2,814 research outputs found
Asymptotic directional structure of radiation for fields of algebraic type D
The directional behavior of dominant components of algebraically special
spin-s fields near a spacelike, timelike or null conformal infinity is studied.
By extending our previous general investigations we concentrate on fields which
admit a pair of equivalent algebraically special null directions, such as the
Petrov type D gravitational fields or algebraically general electromagnetic
fields. We introduce and discuss a canonical choice of the reference tetrad
near infinity in all possible situations, and we present the corresponding
asymptotic directional structures using the most natural parametrizations.Comment: 20 pages, 6 figure
Doubly Heavy Tetraquarks and Baryons
During the last three years strong experimental evidence from and charm
factories has been accumulating for the existence of exotic hadronic quarkonia,
narrow resonances which cannot be made from a quark and an antiquark. Their
masses and decay modes show that they contain a heavy quark-antiquark pair, but
their quantum numbers are such that they must also contain a light
quark-antiquark pair. The theoretical challenge has been to determine the
nature of these resonances. The main possibilities are that they are either
"genuine tetraquarks", i.e. two quarks and two antiquarks within one
confinement volume, or "hadronic molecules" of two heavy-light mesons. In the
last few months there as been more and more evidence in favor of the latter. I
discuss the experimental data and its interpretation and provide fairly precise
predictions for masses and quantum numbers of the additional exotic states
which are naturally expected in the molecular picture but have yet to be
observed. In addition, I provide arguments in favor of the existence of an even
more exotic state - a hypothetical deuteron-like bound state of two heavy
baryons. I also consider "baryon-like" states , which if
found will be direct evidence not just for near-threshold binding of two heavy
mesons, but for genuine tetraquarks with novel color networks. I stress the
importance of experimental search for doubly-heavy baryons in this context.Comment: Invited plenary lecture at International Conference on New Frontiers
in Physics, Aug. 28-Sep. 2013, Kolymbari, Crete, Greec
Review of Rotational Symmetry Breaking in Baby Skyrme Models
We discuss one of the most interesting phenomena exhibited by baby skyrmions
-- breaking of rotational symmetry. The topics we will deal with here include
the appearance of rotational symmetry breaking in the static solutions of baby
Skyrme models, both in flat as well as in curved spaces, the zero-temperature
crystalline structure of baby skyrmions, and finally, the appearance of
spontaneous breaking of rotational symmetry in rotating baby skyrmions.Comment: 35 pages, 16 figures. A version of this manuscript with
higher-resolution figures is available at
http://www.tau.ac.il/~itayhe/SkReview/SkReview.ra
Spin Structure Functions
We review the theory and phenomenology of deep inelastic polarized
lepton-nucleon scattering in the light of recent data with a deuteron target
from the SMC at CERN and a Helium 3 target from the E142 experiment at SLAC.
After including higher-order perturbative QCD corrections, mass corrections and
updated estimates of higher-twist effects, we find good agreement with the
basic Bjorken sum rule, and extract a consistent set of values for the quark
contributions to the proton spin: which are consistent with chiral soliton
models and indications from lattice estimates. We also mention the prospects
for future experiments on the spin structure of the nucleon.Comment: 18 pages (LateX) + 6 postscript figures encoded through uufiles;
CERN-TH-7022/93, TAUP-2094-93; Plenary talk at the PANIC '93 (corrected eq.
(3)
A diquark model for the d*(2380) dibaryon resonance?
Diquark models have been applied with varying degree of success to tetraquark
and pentaquark states involving both light and heavy quark degrees of freedom.
We discuss the applicability of such models to light quark dibaryons, viewed as
three-diquark objects. Highlighting the case of the d*(2380) dibaryon
resonance, we demonstrate the inapplicability of diquark models in the light
quark sector.Comment: v2 -- inessential minor changes with respect to v1, matches published
EPJC versio
Scattering and Resonances in QCD_2
Extending previous works on the spectrum of QCD_2, we now investigate the 2D
analogue of meson-baryon scattering. We use semi-classical methods, perturbing
around classical soliton solutions. We start with the abelian case,
corresponding to one flavor,and find that in this case the effective potential
is reflectionless. We obtain an explicit expression for the forward phase
shift. In the non-abelian case of several flavors, the method yields a
potential which depends on the momentum of the incoming particle. In this case
there is both transmission and reflection. In both cases no resonances appear.
As a byproduct, we derive the general conditions for a 2D quantum field
theoretical action to yield a reflectionless effective potential when one
expands in small fluctuations about the classical solution.Comment: Figs. 2 and 3 and the accompanying text amended, following sign
correction in the first term in the potential V in eq. (58). Qualitative
conclusions unchange
Diquarks and antiquarks in exotics: a menage a trois and a menage a quatre
A menage a trois is very different from an ordinary family. Similarly, exotic
hadrons with both qq and qbar q pairs have important color-space correlations
that are completely absent in ordinary mesons and baryons. The presence of both
types of pairs requires attention to the basic QCD physics that the q qbar
interaction is much stronger than the qq interaction. This new physics in
multiquark systems produces color structures totally different from those of
normal hadrons, for example the ud system is utterly unlike the ud diquark in
the uds Lambda baryon. The color-space correlations produce unusual
experimental properties in tetraquarks with heavy quark pairs which may be
relevant for newly discovered mesons like the X(3872) resonance. Tetraquark
masses can be below the two-meson threshold for sufficiently high quark masses.
A simple model calculation shows the b q bbar ubar and b q cbar qbar
tetraquarks below the B Bbar and B Dbar thresholds. Some of these states have
exotic electric charge and their decays might have striking signatures
involving monoenergetic photons and/or pions.Comment: Journal version (PLB
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