480 research outputs found
Two-loop matching of the dipole operators for and
The order corrections to the Wilson coefficients of the dipole
operators () at the matching scale are a crucial ingredient
for a complete next- to-leading logarithmic calculation of the branching ratio
for . Given the phenomenological relevance and the fact that
this two-loop calculation has been done so far only by one group [1], we
present a detailed re-calculation using a different method. Our results are in
complete agreement with those in ref. [1].Comment: 24 pages, latex, 6 figures include
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
A glassy contribution to the heat capacity of hcp He solids
We model the low-temperature specific heat of solid He in the hexagonal
closed packed structure by invoking two-level tunneling states in addition to
the usual phonon contribution of a Debye crystal for temperatures far below the
Debye temperature, . By introducing a cutoff energy in the
two-level tunneling density of states, we can describe the excess specific heat
observed in solid hcp He, as well as the low-temperature linear term in the
specific heat. Agreement is found with recent measurements of the temperature
behavior of both specific heat and pressure. These results suggest the presence
of a very small fraction, at the parts-per-million (ppm) level, of two-level
tunneling systems in solid He, irrespective of the existence of
supersolidity.Comment: 11 pages, 4 figure
The KASCADE-Grande Experiment and the LOPES Project
KASCADE-Grande is the extension of the multi-detector setup KASCADE to cover
a primary cosmic ray energy range from 100 TeV to 1 EeV. The enlarged EAS
experiment provides comprehensive observations of cosmic rays in the energy
region around the knee. Grande is an array of 700 x 700 sqm equipped with 37
plastic scintillator stations sensitive to measure energy deposits and arrival
times of air shower particles. LOPES is a small radio antenna array to operate
in conjunction with KASCADE-Grande in order to calibrate the radio emission
from cosmic ray air showers. Status and capabilities of the KASCADE-Grande
experiment and the LOPES project are presented.Comment: To appear in Nuclear Physics B, Proceedings Supplements, as part of
the volume for the CRIS 2004, Cosmic Ray International Seminar: GZK and
Surrounding
Radio detection of cosmic ray air showers with LOPES
In the last few years, radio detection of cosmic ray air showers has
experienced a true renaissance, becoming manifest in a number of new
experiments and simulation efforts. In particular, the LOPES project has
successfully implemented modern interferometric methods to measure the radio
emission from extensive air showers. LOPES has confirmed that the emission is
coherent and of geomagnetic origin, as expected by the geosynchrotron
mechanism, and has demonstrated that a large scale application of the radio
technique has great potential to complement current measurements of ultra-high
energy cosmic rays. We describe the current status, most recent results and
open questions regarding radio detection of cosmic rays and give an overview of
ongoing research and development for an application of the radio technique in
the framework of the Pierre Auger Observatory.Comment: 8 pages; Proceedings of the CRIS2006 conference, Catania, Italy; to
be published in Nuclear Physics B, Proceedings Supplement
A self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systems
Local-spin-density functional calculations may be affected by severe errors
when applied to the study of magnetic and strongly-correlated materials. Some
of these faults can be traced back to the presence of the spurious
self-interaction in the density functional. Since the application of a fully
self-consistent self-interaction correction is highly demanding even for
moderately large systems, we pursue a strategy of approximating the
self-interaction corrected potential with a non-local, pseudopotential-like
projector, first generated within the isolated atom and then updated during the
self-consistent cycle in the crystal. This scheme, whose implementation is
totally uncomplicated and particularly suited for the pseudopotental formalism,
dramatically improves the LSDA results for a variety of compounds with a
minimal increase of computing cost.Comment: 18 pages, 14 figure
Massive stars as thermonuclear reactors and their explosions following core collapse
Nuclear reactions transform atomic nuclei inside stars. This is the process
of stellar nucleosynthesis. The basic concepts of determining nuclear reaction
rates inside stars are reviewed. How stars manage to burn their fuel so slowly
most of the time are also considered. Stellar thermonuclear reactions involving
protons in hydrostatic burning are discussed first. Then I discuss triple alpha
reactions in the helium burning stage. Carbon and oxygen survive in red giant
stars because of the nuclear structure of oxygen and neon. Further nuclear
burning of carbon, neon, oxygen and silicon in quiescent conditions are
discussed next. In the subsequent core-collapse phase, neutronization due to
electron capture from the top of the Fermi sea in a degenerate core takes
place. The expected signal of neutrinos from a nearby supernova is calculated.
The supernova often explodes inside a dense circumstellar medium, which is
established due to the progenitor star losing its outermost envelope in a
stellar wind or mass transfer in a binary system. The nature of the
circumstellar medium and the ejecta of the supernova and their dynamics are
revealed by observations in the optical, IR, radio, and X-ray bands, and I
discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure
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