4,574 research outputs found
The Reaction 7Li(pi+,pi-)7B and its Implications for 7B
The reaction 7Li(pi+,pi-)7B has been measured at incident pion energies of
30-90 MeV. 7Li constitutes the lightest target nucleus, where the pionic charge
exchange may proceed as a binary reaction to a discrete final state. Like in
the Delta-resonance region the observed cross sections are much smaller than
expected from the systematics found for heavier nuclei. In analogy to the
neutron halo case of 11Li this cross section suppression is interpreted as
evidence for a proton halo in the particle-unstable nucleus 7B.Comment: 4 pages, 4 figure
Search for Exotic Muon Decays
Recently, it has been proposed that the observed anomaly in the time
distribution of neutrino induced reactions, reported by the KARMEN
collaboration, can be interpreted as a signal from an exotic muon decay branch
mu+ to e+ X. It has been shown that this hypothesis gives an acceptable fit to
the KARMEN data if the boson X has a mass of m_X=103.9MeV/c^2, close to the
kinematical limit. We have performed a search for the X particle by studying
for the first time the very low energy part of the Michel spectrum in mu+
decays. Using a HPGe detector setup at the muE4 beamline at PSI we find
branching ratios BR(mu+ to e+ X)<5.7e-4 (90% C.L.) for most of the region
103MeV/c^2<m_X<105MeV/c^2.Comment: 9 page
Pionic charge exchange on the proton from 40 to 250 MeV
The total cross sections for pionic charge exchange on hydrogen were measured
using a transmission technique on thin CH2 and C targets. Data were taken for
pi- lab energies from 39 to 247 MeV with total errors of typically 2% over the
Delta-resonance and up to 10% at the lowest energies. Deviations from the
predictions of the SAID phase shift analysis in the 60 to 80 MeV region are
interpreted as evidence for isospin-symmetry breaking in the s-wave amplitudes.
The charge dependence of the Delta-resonance properties appears to be smaller
than previously reported
The in-medium isovector pi N amplitude from low energy pion scattering
Differential cross sections for elastic scattering of 21.5 MeV positive and
negative pions by Si, Ca, Ni and Zr have been measured as part of a study of
the pion-nucleus potential across threshold. The `anomalous' repulsion in the
s-wave term was observed, as is the case with pionic atoms. The extra repulsion
can be accounted for by a chiral-motivated model where the pion decay constant
is modified in the medium. Unlike in pionic atoms, the anomaly cannot be
removed by merely introducing an empirical on-shell energy dependence.Comment: 9 pages, 2 figures. Minor changes, to appear in PR
Elastic scattering of low energy pions by nuclei and the in-medium isovector pi N amplitude
Measurements of elastic scattering of 21.5 MeV pi+ and pi- by Si, Ca, Ni and
Zr were made using a single arm magnetic spectrometer. Absolute calibration was
made by parallel measurements of Coulomb scattering of muons. Parameters of a
pion-nucleus optical potential were obtained from fits to all eight angular
distributions put together. The `anomalous' s-wave repulsion known from pionic
atoms is clearly observed and could be removed by introducing a
chiral-motivated density dependence of the isovector scattering amplitude,
which also greatly improved the fits to the data. The empirical energy
dependence of the isoscalar amplitude also improves the fits to the data but,
contrary to what is found with pionic atoms, on its own is incapable of
removing the anomaly.Comment: 20 pages, 5 figures, 5 tables. V2 added details on
uncertainties,extended discussion. To appear in PR
Carburisation of ferritic FeâCr alloys by low carbon activity gases
Model FeâCr alloys were exposed to ArâCO2âH2O gas mixtures at 650 and 800 °C. At equilibrium, these atmospheres are oxidising to the alloys, but decarburising (aC â 10â15 to 10â13). In addition to developing external oxide scales, however, the alloys also carburised. Carbon supersaturation at the scale/alloy interface relative to the gas reflects local equilibrium: a low oxygen potential corresponds to a high pCO/pCO2 ratio, and hence to a high carbon activity. Interfacial carbon activities calculated on the basis of scaleâalloy equilibrium are shown to be in good agreement with measured carburisation rates and precipitate volume fractions, providing support for the validity of the thermodynamic model
Single-Pion Production in pp Collisions at 0.95 GeV/c (II)
The single-pion production reactions , and
were measured at a beam momentum of 0.95 GeV/c (
400 MeV) using the short version of the COSY-TOF spectrometer. The central
calorimeter provided particle identification, energy determination and neutron
detection in addition to time-of-flight and angle measurements from other
detector parts. Thus all pion production channels were recorded with 1-4
overconstraints. Main emphasis is put on the presentation and discussion of the
channel, since the results on the other channels have already been
published previously. The total and differential cross sections obtained are
compared to theoretical calculations. In contrast to the channel we
find in the channel a strong influence of the excitation
already at this energy close to threshold. In particular we find a dependence in the pion angular distribution, typical for a
pure s-channel excitation and identical to that observed in the
channel. Since the latter is understood by a s-channel resonance in
the partial wave, we discuss an analogous scenario for the
channel
Linear optical absorption spectra of mesoscopic structures in intense THz fields: free particle properties
We theoretically study the effect of THz radiation on the linear optical
absorption spectra of semiconductor structures. A general theoretical
framework, based on non-equilibrium Green functions, is formulated, and applied
to the calculation of linear optical absorption spectrum for several
non-equilibrium mesoscopic structures. We show that a blue-shift occurs and
sidebands appear in bulk-like structures, i.e., the dynamical Franz-Keldysh
effect [A.-P. Jauho and K. Johnsen, Phys. Rev. Lett. 76, 4576 (1996)]. An
analytic calculation leads to the prediction that in the case of superlattices
distinct stable steps appear in the absorption spectrum when conditions for
dynamical localization are met.Comment: 13 Pages, RevTex using epsf to include 8 ps figures. Submitted to
Phys. Rev. B (3 April 97
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