17,686 research outputs found
The Unhiggs
We examine a scenario where the Higgs is part of an approximate conformal
field theory, and has a scaling dimension greater than one. Such an unparticle
Higgs (or Unhiggs) can still break electroweak symmetry and unitarize WW
scattering, but its gauge couplings are suppressed. An Unhiggs model has a
reduced sensitivity of the weak scale to the cutoff, and can thus provide a
solution to the little hierarchy problem.Comment: 21 pages, 9 figures; v2: further discussion, references added,
version published in JHE
Multiscale modelling of liquids with molecular specificity
The separation between molecular and mesoscopic length and time scales poses
a severe limit to molecular simulations of mesoscale phenomena. We describe a
hybrid multiscale computational technique which address this problem by keeping
the full molecular nature of the system where it is of interest and
coarse-graining it elsewhere. This is made possible by coupling molecular
dynamics with a mesoscopic description of realistic liquids based on Landau's
fluctuating hydrodynamics. We show that our scheme correctly couples
hydrodynamics and that fluctuations, at both the molecular and continuum
levels, are thermodynamically consistent. Hybrid simulations of sound waves in
bulk water and reflected by a lipid monolayer are presented as illustrations of
the scheme
Raman-scattering study of the phonon dispersion in twisted bi-layer graphene
Bi-layer graphene with a twist angle \theta\ between the layers generates a
superlattice structure known as Moir\'{e} pattern. This superlattice provides a
\theta-dependent q wavevector that activates phonons in the interior of the
Brillouin zone. Here we show that this superlattice-induced Raman scattering
can be used to probe the phonon dispersion in twisted bi-layer graphene (tBLG).
The effect reported here is different from the broadly studied double-resonance
in graphene-related materials in many aspects, and despite the absence of
stacking order in tBLG, layer breathing vibrations (namely ZO' phonons) are
observed.Comment: 18 pages, 4 figures, research articl
Determination of the chemical potential using energy-biased sampling
An energy-biased method to evaluate ensemble averages requiring test-particle
insertion is presented. The method is based on biasing the sampling within the
subdomains of the test-particle configurational space with energies smaller
than a given value freely assigned. These energy-wells are located via unbiased
random insertion over the whole configurational space and are sampled using the
so called Hit&Run algorithm, which uniformly samples compact regions of any
shape immersed in a space of arbitrary dimensions. Because the bias is defined
in terms of the energy landscape it can be exactly corrected to obtain the
unbiased distribution. The test-particle energy distribution is then combined
with the Bennett relation for the evaluation of the chemical potential. We
apply this protocol to a system with relatively small probability of low-energy
test-particle insertion, liquid argon at high density and low temperature, and
show that the energy-biased Bennett method is around five times more efficient
than the standard Bennett method. A similar performance gain is observed in the
reconstruction of the energy distribution.Comment: 10 pages, 4 figure
Probing O-enrichment in C-rich dust planetary nebulae
The abundance of O in planetary nebulae (PNe) has been historically used as a
metallicity indicator of the interstellar medium (ISM) where they originated;
e.g., it has been widely used to study metallicity gradients in our Galaxy and
beyond. However, clear observational evidence for O self enrichment in
low-metallicity Galactic PNe with C-rich dust has been recently reported. Here
we report asymptotic giant branch (AGB) nucleosynthesis predictions for the
abundances of the CNO elements and helium in the metallicity range Zsun/4 < Z <
2Zsun. Our AGB models, with diffusive overshooting from all the convective
borders, predict that O is overproduced in low-Z low-mass (~1-3 Msun) AGB stars
and nicely reproduce the recent O overabundances observed in C-rich dust PNe.
This confirms that O is not always a good proxy of the original ISM metallicity
and another chemical elements such as Cl or Ar should be used instead. The
production of oxygen by low-mass stars should be thus considered in
galactic-evolution models.Comment: Accepted for publication in MNRAS Letters (5 pages, 1 figure, and 1
table
Galactic planetary nebulae with precise nebular abundances as a tool to understand the evolution of asymptotic giant branch stars
We present nucleosynthesis predictions (HeCNOCl) from asymptotic giant branch
(AGB) models, with diffusive overshooting from all the convective borders, in
the metallicity range Z/4 < Z < 2Zsun. They are compared to recent precise
nebular abundances in a sample of Galactic planetary nebulae (PNe) that is
divided among double-dust chemistry (DC) and oxygen-dust chemistry (OC)
according to the infrared dust features. Unlike the similar subsample of
Galactic carbon-dust chemistry PNe recently analysed by us, here the individual
abundance errors, the higher metallicity spread, and the uncertain dust
types/subtypes in some PNe do not allow a clear determination of the AGB
progenitor masses (and formation epochs) for both PNe samples; the comparison
is thus more focussed on a object-by-object basis. The lowest metallicity OC
PNe evolve from low-mass (~1 Msun) O-rich AGBs, while the higher metallicity
ones (all with uncertain dust classifications) display a chemical pattern
similar to the DC PNe. In agreement with recent literature, the DC PNe mostly
descend from high-mass (M > 3.5 Msun) solar/supersolar metallicity AGBs that
experience hot bottom burning (HBB), but other formation channels in low-mass
AGBs like extra mixing, stellar rotation, binary interaction, or He
pre-enrichment cannot be disregarded until more accurate C/O ratios would be
obtained. Two objects among the DC PNe show the imprint of advanced CNO
processing and deep second dredge-up, suggesting progenitors masses close to
the limit to evolve as core collapse supernovae (above 6 Msun). Their actual
C/O ratio, if confirmed, indicate contamination from the third dredge-up,
rejecting the hypothesis that the chemical composition of such high-metallicity
massive AGBs is modified exclusively by HBB.Comment: Accepted for publication in MNRAS (11 pages, 3 figures, and 2 tables
Solar analogs with and without planets: T trends and galactic evolution
We explore a sample of 148 solar-like stars to search for a possible
correlation between the slopes of the abundance trends versus condensation
temperature (known as the Tc slope) both with stellar parameters and Galactic
orbital parameters in order to understand the nature of the peculiar chemical
signatures of these stars and the possible connection with planet formation. We
find that the Tc slope correlates at a significant level with the stellar age
and the stellar surface gravity. We also find tentative evidence that the Tc
slope correlates with the mean galactocentric distance of the stars (Rmean),
suggesting that stars that originated in the inner Galaxy have fewer refractory
elements relative to the volatile ones. We found that the chemical
peculiarities (small refractory-to-volatile ratio) of planet-hosting stars is
probably a reflection of their older age and their inner Galaxy origin. We
conclude that the stellar age and probably Galactic birth place are key to
establish the abundances of some specific elements.Comment: Proceedings of the GREAT-ITN conference: The Milky Way Unravelled by
Gaia. Will be published in the "EAS Publications Series
Ambiguities of arrival-time distributions in quantum theory
We consider the definition that might be given to the time at which a
particle arrives at a given place, both in standard quantum theory and also in
Bohmian mechanics. We discuss an ambiguity that arises in the standard theory
in three, but not in one, spatial dimension.Comment: LaTex, 12 pages, no figure
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