310 research outputs found
Threshold Hyperon Production at COSY-11
The Lambda, Sigma0 and Sigma+ hyperon production in NN collisions is studied
at the COSY - 11 installation in order to investigate the production mechanism
as well as to extract information about the Y-N interaction.Comment: 3 pages, 2 figure
Nonbackscattering Contribution to the Weak Localization
We show that the enhancement of backscattering responsible for the weak
localization is accompanied by reduction of the scattering in other directions.
A simple quasiclassical interpretation of this phenomenon is presented in terms
of a small change in the effective differential cross-section for a single
impurity. The reduction of the scattering at the arbitrary angles leads to the
decrease of the quantum correction to the conductivity. Within the diffusion
approximation this decrease is small, but it should be taken into account in
the case of a relatively strong magnetic field when the diffusion approximation
is not valid.Comment: 18 pages, 6 figures, Submitted to PR
Threshold hyperon production in proton-proton collisions at COSY-11
Sigma+ hyperon production was measured at the COSY-11 spectrometer via the p
p --> n K+ Sigma+ reaction at excess energies of Q = 13 MeV and Q = 60 MeV.
These measurements continue systematic hyperon production studies via the p p
--> p K+ Lambda/Sigma0 reactions where a strong decrease of the cross section
ratio close-to-threshold was observed. In order to verify models developed for
the description of the Lambda and Sigma0 production we have performed the
measurement on the Sigma+ hyperon and found unexpectedly that the total cross
section is by more than one order of magnitude larger than predicted by all
anticipated models.
After the reconstruction of the kaon and neutron four momenta, the Sigma+ is
identified via the missing mass technique. Details of the method and the
measurement will be given and discussed in view of theoretical models.Comment: 7 pages, 8 figures, Phys. Lett. B in print, in the second version a
minor editorial changes have been made by a native speaking proofreader and a
note in proof has been added with an additional referenc
Single parameter scaling in one-dimensional localization revisited
The variance of the Lyapunov exponent is calculated exactly in the
one-dimensional Anderson model with random site energies distributed according
to the Cauchy distribution. We find a new significant scaling parameter in the
system, and derive an exact analytical criterion for single parameter scaling
which differs from the commonly used condition of phase randomization. The
results obtained are applied to the Kronig-Penney model with the potential in
the form of periodically positioned -functions with random strength.Comment: Phys. Rev. Lett. 84, 2678 (2000
Mean Free Path and Energy Fluctuations in Quantum Chaotic Billiards
The elastic mean free path of carriers in a recently introduced model of
quantum chaotic billiards in two and three dimensions is calculated. The model
incorporates surface roughness at a microscopic scale by randomly choosing the
atomic levels at the surface sites between -W/2 and W/2. Surface roughness
yields a mean free path l that decreases as L/W^2 as W increases, L being the
linear size of the system. But this diminution ceases when the surface layer
begins to decouple from the bulk for large enough values of W, leaving more or
less unperturbed states on the bulk. Consequently, the mean free path shows a
minimum of about L/2 for W of the order of the band width. Energy fluctuations
reflect the behavior of the mean free path. At small energy scales, strong
level correlations manifest themselves by small values of the number of levels
variance Sigma^2(E) that are close to Random Matrix Theory (RMT) in all cases.
At larger energy scales, fluctuations are below the logarithmic behavior of RMT
for l > L, and above RMT value when l < L.Comment: 8 twocolumn pages, seven figures, revtex and epsf macros. To be
published in Physical Review B
A New Measurement of the Radiative Decay Width
High precision measurements of the differential cross sections for
photoproduction at forward angles for two nuclei, C and Pb, have
been performed for incident photon energies of 4.9 - 5.5 GeV to extract the
decay width. The experiment was done at Jefferson
Lab using the Hall B photon tagger and a high-resolution multichannel
calorimeter. The decay width was extracted by
fitting the measured cross sections using recently updated theoretical models
for the process. The resulting value for the decay width is . With the 2.8% total uncertainty, this result is a factor of 2.5 more
precise than the current PDG average of this fundamental quantity and it is
consistent with current theoretical predictions.Comment: 4 pages, 5 figure
Associated strangeness production at threshold
The associated strangeness dissociation at threshold has been studied at the
COSY-11 facility measuring the hyperon - and the K+K- meson pair production.
Measurements of the near threshold Lambda and Sigma0 production via the pp ->
pK+ Lambda / Sigma0 reaction at COSY-11 have shown that the Lambda / Sigma0
cross section ratio exceeds the value at high excess energies (Q >= 300 MeV) by
an order of magnitude. For a better understanding additional data have been
taken between 13 MeV and 60 MeV excess energy.
The near threshold production of the charged kaon-antikaon pair is related to
the discussion about the nature of the scalar states in the 1 GeV/c^2 mass
range, i.e. the f0(980) and a0(980). The interpretation as a K anti-K molecule
is strongly dependent on the K anti-K interaction which can be studied via the
production channel. A first total cross section value on the reaction pp ->
ppK+K- at an excess energy of 17 MeV i.e. below the phi production threshold
was measured.Comment: 3 pages, 2 figures, Presented at Conference on Quarks and Nuclear
Physics (QNP 2002), Julich, Germany, 9-14 Jun 200
The proton and deuteron F_2 structure function at low Q^2
Measurements of the proton and deuteron structure functions are
presented. The data, taken at Jefferson Lab Hall C, span the four-momentum
transfer range GeV, and Bjorken values from 0.009 to
0.45, thus extending the knowledge of to low values of at low .
Next-to-next-to-leading order calculations using recent parton distribution
functions start to deviate from the data for GeV at the low and
high -values. Down to the lowest value of , the structure function is
in good agreement with a parameterization of based on data that have been
taken at much higher values of or much lower values of , and which is
constrained by data at the photon point. The ratio of the deuteron and proton
structure functions at low remains well described by a logarithmic
dependence on at low .Comment: 3 figures, submitted pape
Distribution of time-constants for tunneling through a 1D Disordered Chain
The dynamics of electronic tunneling through a disordered 1D chain of finite
length is considered. We calculate distributions of the transmission
coefficient T, Wigner delay time and, and the transport time,
. The central bodies of these distributions have a power-law
form, what can be understood in terms of the resonant tunneling through
localised states.Comment: 5 pages, 3 figures, submitted to PR
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