1,726 research outputs found
Leading and higher twists in proton, neutron and deuteron unpolarized structure functions
We summarize the results of a recent global analysis of proton and deuteron
F2 structure function world data performed over a large range of kinematics,
including recent measurements done at JLab with the CLAS detector. From these
data the lowest moments (n <= 10) of the unpolarized structure functions are
determined with good statistics and systematics. The Q**2 evolution of the
extracted moments is analyzed in terms of an OPE based twist expansion, taking
into account soft-gluon effects at large x. A clean separation among the
Leading and Higher-Twist terms is achieved. By combining proton and deuteron
measurements the lowest moments of the neutron F2 structure function are
determined and its leading twist term is extracted. Particular attention is
paid to nuclear effects in the deuteron, which become increasingly important
for the higher moments. Our results for the non-singlet, isovector (p - n)
combination of the leading twist moments are used to test recent lattice
simulations. We also determine the lowest few moments of the higher twist
contributions, and find these to be approximately isospin independent,
suggesting the possible dominance of ud correlations over uu and dd in the
nucleon.Comment: 8 pages, 4 figures; to appear in the Proceedings of the IVth
International Conference on Quark and Nuclear Physics (QNP06), Madrid
(Spain), June 5-10, 200
Vortex Mass in a Superfluid
We consider the inertial mass of a vortex in a superfluid. We obtain a vortex
mass that is well defined and is determined microscopically and
self-consistently by the elementary excitation energy of the kelvon
quasiparticle localised within the vortex core. The obtained result for the
vortex mass is found to be consistent with experimental observations on
superfluid quantum gases and vortex rings in water. We propose a method to
measure the inertial rest mass and Berry phase of a vortex in superfluid Bose
and Fermi gases.Comment: 12 pages, 1 figur
Relativistic quark models
The application of relativistic constituent quark models to the evaluation of
the electromagnetic properties of the nucleon and its resonances is addressed.
The role of the pair creation process in the Feynmann triangle diagram is
discussed and the importance both of choosing the light-front formalism and of
using a Breit frame where the plus component of the four-momentum transfer is
vanishing, is stressed. The nucleon elastic form factors are calculated free of
spurious effects related to the loss of rotational covariance. The effect of a
finite constituent size is considered and the parameters describing the
constituent form factors are fixed using only the nucleon data up to Q**2 ~ 1
(GeV/c)**2; a constituent charge radius of ~ 0.45 fm is obtained in this way.
Above Q**2 ~ 1 (GeV/c)**2 our parameter-free predictions are in good agreement
with the experimental data, showing that soft physics may be dominant up to
Q**2 ~ 10 (GeV/c)**2. Our new results for the longitudinal and transverse
helicity amplitudes of the N - P_{11}(1440) transition are presented.Comment: to appear in the Procedings of the Int'l Conf. on "The Physics of
Excited Nucleons" (N* 2001), Mainz, March 200
Generalized Parton Distributions in the Light-Front Constituent Quark Model
The Generalized Parton Distributions (GPDs) of the nucleon are analyzed
within the relativistic constituent quark model formulated on the light-front.
It is shown that the matrix elements of the plus component of the one-body
vector current are plagued by spurious effects related to the dependence on the
hyperplane where the nucleon wave function is defined in terms of its
constituents. The physical GPDs can be extracted only from the matrix elements
of a transverse component of the one-body current. The loss of the
polinomiality property is then related to the neglect of the pair creation
process for non-vanishing values of the skewness. The need of implementing
effective many-body currents corresponding to the Z-graph is stressed and a
possible approach to achieve such a goal is proposed.Comment: latex 8 pages; one reference corrected; to appear in the Proc. of the
Int'l Conf. on "The Physics of Excited Nucleons" (NSTAR '04), Grenoble
(France), March 24-027, 200
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