914 research outputs found
Nucleon-Nucleon Scattering under Spin-Isospin Reversal in Large-N_c QCD
The spin-flavor structure of certain nucleon-nucleon scattering observables
derived from the large N_c limit of QCD in the kinematical regime where
time-dependent mean-field theory is valid is discussed. In previous work, this
regime was taken to be where the external momentum was of order N_c which
precluded the study of differential cross sections in elastic scattering. Here
it is shown that the regime extends down to order N_c^{1/2} which includes the
higher end of the elastic regime. The prediction is that in the large N_c
limit, observables describable via mean-field theory are unchanged when the
spin and isospin of either nucleon are both flipped. This prediction is tested
for proton-proton and neutron-proton elastic scattering data and found to fail
badly. We argue that this failure can be traced to a lack of a clear separation
of scales between momentum of order N_c^{1/2} and N_c^1 when N_c is as small as
three. The situation is compounded by an anomalously low particle production
threshold due to approximate chiral symmetry.Comment: 5 pages, 1 figur
Excited Baryon Decay Widths in Large N_c QCD
We study excited baryon decay widths in large N_c QCD. It was suggested
previously that some spin-flavor mixed-symmetric baryon states have strong
couplings of O(N_c^{-1/2}) to nucleons [implying narrow widths of O(1/N_c)], as
opposed to the generic expectation based on Witten's counting rules of an
O(N_c^0) coupling. The calculation obtaining these narrow widths was performed
in the context of a simple quark-shell model. This paper addresses the question
of whether the existence of such narrow states is a general property of large
N_c QCD. We show that a general large N_c QCD analysis does not predict such
narrow states; rather they are a consequence of the extreme simplicity of the
quark model.Comment: 9 page
The large-N(c) nuclear potential puzzle
An analysis of the baryon-baryon potential from the point of view of
large-N(c) QCD is performed. A comparison is made between the N(c)-scaling
behavior directly obtained from an analysis at the quark-gluon level to the
N(c)-scaling of the potential for a generic hadronic field theory in which it
arises via meson exchanges and for which the parameters of the theory are given
by their canonical large-N(c) scaling behavior. The purpose of this comparison
is to use large-N(c) consistency to test the widespread view that the
interaction between nuclei arises from QCD through the exchange of mesons.
Although at the one- and two-meson exchange level the scaling rules for the
potential derived from the hadronic theory matches the quark-gluon level
prediction, at the three- and higher-meson exchange level a generic hadronic
theory yields a potential which scales with N(c) faster than that of the
quark-gluon theory.Comment: 17 pages, LaTeX, 5 figure
1/N Expansion for Exotic Baryons
The 1/N expansion for exotic baryons is developed, and applied to the masses,
meson couplings and decay widths. Masses and widths of the 27 and 35 pentaquark
states in the same tower as the Theta+ are related by spin-flavor symmetry. The
27 and 35 states can decay within the pentaquark tower, as well as to normal
baryons, and so have larger decay widths than the lightest pentaquark Theta.
The 1/N expansion also is applied to baryon exotics containing a single heavy
antiquark. The decay widths of heavy pentaquarks via pion emission, and to
normal baryons plus heavy D^(*),B^(*) mesons are studied, and relations
following from large-N spin-flavor symmetry and from heavy quark symmetry are
derived.Comment: Major additions: plots of widths and branching ratios, discussion of
strong decays of heavy pentaquarks, including consequences of heavy quark
symmetr
Excited Baryons in Large N_c QCD Revisited: The Resonance Picture Versus Single-Quark Excitations
We analyze excited baryon properties via a 1/N_c expansion from two
perspectives: as resonances in meson-nucleon scattering, and as single-quark
excitations in the context of a simple quark model. For both types of analysis
one can derive novel patterns of degeneracy that emerge as N_c --> \infty, and
that are shown to be compatible with one another. This helps justify the
single-quark excitation picture and may give some insight into its successes.
