27,407 research outputs found
Computation of and with Lattice QCD
We pursue a new method, based on lattice QCD, for determining the quantities
, , and of heavy-quark effective theory.
We combine Monte Carlo data for the meson mass spectrum with perturbative
calculations of the short-distance behavior, to extract and
from a formula from HQET. Taking into account uncertainties from
fitting the mass dependence and from taking the continuum limit, we find
and in the quenched approximation.Comment: 7 pp, 4 figs (in v2 Fig. 4 now shows Ref. 13, as advertised); in v3
error in BLM scale is correcte
Excitons in van der Waals heterostructures: The important role of dielectric screening
The existence of strongly bound excitons is one of the hallmarks of the newly
discovered atomically thin semi-conductors. While it is understood that the
large binding energy is mainly due to the weak dielectric screening in two
dimensions (2D), a systematic investigation of the role of screening on 2D
excitons is still lacking. Here we provide a critical assessment of a widely
used 2D hydrogenic exciton model which assumes a dielectric function of the
form {\epsilon}(q) = 1 + 2{\pi}{\alpha}q, and we develop a quasi-2D model with
a much broader applicability. Within the quasi-2D picture, electrons and holes
are described as in-plane point charges with a finite extension in the
perpendicular direction and their interaction is screened by a dielectric
function with a non-linear q-dependence which is computed ab-initio. The
screened interaction is used in a generalized Mott-Wannier model to calculate
exciton binding energies in both isolated and supported 2D materials. For
isolated 2D materials, the quasi-2D treatment yields results almost identical
to those of the strict 2D model and both are in good agreement with ab-initio
many-body calculations. On the other hand, for more complex structures such as
supported layers or layers embedded in a van der Waals heterostructure, the
size of the exciton in reciprocal space extends well beyond the linear regime
of the dielectric function and a quasi-2D description has to replace the 2D
one. Our methodology has the merit of providing a seamless connection between
the strict 2D limit of isolated monolayer materials and the more bulk-like
screening characteristics of supported 2D materials or van der Waals
heterostructures.Comment: 14 pages, 13 figure
Quantum theory of electron tunneling into intersubband cavity polariton states
Through a non-perturbative quantum theory, we investigate how the
quasi-electron excitations of a two-dimensional electron gas are modified by
strong coupling to the vacuum field of a microcavity. We show that the
electronic dressed states originate from a Fano-like coupling between the bare
electron states and the continuum of intersubband cavity polariton excitations.
In particular, we calculate the electron spectral function modified by
light-matter interactions and its impact on the electronic injection of
intersubband cavity polaritons. The domain of validity of the present
theoretical results is critically discussed. We show that resonant electron
tunneling from a narrow-band injector can selectively excite superradiant
states and produce efficient intersubband polariton electroluminescence
B -> pi l nu at three lattice spacings
The increasing accuracy of experimental results for the exclusive,
semileptonic decay B -> pi l nu requires a similarly accurate calculation of
the hadronic matrix elements, to determine |Vub|. We present preliminary
results for the form factors of the B to light meson decay mode. Using results
from three lattices in the range 5.7 <= beta <= 6.1 we study the dependence on
the lattice spacing.Comment: LATTICE98(heavyqk), LaTeX, 3 pages, 4 postscript figures, uses
espcrc2.st
Modifications in the Spectrum of Primordial Gravitational Waves Induced by Instantonic Fluctuations
Vacuum to vacuum instantonic transitions modify the power spectrum of
primordial gravitational waves. We evaluate the new form of the power spectrum
for ordinary gravity as well as the parity violation induced in the spectrum by
a modification of General Relativity known as Holst term and we outline the
possible experimental consequences.Comment: V1: 8 pages. V2: 8 pages, some points clarified, typos corrected,
some references added, final result unchanged. V3: 8 pages, title changed,
presentation improved, discussion of phenomenological consequences added,
comments very welcome. V4: Discussion further improved, comments very very
welcom
A notion of graph likelihood and an infinite monkey theorem
We play with a graph-theoretic analogue of the folklore infinite monkey
theorem. We define a notion of graph likelihood as the probability that a given
graph is constructed by a monkey in a number of time steps equal to the number
of vertices. We present an algorithm to compute this graph invariant and closed
formulas for some infinite classes. We have to leave the computational
complexity of the likelihood as an open problem.Comment: 6 pages, 1 EPS figur
Cluster Abundance in f(R) Gravity Models
As one of the most powerful probes of cosmological structure formation, the
abundance of massive galaxy clusters is a sensitive probe of modifications to
gravity on cosmological scales. In this paper, we present results from N-body
simulations of a general class of f(R) models, which self-consistently solve
the non-linear field equation for the enhanced forces. Within this class we
vary the amplitude of the field, which controls the range of the enhanced
gravitational forces, both at the present epoch and as a function of redshift.
Most models in the literature can be mapped onto the parameter space of this
class. Focusing on the abundance of massive dark matter halos, we compare the
simulation results to a simple spherical collapse model. Current constraints
lie in the large-field regime, where the chameleon mechanism is not important.
In this regime, the spherical collapse model works equally well for a wide
range of models and can serve as a model-independent tool for placing
constraints on f(R) gravity from cluster abundance. Using these results, we
show how constraints from the observed local abundance of X-ray clusters on a
specific f(R) model can be mapped onto other members of this general class of
models.Comment: 8 pages, 6 figure
3D Laparoscopy. A potential cutting edge in minimal invasive digestive surgery
Laparoscopic surgery has changed surgical landscape, providing reduced surgical trauma, shorter hospital stays, less postoperative pain and
better outcomes than open surgery. Since its first development in the 90âs, 3D technology applied to laparoscopic surgery has had several technical
improvements and now it represents, together with high definition technology, the best option in minimal invasive digestive surgery, providing shorter
operative times and lower blood loss, making easier to perform surgical tasks both for trainees than for skilled surgeons. It remains a little bit more
expensive than standard 2D laparoscopic devices but even cheaper than robotic equipment
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