1,458 research outputs found
Unitarity constraints on chiral perturbative amplitudes
Low lying scalar resonances emerge as a necessary part to adjust chiral
perturbation theory to experimental data once unitarity constraint is taken
into consideration. I review recent progress made in this direction in a model
independent approach. Also I briefly review studies on the odd physical
properties of these low lying scalar resonances, including in the
processes.Comment: Talk given at: International Workshop on Effective Field Theories:
from the pion to the upsilon, February 2-6 2009, Valencia, Spai
Scalar resonance at 750 GeV as composite of heavy vector-like fermions
We study a model of scalars which includes both the SM Higgs and a scalar
singlet as composites of heavy vector-like fermions. The vector-like fermions
are bounded by the super-strong four-fermion interactions. The scalar singlet
decays to SM vector bosons through loop of heavy vector-like fermions. We show
that the surprisingly large production cross section of di-photon events at 750
GeV resonance and the odd decay properties can all be explained. This model
serves as a good model for both SM Higgs and a scalar resonance at 750 GeV.Comment: 12 pages, no figure, references updated, version for publicatio
Studies on X(4260) and X(4660) particles
Studies on the X(4260) and X(4660) resonant states in an effective lagrangian
approach are reviewed. Using a Breit--Wigner propagator to describe their
propagation, we find that the X(4260) has a sizable coupling to the
channel, while other couplings are found to be negligible.
Besides, it couples much stronger to than to : As an approximate result for
X(4660), we obtain that the ratio of
. Finally, taking X(3872) as an example, we also point out a possible way to
extend the previous method to a more general one in the effective lagrangian
approach.Comment: Talk given by H. Q. Zheng at "Xth Quark Confinement and the Hadron
Spectrum", October 8-12, 2012, TUM Campus Garching, Munich, Germany. 6 pages,
3 figures, 3 table
New Insights on Low Energy Scattering Amplitudes
The - and - wave phase shifts of low-energy pion-nucleon scatterings
are analysed using Peking University representation, in which they are
decomposed into various terms contributing either from poles or branch cuts. We
estimate the left-hand cut contributions with the help of tree-level
perturbative amplitudes derived in relativistic baryon chiral perturbation
theory up to . It is found that in and
channels, contributions from known resonances and cuts are far from enough to
saturate experimental phase shift data -- strongly indicating contributions
from low lying poles undiscovered before, and we fully explore possible physics
behind. On the other side, no serious disagreements are observed in the other
channels.Comment: slightly chnaged version, a few more figures added. Physical
conclusions unchange
Positivity constraints on the low-energy constants of the chiral pion-nucleon Lagrangian
Positivity constraints on the pion-nucleon scattering amplitude are derived
in this article with the help of general S-matrix arguments, such as
analyticity, crossing symmetry and unitarity, in the upper part of Mandelstam
triangle, R. Scanning inside the region R, the most stringent bounds on the
chiral low energy constants of the pion-nucleon Lagrangian are determined. When
just considering the central values of the fit results from covariant baryon
chiral perturbation theory using extended-on-mass-shell scheme, it is found
that these bounds are well respected numerically both at O(p^3) and O(p^4)
level. Nevertheless, when taking the errors into account, only the O(p^4)
bounds are obeyed in the full error interval, while the bounds on O(p^3) fits
are slightly violated. If one disregards loop contributions, the bounds always
fail in certain regions of R. Thus, at a given chiral order these terms are not
numerically negligible and one needs to consider all possible contributions,
i.e., both tree-level and loop diagrams. We have provided the constraints for
special points in R where the bounds are nearly optimal in terms of just a few
chiral couplings, which can be easily implemented and employed to constrain
future analyses. Some issues about calculations with an explicit Delta(1232)
resonance are also discussed.Comment: 15 pages, 13 eps figures, 2 table
Transporting Robotic Swarms via Mean-Field Feedback Control
With the rapid development of AI and robotics, transporting a large swarm of
networked robots has foreseeable applications in the near future. Existing
research in swarm robotics has mainly followed a bottom-up philosophy with
predefined local coordination and control rules. However, it is arduous to
verify the global requirements and analyze their performance. This motivates us
to pursue a top-down approach, and develop a provable control strategy for
deploying a robotic swarm to achieve a desired global configuration.
Specifically, we use mean-field partial differential equations (PDEs) to model
the swarm and control its mean-field density (i.e., probability density) over a
bounded spatial domain using mean-field feedback. The presented control law
uses density estimates as feedback signals and generates corresponding velocity
fields that, by acting locally on individual robots, guide their global
distribution to a target profile. The design of the velocity field is therefore
centralized, but the implementation of the controller can be fully distributed
-- individual robots sense the velocity field and derive their own velocity
control signals accordingly. The key contribution lies in applying the concept
of input-to-state stability (ISS) to show that the perturbed closed-loop system
(a nonlinear and time-varying PDE) is locally ISS with respect to density
estimation errors. The effectiveness of the proposed control laws is verified
using agent-based simulations
Singularities and Accumulation of Singularities of N Scattering amplitudes
It is demonstrated that for the isospin N scattering amplitude,
, and are
two accumulation points of poles on the second sheet of complex plane, and
are hence accumulation of singularities of . For
, is the accumulation point of
poles on the second sheet of complex plane. The proof is valid up to all
orders of chiral expansions.Comment: 6 pages, one reference added, a bug removed, major conclusions remain
unchange
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