12,638 research outputs found
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
Ward-Green-Takahashi identities and the axial-vector vertex
The colour-singlet axial-vector vertex plays a pivotal role in understanding
dynamical chiral symmetry breaking and numerous hadronic weak interactions, yet
scant model-independent information is available. We therefore use longitudinal
and transverse Ward-Green-Takahashi (WGT) identities, together with kinematic
constraints, in order to ameliorate this situation and expose novel features of
the axial vertex: amongst them, Ward-like identities for elements in the
transverse piece of the vertex, which complement and shed new light on
identities determined previously for components in its longitudinal part. Such
algebraic results are verified via solutions of the Bethe-Salpeter equation for
the axial vertex obtained using two materially different kernels for the
relevant Dyson-Schwinger equations. The solutions also provide insights that
suggest a practical Ansatz for the axial-vector vertex.Comment: 7 pages, 3 figure
Nucleon spin structure at very high-x
Dyson-Schwinger equation treatments of the strong interaction show that the
presence and importance of nonpointlike diquark correlations within the nucleon
are a natural consequence of dynamical chiral symmetry breaking. Using this
foundation, we deduce a collection of simple formulae, expressed in terms of
diquark appearance and mixing probabilities, from which one may compute ratios
of longitudinal-spin-dependent u- and d-quark parton distribution functions on
the domain x =1. A comparison with predictions from other approaches plus a
consideration of extant and planned experiments shows that the measurement of
nucleon longitudinal spin asymmetries on x =1 can add considerably to our
capacity for discriminating between contemporary pictures of nucleon structure.Comment: 6 pages, 1 table, 3 figures. To appear in Phys. Lett.
Survey of vector-like fermion extensions of the Standard Model and their phenomenological implications
With the renewed interest in vector-like fermion extensions of the Standard
Model, we present here a study of multiple vector-like theories and their
phenomenological implications. Our focus is mostly on minimal flavor conserving
theories that couple the vector-like fermions to the SM gauge fields and mix
only weakly with SM fermions so as to avoid flavor problems. We present
calculations for precision electroweak and vector-like state decays, which are
needed to investigate compatibility with currently known data. We investigate
the impact of vector-like fermions on Higgs boson production and decay,
including loop contributions, in a wide variety of vector-like extensions and
their parameter spaces.Comment: 43 pages, 17 figures; v2: text modified to improve readability,
references added, journal versio
The power of low-resolution spectroscopy: On the spectral classification of planet candidates in the ground-based CoRoT follow-up
Planetary transits detected by the CoRoT mission can be mimicked by a
low-mass star in orbit around a giant star. Spectral classification helps to
identify the giant stars and also early-type stars which are often excluded
from further follow-up.
We study the potential and the limitations of low-resolution spectroscopy to
improve the photometric spectral types of CoRoT candidates. In particular, we
want to study the influence of the signal-to-noise ratio (SNR) of the target
spectrum in a quantitative way. We built an own template library and
investigate whether a template library from the literature is able to reproduce
the classifications. Including previous photometric estimates, we show how the
additional spectroscopic information improves the constraints on spectral type.
Low-resolution spectroscopy (1000) of 42 CoRoT targets covering a
wide range in SNR (1-437) and of 149 templates was obtained in 2012-2013 with
the Nasmyth spectrograph at the Tautenburg 2m telescope. Spectral types have
been derived automatically by comparing with the observed template spectra. The
classification has been repeated with the external CFLIB library.
The spectral class obtained with the external library agrees within a few
sub-classes when the target spectrum has a SNR of about 100 at least. While the
photometric spectral type can deviate by an entire spectral class, the
photometric luminosity classification is as close as a spectroscopic
classification with the external library. A low SNR of the target spectrum
limits the attainable accuracy of classification more strongly than the use of
external templates or photometry. Furthermore we found that low-resolution
reconnaissance spectroscopy ensures that good planet candidates are kept that
would otherwise be discarded based on photometric spectral type alone.Comment: accepted for publication in Astronomische Nachrichten; 12 pages, 4
figures, 7 table
Parton distribution amplitudes of light vector mesons
A rainbow-ladder truncation of QCD's Dyson-Schwinger equations is used to
calculate rho- and phi-meson valence-quark (twist-two parton) distribution
amplitudes (PDAs) via a light-front projection of their Bethe-Salpeter wave
functions, which possess S- and D-wave components of comparable size in the
meson rest frame. All computed PDAs are broad concave functions, whose dilation
with respect to the asymptotic distribution is an expression of dynamical
chiral symmetry breaking. The PDAs can be used to define an ordering of
valence-quark light-front spatial-extent within mesons: this size is smallest
within the pion and increases through the perp-polarisation to the
parallel-polarisation of the vector mesons; effects associated with the
breaking of SU(3)-flavour symmetry are significantly smaller than those
associated with altering the polarisation of vector mesons. Notably, the
predicted pointwise behaviour of the rho-meson PDAs is in quantitative
agreement with that inferred recently via an analysis of diffractive
vector-meson photoproduction experiments.Comment: 13 pages, 4 figures, 4 table
Optimal Renormalization Group Transformation from Information Theory
Recently a novel real-space RG algorithm was introduced, identifying the
relevant degrees of freedom of a system by maximizing an information-theoretic
quantity, the real-space mutual information (RSMI), with machine learning
methods. Motivated by this, we investigate the information theoretic properties
of coarse-graining procedures, for both translationally invariant and
disordered systems. We prove that a perfect RSMI coarse-graining does not
increase the range of interactions in the renormalized Hamiltonian, and, for
disordered systems, suppresses generation of correlations in the renormalized
disorder distribution, being in this sense optimal. We empirically verify decay
of those measures of complexity, as a function of information retained by the
RG, on the examples of arbitrary coarse-grainings of the clean and random Ising
chain. The results establish a direct and quantifiable connection between
properties of RG viewed as a compression scheme, and those of physical objects
i.e. Hamiltonians and disorder distributions. We also study the effect of
constraints on the number and type of coarse-grained degrees of freedom on a
generic RG procedure.Comment: Updated manuscript with new results on disordered system
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