23,586 research outputs found
Flavour decomposition of electromagnetic transition form factors of nucleon resonances
In Poincar\'e-covariant continuum treatments of the three valence-quark
bound-state problem, the force behind dynamical chiral symmetry breaking also
generates nonpointlike, interacting diquark correlations in the nucleon and its
resonances. We detail the impact of these correlations on the nucleon's elastic
and nucleon-to-Roper transition electromagnetic form factors, providing
flavour-separation versions that can be tested at modern facilities.Comment: Contribution to the proceedings of the 12th Quark Confinement and the
Hadron Spectrum (CONF12). Aug. 28 - Sep. 4, 2016. Thessaloniki, Greece. arXiv
admin note: text overlap with arXiv:1602.02768, arXiv:1508.0240
[C II] emission from galactic nuclei in the presence of X-rays
The luminosity of [C II] is used to probe the star formation rate in
galaxies, but the correlation breaks down in some active galactic nuclei
(AGNs). Models of the [C II] emission from galactic nuclei do not include the
influence of X-rays on the carbon ionization balance, which may be a factor in
reducing the [C II] luminosity. We calculate the [C II] luminosity in galactic
nuclei under the influence of bright sources of X-rays. We solve the balance
equation of the ionization states of carbon as a function of X-ray flux,
electron, atomic hydrogen, and molecular hydrogen density. These are input to
models of [CII] emission from the interstellar medium (ISM) in galactic nuclei.
We also solve the distribution of the ionization states of oxygen and nitrogen
in highly ionized regions. We find that the dense warm ionized medium (WIM) and
dense photon dominated regions (PDRs) dominate the [C II] emission when no
X-rays are present. The X-rays in galactic nuclei can affect strongly the C
abundance in the WIM converting some fraction to C and higher ionization
states and thus reducing its [C II] luminosity. For an X-ray luminosity >
10 erg/s the [C II] luminosity can be suppressed by a factor of a few,
and for very strong sources, >10 erg/s, such as found for many AGNs by
an order of magnitude. Comparison of the model with extragalactic sources shows
that the [C II] to far-infrared ratio declines for an X-ray luminosity
>10 erg/s, in reasonable agreement with our model.Comment: 16 pages and 14 figures, accepted for publication in A&
Effective-Hamiltonian modeling of external pressures in ferroelectric perovskites
The phase-transition sequence of a ferroelectric perovskite such as BaTiO_3
can be simulated by computing the statistical mechanics of a first-principles
derived effective Hamiltonian [Zhong, Vanderbilt and Rabe, Phys. Rev. Lett. 73,
1861 (1994)]. Within this method, the effect of an external pressure (in
general, of any external field) can be studied by considering the appropriate
"enthalpy" instead of the effective Hamiltonian itself. The legitimacy of this
approach relies on two critical assumptions that, to the best of our knowledge,
have not been adequately discussed in the literature to date: (i) that the
zero-pressure relevant degrees of freedom are still the only relevant degrees
of freedom at finite pressures, and (ii) that the truncation of the Taylor
expansion of the energy considered in the effective Hamiltonian remains a good
approximation at finite pressures. Here we address these issues in detail and
present illustrative first-principles results for BaTiO_3. We also discuss how
to construct effective Hamiltonians in cases in which these assumptions do not
hold.Comment: 5 pages, with 2 postscript figures embedded. Proceedings of
"Fundamental Physics of Ferroelectrics, 2002", R. Cohen and T. Egami, eds.
(AIP, Melville, New York, 2002). Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_effp/index.htm
Local supersymmetry without SUSY partners
A gauge theory for a superalgebra that includes an internal gauge (G) and
local Lorentz algebras, and that could describe the low energy particle
phenomenology is constructed. These two symmetries are connected by fermionic
supercharges. The system includes an internal gauge connection 1-form , a
spin-1/2 Dirac spinor , the Lorentz connection , and the vielbein
. The connection one-form is in the adjoint representation of G, while
is in the fundamental. In contrast to standard supergravity, the metric
is not a fundamental field and is in the center of the superalgebra: it is not
only invariant under the internal gauge group and under Lorentz
transformations, but is also invariant under supersymmetry. The features of
this theory that mark the difference with standard supersymmetry are: A) The
number of fermionic and bosonic states is not necessarily the same; B) There
are no superpartners with equal mass, "bosoninos", sleptons and squarks are
absent; C) Although this supersymmetry originates in a local gauge theory and
gravity is included, there is no gravitino; D) Fermions acquire mass from their
coupling to the background or from self-couplings, while bosons remain
massless. In odd dimensions, the Chern-Simons form provides an action that is
quasi-invariant under the entire superalgebra. In even dimensions, the
Yang-Mills form is the only natural option, and the symmetry breaks
down to [G x SO(1,D-1)]. In 4D, the construction follows the Townsend - Mac
Dowell-Mansouri approach. Due to the absence of osp(4|2)-invariant traces in
four dimensions, the resulting Lagrangian is only invariant under [U(1) x
SO(3,1)], and includes a Nambu--Jona-Lasinio term. In this case, the Lagrangian
depends on a single dimensionful parameter that fixes Newton's constant, the
cosmological constant and the NJL coupling.Comment: 24 pages, no figures. Title changed in journal version to
"Unconventional supersymmetry and its breaking". Few references added and
some paragraphs rewritten from v.1. This version includes two appendices that
are not found in the journal versio
Understanding the nucleon as a Borromean bound-state
Analyses of the three valence-quark bound-state problem in relativistic
quantum field theory predict that the nucleon may be understood primarily as a
Borromean bound-state, in which binding arises mainly from two separate
effects. One originates in non-Abelian facets of QCD that are expressed in the
strong running coupling and generate confined but strongly-correlated
colour-antitriplet diquark clusters in both the scalar-isoscalar and
pseudovector-isotriplet channels. That attraction is magnified by quark
exchange associated with diquark breakup and reformation. Diquark clustering is
driven by the same mechanism which dynamically breaks chiral symmetry in the
Standard Model. It has numerous observable consequences, the complete
elucidation of which requires a framework that also simultaneously expresses
the running of the coupling and masses in the strong interaction. Planned
experiments are capable of validating this picture.Comment: 7 pages, 7 figure
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