21,309 research outputs found

### Field-induced long-range order in the S=1 antiferromagnetic chain

The quasi-one dimensional S=1 antiferromagnet in magnetic field H is
investigated with the exact diagonalization of finite chains and the mean field
approximation for the interchain interaction. In the presence of the single-ion
anisotropy D, the full phase diagram in the $HT$ plane is presented for H
\parallel D and H \perp D. The shape of the field-induced long-range ordered
phase is revealed to be quite different between the two cases, as observed in
the recent experiment of NDMAP. The estimated ratio of the interchain and
intrachain couplings of NDMAP (J'/J ~ 10^{-3}) is consistent with the neutron
scattering measurement.Comment: 4 pages, Revtex, with 6 eps figure

### More on a holographic dual of QCD

We investigate the interactions among the pion, vector mesons and external
gauge fields in the holographic dual of massless QCD proposed in a previous
paper, hep-th/0412141, on the basis of probe D8-branes embedded in a D4-brane
background in type IIA string theory. We obtain the coupling constants by
performing both analytic and numerical calculations, and compare them with
experimental data. It is found that the vector meson dominance in the pion form
factor as well as in the Wess-Zumino-Witten term holds in an intriguing manner.
We also study the \omega to \pi\gamma and \omega to 3\pi decay amplitudes. It
is shown that the interactions relevant to these decay amplitudes have the same
structure as that proposed by Fujiwara et al. Various relations among the
masses and the coupling constants of an infinite tower of mesons are derived.
These relations play crucial roles in the analysis. We find that most of the
results are consistent with experiments.Comment: Latex2e, 40 pages, 6 figures, v2: references added, typos corrected,
v3: PTP-style, typos corrected, v4: minor correction

### How does gravity save or kill Q-balls?

We explore stability of gravitating Q-balls with potential
$V_4(\phi)={m^2\over2}\phi^2-\lambda\phi^4+\frac{\phi^6}{M^2}$ via catastrophe
theory, as an extension of our previous work on Q-balls with potential
$V_3(\phi)={m^2\over2}\phi^2-\mu\phi^3+\lambda\phi^4$. In flat spacetime
Q-balls with $V_4$ in the thick-wall limit are unstable and there is a minimum
charge $Q_{{\rm min}}$, where Q-balls with $Q<Q_{{\rm min}}$ are nonexistent.
If we take self-gravity into account, on the other hand, there exist stable
Q-balls with arbitrarily small charge, no matter how weak gravity is. That is,
gravity saves Q-balls with small charge. We also show how stability of Q-balls
changes as gravity becomes strong.Comment: 10 pages, 10 figure

### Unified Analysis of Spin Isospin Responses of Nuclei

We investigated the Gamow-Teller (GT) strength distribution, especially the
quenching with respect to the GT sum rule, and the enhancement of the pionic
responses in the quasielasic scattering region, in the same theoretical
framework. That is the continuum random phase approximation with the pi+rho+g'
model interaction, incorporated with distorted wave impulse approximation and
two-step calculations. From this analysis we searched the Landau-Migdal
parameters, g'NN and g'ND, through the comparison with the experimental data of
the GT strength distribution obtained at 300 MeV and the spin-longitudinal
(pionic) cross sections IDq of (p,n) at 350 and 500 MeV. This comprehensive and
sophisticated study gave a common set of g'NN=0.6-0.7 and g'ND=0.2-0.4, for
both low and high momentum transfers.Comment: 11 pages, 4 figures, submitted to Phys. Rev.

### Low energy hadron physics in holographic QCD

We present a holographic dual of four-dimensional, large N_c QCD with
massless flavors. This model is constructed by placing N_f probe D8-branes into
a D4 background, where supersymmetry is completely broken. The chiral symmetry
breaking in QCD is manifested as a smooth interpolation of D8 - anti-D8 pairs
in the supergravity background. The meson spectrum is examined by analyzing a
five-dimensional Yang-Mills theory that originates from the non-Abelian DBI
action of the probe D8-brane. It is found that our model yields massless pions,
which are identified with Nambu-Goldstone bosons associated with the chiral
symmetry breaking. We obtain the low-energy effective action of the pion field
and show that it contains the usual kinetic term of the chiral Lagrangian and
the Skyrme term. A brane configuration that defines a dynamical baryon is
identified with the Skyrmion. We also derive the effective action including the
lightest vector meson. Our model is closely related to that in the hidden local
symmetry approach, and we obtain a
Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin-type relation among the couplings.
Furthermore, we investigate the Chern-Simons term on the probe brane and show
that it leads to the Wess-Zumino-Witten term. The mass of the \eta' meson is
also considered, and we formulate a simple derivation of the \eta' mass term
satisfying the Witten-Veneziano formula from supergravity.Comment: 46 pages, 1 figure, minor corrections, version to appear in Prog.
Theor. Phys, v4: typos corrected, v5: The normalization of the 5d Yang-Mills
action given in Eq. (5.1) is corrected. Some of the expressions in section 5
are modified accordingly, but all the conclusions remain unchange

### Unified pictures of Q-balls and Q-tubes

While Q-balls have been investigated intensively for many years, another type
of nontopological solutions, Q-tubes, have not been understood very well. In
this paper we make a comparative study of Q-balls and Q-tubes. First, we
investigate their equilibrium solutions for four types of potentials. We find,
for example, that in some models the charge-energy relation is similar between
Q-balls and Q-tubes while in other models the relation is quite different
between them. To understand what determines the charge-energy relation, which
is a key of stability of the equilibrium solutions, we establish an analytical
method to obtain the two limit values of the energy and the charge. Our
prescription indicates how the existent domain of solutions and their stability
depends on their shape as well as potentials, which would also be useful for a
future study of Q-objects in higher-dimensional spacetime.Comment: 11 pages, 14 figure

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