683 research outputs found
Pion form factors with improved infrared factorization
We calculate electromagnetic pion form factors with an analytic model for
which is infrared (IR) finite without invoking a
``freezing'' hypothesis. We show that for the asymptotic pion distribution
amplitude, agrees well with the data, whereas
the IR-enhanced hard contribution to and the soft (nonfactorizing)
part can jointly account for the data.Comment: 12 pages; 3 figures as PS files (1 figure added); modified text;
added references. To appear in Phys. Lett.
transition form factors in Quenched and QCD
Calculations of the magnetic dipole, electric quadrupole and Coulomb
quadrupole amplitudes for the transition are presented
both in quenched QCD and with two flavours of degenerate dynamical quarks.Comment: Lattice2003(Matrix), 3 page
A Chern-Simons approach to Galilean quantum gravity in 2+1 dimensions
We define and discuss classical and quantum gravity in 2+1 dimensions in the
Galilean limit. Although there are no Newtonian forces between massive objects
in (2+1)-dimensional gravity, the Galilean limit is not trivial. Depending on
the topology of spacetime there are typically finitely many topological degrees
of freedom as well as topological interactions of Aharonov-Bohm type between
massive objects. In order to capture these topological aspects we consider a
two-fold central extension of the Galilei group whose Lie algebra possesses an
invariant and non-degenerate inner product. Using this inner product we define
Galilean gravity as a Chern-Simons theory of the doubly-extended Galilei group.
The particular extension of the Galilei group we consider is the classical
double of a much studied group, the extended homogeneous Galilei group, which
is also often called Nappi-Witten group. We exhibit the Poisson-Lie structure
of the doubly extended Galilei group, and quantise the Chern-Simons theory
using a Hamiltonian approach. Many aspects of the quantum theory are determined
by the quantum double of the extended homogenous Galilei group, or Galilei
double for short. We study the representation theory of the Galilei double,
explain how associated braid group representations account for the topological
interactions in the theory, and briefly comment on an associated
non-commutative Galilean spacetime.Comment: 38 pages, 1 figure, references update
Calculation of the N to Delta electromagnetic transition matrix element
We present results on the ratio of electric quadrupole to magnetic dipole
amplitudes, , for the transition from lattice QCD. We consider both the quenched and the 2-flavor
theory.Comment: 3 pages, 4 figures, talk presented at Lattice2002(matrixel); Layout
of figures adjuste
Meson decay constants from Nf=2 clover fermions
We present recent results for meson decay constants calculated on
configurations with two flavours of O(a)-improved Wilson fermions.
Non-perturbative renormalisation is applied and quark mass dependencies as well
as finite volume and discretisation effects are investigated. In this work we
also present a computation of the coupling of the light vector mesons to the
tensor current using dynamical fermions.Comment: 6 pages, contribution to Lattice2005(Hadron spectrum and quark
masses
The Consequences of Non-Normality
The non-normality of Wilson-type lattice Dirac operators has important consequences - the application of the usual concepts from the textbook (hermitian) quantum mechanics should be reconsidered. This includes an appropriate definition of observables and the refinement of computational tools. We show that the truncated singular value expansion is the optimal approximation to the inverse operator D^{-1} and we prove that due to the gamma_5-hermiticity it is equivalent to gamma_5 times the truncated eigenmode expansion of the hermitian Wilson-Dirac operator
Quaternionic and Poisson-Lie structures in 3d gravity: the cosmological constant as deformation parameter
Each of the local isometry groups arising in 3d gravity can be viewed as the
group of unit (split) quaternions over a ring which depends on the cosmological
constant. In this paper we explain and prove this statement, and use it as a
unifying framework for studying Poisson structures associated with the local
isometry groups. We show that, in all cases except for Euclidean signature with
positive cosmological constant, the local isometry groups are equipped with the
Poisson-Lie structure of a classical double. We calculate the dressing action
of the factor groups on each other and find, amongst others, a simple and
unified description of the symplectic leaves of SU(2) and SL(2,R). We also
compute the Poisson structure on the dual Poisson-Lie groups of the local
isometry groups and on their Heisenberg doubles; together, they determine the
Poisson structure of the phase space of 3d gravity in the so-called
combinatorial description.Comment: 34 pages, minor corrections, references adde
Momentum dependence of the N to Delta transition form factors
We present a new method to determine the momentum dependence of the N to
Delta transition form factors and demonstrate its effectiveness in the quenched
theory at on a lattice. We address a number of
technical issues such as the optimal combination of matrix elements and the
simultaneous overconstrained analysis of all lattice vector momenta
contributing to a given momentum transfer squared, .Comment: Talk presented at Lattice 2004 (spectrum), Fermilab, 21-26 Jun. 2004.
3 pages, 3 figures. One typo in phenomenological Ansatz correcte
Distribution Amplitudes of Pseudoscalar Mesons
We present results for the first two moments of the distribution amplitudes
of pseudoscalar mesons. Using two flavors of non-perturbatively improved clover
fermions and non-perturbative renormalization of the matrix elements we perform
both chiral and continuum extrapolations and compare with recent results from
models and experiments.Comment: 7 pages, 4 figures, based on presentation at Lattice 200
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