7,171 research outputs found
Nucleon electromagnetic form factors in two-flavour QCD
We present results for the nucleon electromagnetic form factors, including
the momentum transfer dependence and derived quantities (charge radii and
magnetic moment). The analysis is performed using O(a) improved Wilson fermions
in Nf=2 QCD measured on the CLS ensembles. Particular focus is placed on a
systematic evaluation of the influence of excited states in three-point
correlation functions, which lead to a biased evaluation, if not accounted for
correctly. We argue that the use of summed operator insertions and fit
ans\"atze including excited states allow us to suppress and control this
effect. We employ a novel method to perform joint chiral and continuum
extrapolations, by fitting the form factors directly to the expressions of
covariant baryonic chiral effective field theory. The final results for the
charge radii and magnetic moment from our lattice calculations include, for the
first time, a full error budget. We find that our estimates are compatible with
experimental results within their overall uncertainties.Comment: 22 pages, 10 figures, citations modifie
Two-photon physics
It is reviewed how Compton scattering sum rules relate low-energy nucleon
structure quantities to the nucleon excitation spectrum. In particular, the GDH
sum rule and recently proposed extensions of it will be discussed. These
extensions are sometimes more calculationally robust, which may be an advantage
when estimating the chiral extrapolations of lattice QCD results, such as for
anomalous magnetic moments. Subsequently, new developments in our description
of the nucleon excitation spectrum will be discussed, in particular a recently
developed chiral effective field theory framework for the
-resonance region. Within this framework, we discuss results on
and masses, the transition and the
magnetic dipole moment.Comment: 10 pages, prepared for proceedings of Symposium on 20 Years of
Physics at the Mainz Mikrotro
Radiative corrections in K --> 3 pi decays
We investigate radiative corrections to K --> 3 pi decays. In particular, we
extend the non-relativistic framework developed recently to include real and
virtual photons and show that, in a well-defined power counting scheme, the
results reproduce corrections obtained in the relativistic calculation. Real
photons are included exactly, beyond the soft-photon approximation, and we
compare the result with the latter. The singularities generated by pionium near
threshold are investigated, and a region is identified where standard
perturbation theory in the fine structure constant alpha may be applied. We
expect that the formulae provided allow one to extract S-wave pi pi scattering
lengths from the cusp effect in these decays with high precision.Comment: 57 pages, 17 figure
Neutron-proton mass difference in nuclear matter
Isospin-breaking effects in nuclear matter are studied in the framework of a
medium-modified Skyrme model. The proposed effective Lagrangian incorporates
both the medium influence of the surrounding nuclear environment on the single
nucleon properties and an explicit isospin-breaking effect in the mesonic
sector. The approach predicts that the neutron-proton mass difference decreases
in isospin-symmetric nuclear matter but by a very small amount only.Comment: 8 pages, 4 figures, revised versio
Nucleon structure from mixed action calculations using 2+1 flavors of asqtad sea and domain wall valence fermions
We present high statistics results for the structure of the nucleon from a
mixed-action calculation using 2+1 flavors of asqtad sea and domain wall
valence fermions. We perform extrapolations of our data based on different
chiral effective field theory schemes and compare our results with available
information from phenomenology. We discuss vector and axial form factors of the
nucleon, moments of generalized parton distributions, including moments of
forward parton distributions, and implications for the decomposition of the
nucleon spin.Comment: 68 pages, 47 figures. Main revision points: improved discussion of
chiral fits and systematic uncertainties, several minor refinements. Accepted
for publication in Phys.Rev.
Electromagnetic form factor of pion from N_f=2+1 dynamical flavor QCD
We present a calculation of the electromagnetic form factor of the pion in
flavor lattice QCD. Calculations are made on the PACS-CS gauge field
configurations generated using Iwasaki gauge action and Wilson-clover quark
action on a lattice volume with the lattice spacing estimated as
fm at the physical point. Measurements of the form factor are
made using the technique of partially twisted boundary condition to reach small
momentum transfer as well as periodic boundary condition with integer momenta.
Additional improvements including random wall source techniques and a judicious
choice of momenta carried by the incoming and outgoing quarks are employed for
error reduction. Analyzing the form factor data for the pion mass at MeV and 296 MeV, we find that the NNLO SU(2) chiral perturbation
theory fit yields for the pion charge radius
at the physical pion mass. Albeit the error is quite large, this is consistent
with the experimental value of . Below MeV, we find that statistical fluctuations in the pion two- and
three-point functions become too large to extract statistically meaningful
averages on a spatial volume. We carry out a sample calculation on a
lattice with the quark masses close to the physical point, which
suggests that form factor calculations at the physical point become feasible by
enlarging lattice sizes to .Comment: 28 pages, 14 figure
Generalized parton distributions of the pion in a Bethe-Salpeter approach
We calculate generalized parton distribution functions in a field theoretic
formalism using a covariant Bethe-Salpeter approach for the determination of
the bound-state wave function. We describe the procedure in an exact
calculation in scalar Electrodynamics proving that the relevant corrections
outside our scheme vanish. We extend the formalism to the Nambu--Jona-Lasinio
model, a realistic theory of the pion. We go in both cases beyond all previous
calculations and discover that all important features required by general
physical considerations, like symmetry properties, sum rules and the
polynomiality condition, are explicitly verified. We perform a numerical study
of their behavior in the weak and strong coupling limits.Comment: 19 pages, 21 eps figures, accepted for publication in EPJ
Determination of Vus: Recent Input from the Lattice
The two most precise determinations of the CKM matrix element V_us are based
on the analyses of leptonic and semileptonic kaon decays. These studies also
rely on the lattice QCD calculations of two hadronic parameters, namely the
ratio of the kaon and pion decay constants, f_K+/f_pi+, and the kaon
semileptonic vector form factor at zero momentum transfer, f_+(0). In this
talk, I review the recent lattice results for these quantities, by showing that
the sub-percent accuracy required by the phenomenological analyses has been
reached by lattice QCD. As best estimates of the lattice calculations I quote
f_K+/f_pi+ = 1.193(4) and f_+(0)=0.965(3). I also discuss some recent
theoretical progress in the evaluation of the small, but phenomenologically
relevant, SU(2) isospin breaking corrections.Comment: Kaon 2013 conference proceeding
- …