2,214 research outputs found
Electroexcitation of the Roper resonance from CLAS data
The helicity amplitudes of the electroexcitation of the Roper resonance on
proton are extracted at 1.7 < Q2 < 4.2 GeV2 from recent high precision
JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for
pi+ electroproduction on protons. The analysis is made using two approaches,
dispersion relations and unitary isobar model, which give consistent results.
It is found that the transverse helicity amplitude for the gamma* p -->
P11(1440) transition, which is large and negative at Q2=0, becomes large and
positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. Longitudinal helicity
amplitude, that was previously found from CLAS data as large and positive at
Q2=0.4,0.65 GeV2, drops with Q2. These results rule out the presentation of
P11(1440) as a 3qG hybrid state, and provide strong evidence in favor of this
resonance as a first radial excitation of the 3q ground state.Comment: 3 pages, 2 figures, Talk on the Workshop on "The Physics of Excited
Nucleons", Bonn, Germany, October 200
Baryon Resonance Analysis from SAID
We discuss the analysis of data from piN elastic scattering and single pion
photo- and electroproduction. The main focus is a study of low-lying
non-strange baryon resonances. Here we concentrate on some difficulties
associated with resonance identification, in particular the Roper and higher
P11 states.Comment: 4 pages, 6 figures; Nstar2009 Conf Proceedings; small revisio
Updated resonance photo-decay amplitudes to 2 GeV
We present the results of an energy-dependent and set of single-energy
partial-wave analyses of single-pion photoproduction data. These analyses
extend from threshold to 2 GeV in the laboratory photon energy, and update our
previous analyses to 1.8 GeV. Photo-decay amplitudes are extracted for the
baryon resonances within this energy range. We consider two photoproduction sum
rules and the contributions of two additional resonance candidates found in our
most recent analysis of elastic scattering data. Comparisons are made
with previous analyses.Comment: Revtex, 26 pages, 3 figures. Postscript figures available from
ftp://clsaid.phys.vt.edu/pub/pr or indirectly from
http://clsaid.phys.vt.edu/~CAPS
Study of nucleon resonances with electromagnetic interactions
Recent developments in using electromagnetic meson production reactions to
study the structure of nucleon resonances are reviewed. Possible future works
are discussed.Comment: 25 pages, 19 figure
Phases of a conserved mass model of aggregation with fragmentation at fixed sites
To study the effect of quenched disorder in a class of reaction-diffusion
systems, we introduce a conserved mass model of diffusion and aggregation in
which the mass moves as a whole to a nearest neighbour on most sites while it
fragments off as a single monomer (i.e. chips off) from certain fixed sites.
Once the mass leaves any site, it coalesces with the mass present on its
neighbour. We study in detail the effect of a \emph{single} chipping site on
the steady state in arbitrary dimensions, with and without bias. In the
thermodynamic limit, the system can exist in one of the following phases -- (a)
Pinned Aggregate (PA) phase in which an infinite aggregate (with mass
proportional to the volume of the system) appears with probability one at the
chipping site but not in the bulk. (b) Unpinned Aggregate (UA) phase in which
\emph{both} the chipping site and the bulk can support an infinite aggregate
simultaneously. (c) Non Aggregate (NA) phase in which there is no infinite
cluster. Our analytical and numerical studies show that the system exists in
the UA phase in all cases except in 1d with bias. In the latter case, there is
a phase transition from the NA phase to the PA phase as density is increased. A
variant of the above aggregation model is also considered in which total
particle number is conserved and chipping occurs at a fixed site, but the
particles do not interact with each other at other sites. This model is solved
exactly by mapping it to a Zero Range Process. With increasing density, it
exhibits a phase transition from the NA phase to the PA phase in all
dimensions, irrespective of bias. Finally, we discuss the likely behaviour of
the system in the presence of extensive disorder.Comment: RevTex, 19 pages including 11 figures, submitted to Phys. Rev.
