275 research outputs found
The pion mass dependence of the nucleon form-factors of the energy momentum tensor in the chiral quark-soliton model
The nucleon form factors of the energy-momentum tensor are studied in the
large-Nc limit in the framework of the chiral quark-soliton model for model
parameters that simulate physical situations in which pions are heavy. This
allows for a direct comparison to lattice QCD results.Comment: 17 pages, 12 figure
Collective coordinates of the Skyrme model coupled with fermions
The problem of construction of fiber bundle over the moduli space of the
Skyrme model is considered. We analyse an extension of the original Skyrme
model which includes the minimal interaction with fermions. An analogy with
modili space of the fermion-monopole system is used to construct a fiber bundle
structure over the skyrmion moduli space. The possibility of the non-trivial
holonomy appearance is considered. It is shown that the effect of the fermion
interaction turns the -skyrmion moduli space into a real vector bundle with
natural connection.Comment: 10 page
The Neutron Electric Dipole Moment in the Instanton Vacuum: Quenched Versus Unquenched Simulations
We investigate the role played by the fermionic determinant in the evaluation
of the CP-violating neutron electric dipole moment (EDM) adopting the Instanton
Liquid Model. Significant differences between quenched and unquenched
calculations are found. In the case of unquenched simulations the neutron EDM
decreases linearly with the quark mass and is expected to vanish in the chiral
limit. On the contrary, within the quenched approximation, the neutron EDM
increases as the quark mass decreases and is expected to diverge as (1/m)**Nf
in the chiral limit. We argue that such a qualitatively different behavior is a
parameter-free, semi-classical prediction and occurs because the neutron EDM is
sensitive to the topological structure of the vacuum. The present analysis
suggests that quenched and unquenched lattice QCD simulations of the neutron
EDM as well as of other observables governed by topology might show up
important differences in the quark mass dependence, for mq < Lambda(QCD).Comment: 8 pages, 3 figures, 2 table
Baryon resonances in the mean field approach and a simple explanation of the Theta+ pentaquark
We suggest to classify baryon resonances as single-quark states in a mean
field, and/or as its collective excitations. Identifying the Roper resonance
N(1440), the nucleon resonance N(1535), and the singlet hyperon Lambda(1405) as
single-quark excitations, we find that there must be an exotic S=+1 baryon
resonance Theta+ (the "pentaquark") with a mass about 1440+1535-1405=1570 MeV
and spin-parity one-half-plus. We argue that Theta+ is an analog of the
Gamov--Teller excitation long known in nuclear physics.Comment: 9 p., 5 fig
Correlated two-pion exchange and large-N(C) behavior of nuclear forces
The effect of correlated scalar-isoscalar two-pion exchange (CrTPE) modes is
considered in connection with central and spin-orbit parts of the NN force. The
two-pion correlation function is coupled directly to the scalar form factor of
the nucleon which we calculate in the large-N(C) limit where the nucleon can be
described as a soliton of an effective chiral theory. The results for the
central NN force show a strong repulsive core at short internucleon distances
supplemented by a moderate attraction beyond 1 fm. The long-range tail of the
central NN potential is driven by the the pion-nucleon sigma term and
consistent with the effective meson exchange. The spin-orbit part of
the NN potential is repulsive. The large-N(C) scaling behavior of the
scalar-isoscalar NN interaction is addressed. We show that the spin-orbit part
is O(1/N^2(C)) in strength relative to the central force resulting in the ratio
suggested by the 1/N(C) expansion for N(C)=3. The latter is in
agreement with our numerical analysis and with the Kaplan-Manohar large-N(C)
power counting. Unitarization of the scattering amplitude plays here
an important role and improves the tree level results. Analytical
representations of the CrTPE NN potential in terms of elementary functions are
derived and their chiral content is discussed.Comment: 29 pages, 7 figure
Spin-dependent twist-4 matrix elements from the instanton vacuum: Flavor-singlet and nonsinglet
We estimate the twist-4 spin-1 nucleon matrix element f_2 in an
instanton-based description of the QCD vacuum. In addition to the
flavor-nonsinglet we compute also the flavor-singlet matrix element, which
appears in next-to-leading order of the (1/N_c)-expansion. The corresponding
twist-3 spin-2 matrix elements d_2 are suppressed in the packing fraction of
the instanton medium, (\bar \rho)/(\bar R) << 1. We use our results to estimate
the leading (1/Q^2) power corrections to the first moment of the proton and
neutron spin structure functions G_1, as well as the intrinsic charm
contribution to the nucleon spin.Comment: 17 pages, 4 eps figures include
Polynomiality of unpolarized off-forward distribution functions and the D-term in the chiral quark-soliton model
Mellin moments of off-forward distribution functions are even polynomials of
the skewedness parameter. This constraint, called polynomiality property,
follows from Lorentz- and time-reversal invariance. We prove that the
unpolarized off-forward distribution functions in the chiral quark-soliton
model satisfy the polynomiality property. The proof is an important
contribution to the demonstration that the description of off-forward
distribution functions in the model is consistent. As a byproduct of the proof
we derive explicit model expressions for moments of the D-term and compute the
first coefficient in the Gegenbauer expansion for this term.Comment: 18 pages, no figures. Corrections and improvements in section 6. To
appear in Phys.Rev.
Trying to understand confinement in the Schroedinger picture
We study the gauge-invariant gaussian ansatz for the vacuum wave functional
and show that it potentially possesses many desirable features of the
Yang--Mills theory, like asymptotic freedom, mass generation through the
transmutation of dimensions and a linear potential between static quarks. We
point out that these (and other) features can be studied in a systematic way by
combining perturbative and 1/n expansions. Contrary to the euclidean approach,
confinement can be easily formulated and easily built in, if not derived, in
the variational Schroedinger approach.Comment: 21 pages, 1 figure. Lecture given at the 4th St.Petersburg Winter
School in Theoretical Physics, Feb. 22-28, 199
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