1,155 research outputs found
Model-independent analysis for determining mass splittings of heavy baryons
We study the hyperfine mass differences of heavy hadrons in the heavy quark
effect theory (HQET). The effects of one-gluon exchange interaction are
considered for the heavy mesons and baryons. Base on the known experimental
data, we predict the masses of some heavy baryons in a model-independent way.Comment: 14 pages, 1 figur
Exclusive Decuplet-Baryon Pair Production in Two-Photon Collisions
This work extends our previous studies of two-photon annihilation into
baryon-antibaryon pairs from spin-1/2 octet to spin-3/2 decuplet baryons. Our
approach is based on perturbative QCD and treats baryons as quark-diquark
systems. Using the same model parameters as in our previous work, supplemented
by QCD sum-rule results for decuplet baryon wave functions, we are able to give
absolute predictions for decuplet baryon cross sections without introducing new
parameters. We find that the cross section is of the same order
of magnitude as the proton cross section, well within experimental bounds.Comment: 14 pages, 2 figure
The dispersive contribution of decays and X(1576)
We study whether the broad enhancement X(1576) arises from the final state
interaction (FSI) of decays. We
consider both the absorptive and dispersive contribution of the above
amplitudes since the intermediate states are very close to .
The same mechanism leads to a similar enhancement around 1580 MeV in the
spectrum in the channel, which
can be used to test whether X(1576) can be ascribed to the FSI effect of
.Comment: 4 pages, 4 figure
Nucleon Polarizibilities for Virtual Photons
We generalize the sum rules for the nucleon electric plus magnetic
polarizability and for the nucleon spin-polarizability
, to virtual photons with . The dominant low energy cross
sections are represented in our calculation by one-pion-loop graphs of
relativistic baryon chiral perturbation theory and the -resonance
excitation. For the proton we find good agreement of the calculated
with empirical values obtained from integrating up
electroproduction data for . The proton spin-polarizability
switches sign around and it joins smoothly the
"partonic" curve, extracted from polarized deep-inelastic scattering, around
. For the neutron our predictions of and
agree reasonably well at with existing determinations.
Upcoming (polarized) electroproduction experiments will be able to test the
generalized polarizability sum rules investigated here.Comment: 12 pages, 5 figures, submittes to Nuclear Physics
QCD Sum Rule Calculation of Twist-3 Contributions to Polarized Nucleon Structure Functions
Using the framework of QCD sum rules we predict the twist-3 contribution to
the second moment of the polarized nucleon structure function . As the
relevant local operator depends explicitely on the gluon field, we employ a
recently studied interpolating nucleon current which contains three quark field
and one gluon field operator. Despite the fact that our calculation is based on
the analysis of a completely different correlation function, our estimates are
consitent with those of Balitsky, Braun and Kolesnichenko who used a
three-quark current.Comment: 16pp. , 2 figures (uuencoded eps-files), LateX. Some misprints
corrected, results unchange
Spectra of Free Diquark in the Bethe-Salpeter Approach
In this work, we employ the Bethe-Salpeter (B-S) equation to investigate the
spectra of free diquarks and their B-S wave functions. We find that the B-S
approach can be consistently applied to study the diqaurks with two heavy
quarks or one heavy and one light quarks, but for two light-quark systems, the
results are not reliable. There are a few free parameters in the whole scenario
which can only be fixed phenomenologically. Thus, to determine them, one has to
study baryons which are composed of quarks and diquarks.Comment: 16 pages, no figure
DECam integration tests on telescope simulator
The Dark Energy Survey (DES) is a next generation optical survey aimed at
measuring the expansion history of the universe using four probes: weak
gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and
Type Ia supernovae. To perform the survey, the DES Collaboration is building
the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera
which will be mounted at the Blanco 4-meter telescope at the Cerro Tololo
Inter- American Observatory. DES will survey 5000 square degrees of the
southern galactic cap in 5 filters (g, r, i, z, Y). DECam will be comprised of
74 250 micron thick fully depleted CCDs: 62 2k x 4k CCDs for imaging and 12 2k
x 2k CCDs for guiding and focus. Construction of DECam is nearing completion.
In order to verify that the camera meets technical specifications for DES and
to reduce the time required to commission the instrument, we have constructed a
full sized telescope simulator and performed full system testing and
integration prior to shipping. To complete this comprehensive test phase we
have simulated a DES observing run in which we have collected 4 nights worth of
data. We report on the results of these unique tests performed for the DECam
and its impact on the experiments progress.Comment: Proceedings of the 2nd International Conference on Technology and
Instrumentation in Particle Physics (TIPP 2011). To appear in Physics
Procedia. 8 pages, 3 figure
Pentaquark baryon production from photon-neuteron reactions
Extending the hadronic Lagrangians that we recently introduced for studying
pentaquark baryon production from meson-proton, proton-proton, and
photon-proton reactions near threshold to include the anomalous interaction
between and , we evaluate the cross section for
production from photon-neutron reactions, in which the was first
detected in the SPring-8 experiment in Japan and the CLAS experiment at Thomas
Jefferson National Laboratory. With empirical coupling constants and form
factors, and assuming that the decay width of is 20 MeV, the
predicted cross section is found to have a peak value of about 280 nb, which is
substantially larger than that for production from photon-proton
reactions.Comment: 13 pages, 6 figure
Pion-Nucleon Scattering in a Large-N Sigma Model
We review the large-N_c approach to meson-baryon scattering, including recent
interesting developments. We then study pion-nucleon scattering in a particular
variant of the linear sigma-model, in which the couplings of the sigma and pi
mesons to the nucleon are echoed by couplings to the entire tower of I=J
baryons (including the Delta) as dictated by large-N_c group theory. We sum the
complete set of multi-loop meson-exchange
\pi N --> \pi N and \pi N --> \sigma N Feynman diagrams, to leading order in
1/N_c. The key idea, reviewed in detail, is that large-N_c allows the
approximation of LOOP graphs by TREE graphs, so long as the loops contain at
least one baryon leg; trees, in turn, can be summed by solving classical
equations of motion. We exhibit the resulting partial-wave S-matrix and the
rich nucleon and Delta resonance spectrum of this simple model, comparing not
only to experiment but also to pion-nucleon scattering in the Skyrme model. The
moral is that much of the detailed structure of the meson-baryon S-matrix which
hitherto has been uncovered only with skyrmion methods, can also be described
by models with explicit baryon fields, thanks to the 1/N_c expansion.Comment: This LaTeX file inputs the ReVTeX macropackage; figures accompany i
Brane Gases in the Early Universe
Over the past decade it has become clear that fundamental strings are not the
only fundamental degrees of freedom in string theory. D-branes are also part of
the spectrum of fundamental states. In this paper we explore some possible
effects of D-branes on early Universe string cosmology, starting with two key
assumptions: firstly that the initial state of the Universe corresponded to a
dense, hot gas in which all degrees of freedom were in thermal equilibrium, and
secondly that the topology of the background space admits one-cycles. We argue
by t-duality that in this context the cosmological singularities are not
present. We derive the equation of state of the brane gases and apply the
results to suggest that, in an expanding background, the winding modes of
fundamental strings will play the most important role at late times. In
particular, we argue that the string winding modes will only allow four
space-time dimensions to become large. The presence of brane winding modes with
may lead to a hierarchy in the sizes of the extra dimensions.Comment: 8 pages, 1 figure; typos corrected; published in PR
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