7,299 research outputs found
Partial decay widths of baryons in the spin-momentum operator expansion method
The cross sections for photo- and pion-induced production of baryon
resonances and their partial decay widths to the two--body and multi--body
final states are calculated in the framework of the operator expansion method.
The approach is fully relativistic invariant, and it allows us to perform
combined analyses of different reactions imposing directly the analyticity and
unitarity constraints. All formulae are given explicitly in the form used by
the Crystal Barrel collaboration in the partial wave analysis.Comment: 15 pages, 6 figure
Rates for the reactions antiproton-proton --> pi phi and gamma phi
We study antiproton-proton annihilation at rest into and
. Rescattering by and
for states is sizable, of
order in the branching ratio, but
smaller than experiment. For the
rescattering contributions are negligible, but the channel is well
explained by a intermediate state combined with vector meson
dominance.Comment: 12 pages, plain latex, 2 postscript figures available upon request,
PSI-PR-93-2
Coupling Constants for Scalar Glueball Decay
We evaluate the partial decay widths of the lightest scalar glueball to pairs
of pseudoscalar quark-antiquark states. The calculation is done in the valence
(quenched) approximation on a 16^3 \time 24 lattice at . These
predictions and values obtained earlier for the infinite volume continuum limit
of the scalar glueball's mass are in good agreement with the observed
properties of and inconsistent with all other observed meson
resonances.Comment: 8 pages of Latex, 12 PostScript figures, 2 macros included, plenary
talk given by D. Weingarten at Lattice 95, to appear in conference
proceeding
Prediction of a novel monoclinic carbon allotrope
A novel allotrope of carbon with symmetry was identified during an
\emph{ab-initio} minima-hopping structural search which we call -carbon.
This structure is predicted to be more stable than graphite at pressures above
14.4 GPa and consists purely of bonds. It has a high bulk modulus and is
almost as hard as diamond. A comparison of the simulated X-ray diffraction
pattern shows a good agreement with experimental results from cold compressed
graphite.Comment: 3 pages, 3 figure
Scalar Quarkonium Masses and Mixing with the Lightest Scalar Glueball
We evaluate the continuum limit of the valence (quenched) approximation to
the mass of the lightest scalar quarkonium state, for a range of different
quark masses, and to the mixing energy between these states and the lightest
scalar glueball. Our results support the interpretation of as
composed mainly of the lightest scalar glueball.Comment: 14 pages of Latex, 5 PostScript figure
Study of nuclear recoils in liquid argon with monoenergetic neutrons
For the development of liquid argon dark matter detectors we assembled a
setup in the laboratory to scatter neutrons on a small liquid argon target. The
neutrons are produced mono-energetically (E_kin=2.45 MeV) by nuclear fusion in
a deuterium plasma and are collimated onto a 3" liquid argon cell operating in
single-phase mode (zero electric field). Organic liquid scintillators are used
to tag scattered neutrons and to provide a time-of-flight measurement. The
setup is designed to study light pulse shapes and scintillation yields from
nuclear and electronic recoils as well as from {\alpha}-particles at working
points relevant to dark matter searches. Liquid argon offers the possibility to
scrutinise scintillation yields in noble liquids with respect to the
populations of the two fundamental excimer states. Here we present experimental
methods and first results from recent data towards such studies.Comment: 9 pages, 8 figures, proceedings of TAUP 2011, to be published in
Journal of Physics: Conference Series (JCPS
Luminescence quenching of the triplet excimer state by air traces in gaseous argon
While developing a liquid argon detector for dark matter searches we
investigate the influence of air contamination on the VUV scintillation yield
in gaseous argon at atmospheric pressure. We determine with a radioactive
alpha-source the photon yield for various partial air pressures and different
reflectors and wavelength shifters. We find for the fast scintillation
component a time constant tau1= 11.3 +- 2.8 ns, independent of gas purity.
However, the decay time of the slow component depends on gas purity and is a
good indicator for the total VUV light yield. This dependence is attributed to
impurities destroying the long-lived argon excimer states. The population ratio
between the slowly and the fast decaying excimer states is determined for
alpha-particles to be 5.5 +-0.6 in argon gas at 1100 mbar and room temperature.
The measured mean life of the slow component is tau2 = 3.140 +- 0.067 microsec
at a partial air pressure of 2 x 10-6 mbar.Comment: 7 pages submitted to NIM
Low-density silicon allotropes for photovoltaic applications
Silicon materials play a key role in many technologically relevant fields,
ranging from the electronic to the photovoltaic industry. A systematic search
for silicon allotropes was performed by employing a modified ab initio minima
hopping crystal structure prediction method. The algorithm was optimized to
specifically investigate the hitherto barely explored low-density regime of the
silicon phase diagram by imitating the guest-host concept of clathrate
compounds. In total 44 metastable phases are presented, of which 11 exhibit
direct or quasi-direct band-gaps in the range of 1.0-1.8 eV, close to
the optimal Shockley-Queisser limit of 1.4 eV, with a stronger overlap
of the absorption spectra with the solar spectrum compared to conventional
diamond silicon. Due to the structural resemblance to known clathrate compounds
it is expected that the predicted phases can be synthesized
Charmed Mesons Have No Discernable Color-Coulomb Attraction
Starting with a confining linear Lorentz scalar potential V_s and a Lorentz
vector potential V_v which is also linear but has in addition a color-Coulomb
attraction piece, -alpha_s/r, we solve the Dirac equation for the ground-state
c- and u-quark wave functions. Then, convolving V_v with the u-quark density,
we find that the Coulomb attraction mostly disappears, making an essentially
linear barV_v for the c-quark. A similar convolution using the c-quark density
also leads to an essentially linear tildeV_v for the u-quark. For bound cbar-c
charmonia, where one must solve using a reduced mass for the c-quarks, we also
find an essentially linear widehatV_v. Thus, the relativistic quark model
describes how the charmed-meson mass spectrum avoids the need for a
color-Coulomb attraction.Comment: 9 pages, 5 PDF figure
Emergence of hidden phases of methylammonium lead-iodide (CHNHPbI) upon compression
We perform a thorough structural search with the minima hopping method (MHM)
to explore low-energy structures of methylammonium lead iodide. By combining
the MHM with a forcefield, we efficiently screen vast portions of the
configurational space with large simulation cells containing up to 96 atoms.
Our search reveals two structures of methylammonium iodide perovskite (MAPI)
that are substantially lower in energy than the well-studied experimentally
observed low-temperature orthorhombic phase according to density
functional calculations. Both structures have not yet been reported in the
literature for MAPI, but our results show that they could emerge as
thermodynamically stable phases via compression at low temperatures. In terms
of the electronic properties, the two phases exhibit larger band gaps than the
standard perovskite-type structures. Hence, pressure induced phase selection at
technologically achievable pressures (i.e., via thin-film strain) is a route
towards the synthesis of several MAPI polymorph with variable band gaps
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