397 research outputs found
reaction near threshold
We analyze the total cross section data for near threshold
measured recently at SATURNE. Using an effective range approximation for the
on-shell S-wave final state interaction we extract from these data the
modulus fm of the threshold transition amplitude
. We present a calculation of various (tree-level) meson exchange
diagrams contributing to . It is essential that -emission from
the anomalous -vertex interferes destructively with
-emission from the proton lines. The contribution of scalar
-meson exchange to turns out to be negligibly small. Without
introducing off-shell meson-nucleon form factors the experimental value
fm can be reproduced with an -coupling constant
of . The results of the present approach agree qualitatively
with the J\"ulich model. We also perform a combined analysis of the reactions
and near threshold.Comment: Latex-file 6 pages, 2 Figure
Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene
The ultrafast relaxation and recombination dynamics of photogenerated
electrons and holes in epitaxial graphene are studied using optical-pump
Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz
frequencies depends on the carrier concentration as well as the carrier
distribution in energy. Time-resolved studies of the conductivity can therefore
be used to probe the dynamics associated with carrier intraband relaxation and
interband recombination. We report the electron-hole recombination times in
epitaxial graphene for the first time. Our results show that carrier cooling
occurs on sub-picosecond time scales and that interband recombination times are
carrier density dependent.Comment: 4 pages, 5 figure
What have we learned from antiproton proton scattering?
From recent charge exchange measurements in the extreme forward direction, an
independent and precise determination of the pion nucleon coupling constant is
possible. This determination has reopened the debate on the value of this
fundamental coupling constant of nuclear physics. Precise measurements of
charge exchange observables at forward angles below 900 MeV/c would also give a
better understanding of the long range part of the two-pion exchange potential.
For example, the confirmation of the coherence of the tensor forces from the
pion exchange and the isovector two-pion exchange would be very valuable. With
the present data first attempts at an \NbarN partial wave analysis have been
made where, as in nucleon nucleon scattering, the antinucleon nucleon high J
partial waves are mainly given by one-pion exchange. Finally a recent \pbarp
atomic cascade calculation and the fraction of P-state annihilation in gas
targets is commented on.Comment: 10 pages, Latex, to be published in Nucl. Phy
Te covered Si(001): a variable surface reconstruction
At a given temperature, clean and adatom covered silicon surfaces usually
exhibit well-defined reconstruction patterns. Our finite temperature ab-initio
molecular dynamics calculations show that the tellurium covered Si(001) surface
is an exception. Soft longitudinal modes of surface phonons due to the strongly
anharmonic potential of the bridged tellurium atoms prevent the reconstruction
structure from attaining any permanent, two dimensional periodic geometry. This
explains why experiments attempting to find a definite model for the
reconstruction have reached conflicting conclusions.Comment: 4 pages, 3 gif figure
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
Functional approach to the electromagnetic response function: the Longitudinal Channel
In this paper we address the (charge) longitudinal electromagnetic response
for a homogeneous system of nucleons interacting via meson exchanges in the
functional framework. This approach warrants consistency if the calculation is
carried on order-by-order in the mesonic loop expansion with RPA-dressed
mesonic propagators. At the 1-loop order and considering pion, rho and omega
exchanges we obtain a quenching of the response, in line with the experimental
results.Comment: RevTeX, 18 figures available upon request - to be published in
Physical Review
Quasi-Elastic Scattering in the Inclusive (He, t) Reaction
The triton energy spectra of the charge-exchange C(He,t) reaction
at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out
region. Considering that this region is mainly populated by the charge-exchange
of a proton in He with a neutron in the target nucleus and the final proton
going in the continuum, the cross-sections are written in the distorted-wave
impulse approximation. The t-matrix for the elementary exchange process is
constructed in the DWBA, using one pion- plus rho-exchange potential for the
spin-isospin nucleon- nucleon potential. This t-matrix reproduces the
experimental data on the elementary pn np process. The calculated
cross-sections for the C(He,t) reaction at to triton
emission angle are compared with the corresponding experimental data, and are
found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at
[email protected], submitted to Phy.Rev.
Spectroscopic evidence of odd frequency superconducting order
Spin filter superconducting S/I/N tunnel junctions (NbN/GdN/TiN) show a
robust and pronounced zero bias conductance peak at low temperatures, the
magnitude of which is several times the normal state conductance of the
junction. Such a conductance anomaly is representative of unconventional
superconductivity and is interpreted as a direct signature of an odd frequency
superconducting order.Comment: 5 pages, 3 figures + supplementary informatio
Role of Vector Mesons in High-Q^2 Lepton-Nucleon Scattering
The possible role played by vector mesons in inclusive deep inelastic
lepton-nucleon scattering is investigated. In the context of the convolution
model, we calculate self-consistently the scaling contribution to the nucleon
structure function using the formalism of time-ordered perturbation theory in
the infinite momentum frame. Our results indicate potentially significant
effects only when the vector meson---nucleon form factor is very hard.
Agreement with the experimental antiquark distributions, however, requires
relatively soft form factors for the , and vertices.Comment: 22 pages, 9 figures (available upon request); accepted for
publication in Phys.Rev.D, ADP-92-197/T12
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