Sympathetic Cooling with Two Atomic Species in an Optical Trap
We simultaneously trap ultracold lithium and cesium atoms in an optical
dipole trap formed by the focus of a CO laser and study the exchange of
thermal energy between the gases. The cesium gas, which is optically cooled to
K, efficiently decreases the temperature of the lithium gas through
sympathetic cooling. The measured cross section for thermalizing
Cs-Li collisions is cm, for both species in
their lowest hyperfine ground state. Besides thermalization, we observe
evaporation of lithium purely through elastic cesium-lithium collisions
(sympathetic evaporation).Comment: 4 pages 3 fig
A quark model framework for the study of nuclear medium effects
A quark-model framework for studying nuclear medium effects on nucleon
resonances is described and applied here to pion photoproduction on the
deuteron, which is the simplest composite nucleon system and serves as a first
test case. Pion photoproduction on nuclei is discussed within a chiral
constituent quark model in which the quark degrees of freedom are explicitly
introduced through an effective chiral Lagrangian for the
quark-pseudoscalar-meson coupling. The advantage of this model is that a
complete set of nucleon resonances can be systematically included with a
limited number of parameters. Also, the systematic description of the nucleon
and its resonances at quark level allows us to self-consistently relate the
nuclear medium's influence on the baryon properties to the intrinsic dynamic
aspects of the baryons. As the simplest composite nucleus, the deuteron
represents the first application of this effective theory for meson
photoproduction on light nuclei. The influence of the medium on the transition
operators for a free nucleon is investigated in the Delta resonance region. No
evidence is found for a change of the Delta properties in the pion
photoproduction reaction on the deuteron since the nuclear medium here involves
just one other nucleon and the low binding energy implies low nuclear density.
However, we show that the reaction mechanism is in principle sensitive to
changes of Delta properties that would be produced by the denser nuclear medium
of heavier nuclei through the modification of the quark model parameters.Comment: Revtex, 8 pages, 4 figure
Proton polarizability contribution to the hydrogen hyperfine splitting
The contribution of the proton polarizability to the hydrogen hyperfine
splitting is evaluated on the basis of modern experimental and theoretical
results on the proton polarized structure functions. The value of this
correction is equal to 1.4 ppm.Comment: 11 pages, LaTeX2.09, 7 figures, uses linedraw.sty, psfig.sty,
epsf.st
Yang-Lee Zeros of the Q-state Potts Model on Recursive Lattices
The Yang-Lee zeros of the Q-state Potts model on recursive lattices are
studied for non-integer values of Q. Considering 1D lattice as a Bethe lattice
with coordination number equal to two, the location of Yang-Lee zeros of 1D
ferromagnetic and antiferromagnetic Potts models is completely analyzed in
terms of neutral periodical points. Three different regimes for Yang-Lee zeros
are found for Q>1 and 0<Q<1. An exact analytical formula for the equation of
phase transition points is derived for the 1D case. It is shown that Yang-Lee
zeros of the Q-state Potts model on a Bethe lattice are located on arcs of
circles with the radius depending on Q and temperature for Q>1. Complex
magnetic field metastability regions are studied for the Q>1 and 0<Q<1 cases.
The Yang-Lee edge singularity exponents are calculated for both 1D and Bethe
lattice Potts models. The dynamics of metastability regions for different
values of Q is studied numerically.Comment: 15 pages, 6 figures, with correction
Constructing Hybrid Baryons with Flux Tubes
Hybrid baryon states are described in quark potential models as having
explicit excitation of the gluon degrees of freedom. Such states are described
in a model motivated by the strong coupling limit of Hamiltonian lattice gauge
theory, where three flux tubes meeting at a junction play the role of the glue.
The adiabatic approximation for the quark motion is used, and the flux tubes
and junction are modeled by beads which are attracted to each other and the
quarks by a linear potential, and vibrate in various string modes. Quantum
numbers and estimates of the energies of the lightest hybrid baryons are
provided.Comment: 4 pages, RevTeX. Submitted to Physical Review Letter
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