrains the allowed domains for pairs or triples of spin
observables in polarised reactions. Various domain shapes in
reactions are displayed. Some
methods to determine these domains are mentioned and a new one based on the
anticommutation between two observables is presented.Comment: Talk given by Xavier Artru at "DSPIN-07", XII Workshop on High-Energy
Spin Physics, Dubna, Sept. 3-7, 2007, to appear in the Proceeding
Atomic hydrogen in the disturbed edge-on galaxy NGC 4631
We present WSRT HI observations of the nearby, disturbed, edge-on galaxy NGC 4631. A low-resolution (45 in. x 87 in.) map shows previously unknown tidal debris at large distances from the plane, and two dwarf companions. A high resolution (12 in. x 22 in.) map reveals a very disturbed gas layer in NGC 4631, with a wealth of small-scale structure. The most striking discovery is a supershell in the eastern half of the disk with a diameter of about 3 kpc, a mass of approximately 10 exp 8 solar mass and a tentative expansion velocity of 45 km/s. If the expansion is real, the energy which must have been injected by supernovae to explain the shell's current parameters is roughly 4 x 10(exp 55) ergs. Such a high energy requirement suggests an alternative formation mechanism, such as a collision with a small companion
Relation between and from QCD
We have studied, using double ratio of QCD (spectral) sum rules, the ratio
between the masses of and X(3872) assuming that they are respectively
described by the and molecular currents. We found
(within our approximation) that the masses of these two states are almost
degenerate. Since the pion exchange interaction between these mesons is exactly
the same, we conclude that if the observed X(3872) meson is a
molecule, then the molecule should also exist with approximately the
same mass. An extension of the analysis to the -quark case leads to the same
conclusion. We also study the SU(3) breakings for the mass
ratios. Motivated by the recent Belle observation of two states, we
revise our determination of by combining results from exponential and
FESR sum rules.Comment: revised version to appear on Phys. Lett.
Graph Laplacians and Stabilization of Vehicle Formations
Control of vehicle formations has emerged as a topic of significant interest to the controls community. In this paper, we merge tools from graph theory and control theory to derive stability criteria for formation stabilization. The interconnection between vehicles (i.e., which vehicles are sensed by other vehicles) is modeled as a graph, and the eigenvalues of the Laplacian matrix of the graph are used in stating a Nyquist-like stability criterion for vehicle formations. The location of the Laplacian eigenvalues can be correlated to the graph structure, and therefore used to identify desirable and undesirable formation interconnection topologies
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