118,077 research outputs found
Meson twist-4 parton distributions in terms of twist-2 distribution amplitudes at large Nc
We show that in the large N_c limit four-quark twist-4 distributions in the
pion can be expressed in terms of twist-2 pion distribution amplitude. This
allows us to compute the isospin-2 structure function of the pion
F_2^{I=2}(x_B) in the large N_c limit. The method can be easily applied to
other mesons as well.Comment: 6 pages, one figur
Electron Mass Operator in a Strong Magnetic Field and Dynamical Chiral Symmetry Breaking
The electron mass operator in a strong magnetic field is calculated. The
contribution of higher Landau levels of virtual electrons, along with the
ground Landau level, is shown to be essential in the leading log approximation.
The effect of the electron dynamical mass generation by a magnetic field is
investigated. In a model with N charged fermions, it is shown that some
critical number N_{cr} exists for any value of the electromagnetic coupling
constant alpha, such that the fermion dynamical mass is generated with a
doublet splitting for N < N_{cr}, and the dynamical mass does not arise at all
for N > N_{cr}, thus leaving the chiral symmetry unbroken.Comment: 4 pages, REVTEX4, 3 figure
The Gluon Spin in the Chiral Bag Model
We study the gluon polarization contribution at the quark model
renormalization scale to the proton spin, , in the chiral bag model. It
is evaluated by taking the expectation value of the forward matrix element of a
local gluon operator in the axial gauge . It is shown that the confining
boundary condition for the color electric field plays an important role. When a
solution satisfying the boundary condition for the color electric field, which
is not the conventionally used but which we favor, is used, the has a
positive value for {\it all} bag radii and its magnitude is comparable to the
quark spin polarization. This results in a significant reduction in the
relative fraction of the proton spin carried by the quark spin, which is
consistent with the small flavor singlet axial current measured in the EMC
experiments.Comment: Corrections to figure
Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations
Using density-functional-theory calculations, we have identified new stable
configurations for tri-, tetra-, and penta-vacancies in silicon. These new
configurations consist of combinations of a ring-hexavacancy with three, two,
or one interstitial atoms, respectively, such that all atoms remain fourfold.
As a result, their formation energies are lower by 0.6, 1.0, and 0.6 eV,
respectively, than the ``part of a hexagonal ring'' configurations, believed up
to now to be the lowest-energy states
Reevaluation of Neutron Electric Dipole Moment with QCD Sum Rules
We study the neutron electric dipole moment in the presence of the
CP-violating operators up to the dimension five in terms of the QCD sum rules.
It is found that the OPE calculation is robust when exploiting a particular
interpolating field for neutron, while there exist some uncertainties on the
phenomenological side. By using input parameters obtained from the lattice
calculation, we derive a conservative limit for the contributions of the CP
violating operators. We also show the detail of the derivation of the sum
rules.Comment: 33 pages, 5 figure
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