We also find that in the large N_c limit one of the S_{11} baryons does not
couple to the pi-N channel but couples to the eta-N channel. This is
empirically observed in the N(1535), which couples very weakly to the pi-N
channel and quite strongly to the eta-N channel. The comparatively strong
coupling of the N(1650) to the pi-N channel and weak coupling to eta-N channel
is also predicted. In the context of the simple quark model picture we
reproduce expressions for mixing angles that are accurate up to O(1/N_c)
corrections and are in good agreement with mixing angles extracted
phenomenologically.Comment: 13 pages, ReVTeX
Conference Discussion of the Nuclear Force
Discussion of the nuclear force, lead by a round table consisting of T.
Cohen, E. Epelbaum, R. Machleidt, and F. Gross (chair). After an invited talk
by Machleidt, published elsewhere in these proceedings, brief remarks are made
by Epelbaum, Cohen, and Gross, followed by discussion from the floor moderated
by the chair. The chair asked the round table and the participants to focus on
the following issues: (i) What does each approach (chiral effective field
theory, large Nc, and relativistic phenomenology) contribute to our knowledge
of the nuclear force? Do we need them all? Is any one transcendent? (ii) How
important for applications (few body, nuclear structure, EMC effect, for
example) are precise fits to the NN data below 350 MeV? How precise do these
fits have to be? (iii) Can we learn anything about nonperturbative QCD from
these studies of the nuclear force? The discussion presented here is based on a
video recording made at the conference and transcribed afterward.Comment: Discussion at the 21st European Conference on Few Body Problems
(EFP21) held at Salamanca, Spain, 30 Aug - 3 Sept 201
noise in variable range hopping conduction
A mechanism of noise due to traps formed by impurities which have no
neighbors with close energies in their vicinity is studied. Such traps slowly
exchange electrons with the rest of conducting media. The concentration of
traps and proportional to it noise exponentially grow with decreasing
temperature in the variable range hopping regime. This theory provides smooth
transition to the nearest neighbor hopping case where it predicts a very weak
temperature dependence
Selection rules for J^PC Exotic Hybrid Meson Decay in Large-N_c
The coupling of a neutral hybrid {1,3,5...}^-+ exotic particle (or current)
to two neutral (hybrid) meson particles with the same J^PC and J=0 is proved to
be sub-leading to the usual large-N_c QCD counting. The coupling of the same
exotic particle to certain two - (hybrid) meson currents with the same J^PC and
J=0 is also sub-leading. The decay of a {1,3,5...}^-+ hybrid to eta pi^0, eta'
pi^0, eta' eta, eta(1295) pi^0, pi(1300)^0 pi0, eta(1440) pi^0, a_0(980)^0
sigma or f_0(980) sigma is sub-leading, assuming that these final state
particles are (hybrid) mesons in the limit of large N_c.Comment: 16 pages, LaTeX. Main paper shortened/rewritten and appendices
expanded. Implications for phenomenology of exotic hybrid mesons clarifie
(Field) Symmetrization Selection Rules
QCD and QED exhibit an infinite set of three-point Green's functions that
contain only OZI rule violating contributions, and (for QCD) are subleading in
the large N_c expansion. The Green's functions describe the ``decay'' of a
J^{PC}={1,3,5 ...}^{-+} exotic hybrid meson current to two J=0 (hybrid) meson
currents with identical P and C. We prove that the QCD amplitude for a neutral
hybrid {1,3,5 ...}^{-+} exotic current to create eta pi0 only comes from OZI
rule violating contributions under certain conditions, and is subleading in
N_c.Comment: 20 pages, LaTeX. Two postscript figures. Final published versio
Superstatistics of Brownian motion: A comparative study
The dynamics of temperature fluctuations of a gas of Brownian particles in
local equilibrium with a nonequilibrium heat bath, are described using an
approach consistent with Boltzmann-Gibbs statistics (BG). We use mesoscopic
nonequilibrium thermodynamics (MNET) to derive a Fokker-Planck equation for the
probability distribution in phase space including the local intensive variables
fluctuations. We contract the description to obtain an effective probability
distribution (EPD) from which the mass density, van Hove's function and the
dynamic structure factor of the system are obtained. The main result is to show
that in the long time limit the EPD exhibits a similar behavior as the
superstatistics distribution of nonextensive statistical mechanics (NESM),
therfore implying that the coarse-graining procedure is responsible for the so
called nonextensive effects.Comment: 14 pages,5 figure
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