2,677 research outputs found
Next--to--Leading Order QCD corrections for the --mixing with an extended Higgs sector
We present a calculation of the B0-B0--mixing including Next--to--Leading
Order (NLO) QCD corrections within the Two Higgs Doublet Model (2HDM). The QCD
corrections at NLO are contained in the factor denoted by eta_2 which modifies
the result obtained at the lowest order of perturbation theory. In the Standard
Model case, we confirm the results for eta_2 obtained by Buras, Jamin and
Weisz. The factor eta_2 is gauge and renormalization prescription invariant and
it does not depend on the infrared behaviour of the theory, which constitutes
an important test of the calculations. The NLO--calculations within the 2HDM
enhance the LO--result up to 18%, which affects the correlation between M_H and
V_{td}.Comment: 22 pages (LaTeX), 22 Postscript figures, version to appear in Nucl.
Phys. B, corrected some typos and a sign in the program, which results in
changes in Eqs. (71), (74) and (75). Due to these changes Eqs. (23) and (34)
may be written in a more compact wa
The Cosmological Constant is Back
A diverse set of observations now compellingly suggest that Universe
possesses a nonzero cosmological constant. In the context of quantum-field
theory a cosmological constant corresponds to the energy density of the vacuum,
and the wanted value for the cosmological constant corresponds to a very tiny
vacuum energy density. We discuss future observational tests for a cosmological
constant as well as the fundamental theoretical challenges---and
opportunities---that this poses for particle physics and for extending our
understanding of the evolution of the Universe back to the earliest moments.Comment: latex, 8 pages plus one ps figure available as separate compressed
uuencoded fil
Numerical Studies on the Magnetism of Fe-Ni-Mn Alloys in the Invar Region
By means of self-consistent semi-empirical LCAO calculations we study the
itinerant magnetism of (Fe_{0.65}Ni_{0.35})_{1-y} Mn_y alloys for y between 0
and 0.22 at T=0 K, neglecting only the transverse spin components. We find that
the magnetic behaviour is quite complicated on a local scale. In addition to
ferromagnetic behaviour, also metastable spin-glass-like configurations are
found. In the same approach, using a direct numerical calculation by the
Kubo-Formalism without any fit parameters, we also calculate the electrical
conductance in the magnetic state and find that the -dependence observed in
the experiments is well reproduced by our calculations, except of an overall
factor of rougly 5, by which our resistivities are too large.Comment: 12 pages (Latex, to be applied 2 times) + 13 figures (eps-files
Atomic effects in astrophysical nuclear reactions
Two models are presented for the description of the electron screening
effects that appear in laboratory nuclear reactions at astrophysical energies.
The two-electron screening energy of the first model agrees very well with the
recent LUNA experimental result for the break-up reaction , which so far defies all available theoretical models.
Moreover, multi-electron effects that enhance laboratory reactions of the CNO
cycle and other advanced nuclear burning stages, are also studied by means of
the Thomas-Fermi model, deriving analytical formulae that establish a lower and
upper limit for the associated screening energy. The results of the second
model, which show a very satisfactory compatibility with the adiabatic
approximation ones, are expected to be particularly useful in future
experiments for a more accurate determination of the CNO astrophysical factors.Comment: 14 RevTex pages + 2 ps (revised) figures. Phys.Rev.C (in production
Astrophysical factors:Zero energy vs. Most effective energy
Effective astrophysical factors for non-resonant astrophysical nuclear
reaction are invariably calculated with respect to a zero energy limit. In the
present work that limit is shown to be very disadvantageous compared to the
more natural effective energy limit. The latter is used in order to modify the
thermonuclear reaction rate formula so that it takes into account both plasma
and laboratory screening effects.Comment: 7 RevTex pages. Accepted for publication in Phys.Rev.
A Model for Neutrino Masses and Dark Matter
We propose a model for neutrino masses that simultaneously results in a new
dark matter candidate, the right-handed neutrino. We derive the dark matter
abundance in this model, show how the hierarchy of neutrino masses is obtained,
and verify that the model is compatible with existing experimental results. The
model provides an economical method of unifying two seemingly separate puzzles
in contemporary particle physics and cosmology.Comment: 4 pages, submitted to PR
New Mechanism for Electronic Energy Relaxation in Nanocrystals
The low-frequency vibrational spectrum of an isolated nanometer-scale solid
differs dramatically from that of a bulk crystal, causing the decay of a
localized electronic state by phonon emission to be inhibited. We show,
however, that an electron can also interact with the rigid translational motion
of a nanocrystal. The form of the coupling is dictated by the equivalence
principle and is independent of the ordinary electron-phonon interaction. We
calculate the rate of nonradiative energy relaxation provided by this mechanism
and establish its experimental observability.Comment: 4 pages, Submitted to Physical Review
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