1,493 research outputs found
The generalized identification of truly interfacial molecules (ITIM) algorithm for nonplanar interfaces
We present a generalized version of the ITIM algorithm for the identification of interfacial molecules, which is able to treat arbitrarily shaped interfaces. The algorithm exploits the similarities between the concept of probe sphere used in ITIM and the circumsphere criterion used in the α-shapes approach, and can be regarded either as a reference-frame independent version of the former, or as an extended version of the latter that includes the atomic excluded volume. The new algorithm is applied to compute the intrinsic orientational order parameters of water around a dodecylphosphocholine and a cholic acid micelle in aqueous environment, and to the identification of solvent-reachable sites in four model structures for soot. The additional algorithm introduced for the calculation of intrinsic density profiles in arbitrary geometries proved to be extremely useful also for planar interfaces, as it allows to solve the paradox of smeared intrinsic profiles far from the interface. © 2013 American Institute of Physics
Some Aspects of New CDM Models and CDM Detection Methods
We briefly review some recent Cold Dark Matter (CDM) models. Our main focus
are charge symmetric models of WIMPs which are not the standard SUSY LSP's
(Lightest Supersymmetric Partners). We indicate which experiments are most
sensitive to certain aspects of the models. In particular we discuss the
manifestations of the new models in neutrino telescopes and other set-ups. We
also discuss some direct detection experiments and comment on measuring the
direction of recoil ions--which is correlated with the direction of the
incoming WIMP. This could yield daily variations providing along with the
annual modulation signatures for CDM.Comment: 14 page
Comment on "First Observation of Ground State Dineutron Decay: 16Be"
A recent measurement [Spyrou et al., PRL 108, 102501 (2012)] of the in-flight
decay of 16Be into 14Be+n+n has been interpreted as the first case of dineutron
emission. Here we point out that the inclusion of the n-n interaction neglected
in the description of the direct three-body decay can generate strong
enhancements at low n-n relative energy and angle, as observed, without any
need to invoke dineutron decay.Comment: Final version, published in Physical Review Letter
Determination of the magnetization profile of Co/Mg periodic multilayers by magneto-optic Kerr effect and X-ray magnetic resonant reflectivity
The resonant magnetic reflectivity of Co/Mg multilayers around the Co L2,3
absorption edge is simulated then measured on a specifically designed sample.
The dichroic signal is obtained when making the difference between the two
reflectivities measured with the magnetic field applied in two opposite
directions parallel to the sample surface. The simulations show that the
existence of magnetic dead layers at the interfaces between the Co and Mg
layers leads to an important increase of the dichroic signal measured in the
vicinity of the third Bragg peak that otherwise should be negligible. The
measurements are in agreement with the model introducing 0.25 nm thick dead
layers. This is attributed to the Co atoms in contact with the Mg layers and
thus we conclude that the Co-Mg interfaces are abrupt from the magnetic point
of view.Comment: 8 page
Generation of Vorticity and Velocity Dispersion by Orbit Crossing
We study the generation of vorticity and velocity dispersion by orbit
crossing using cosmological numerical simulations, and calculate the
backreaction of these effects on the evolution of large-scale density and
velocity divergence power spectra. We use Delaunay tessellations to define the
velocity field, showing that the power spectra of velocity divergence and
vorticity measured in this way are unbiased and have better noise properties
than for standard interpolation methods that deal with mass weighted
velocities. We show that high resolution simulations are required to recover
the correct large-scale vorticity power spectrum, while poor resolution can
spuriously amplify its amplitude by more than one order of magnitude. We
measure the scalar and vector modes of the stress tensor induced by orbit
crossing using an adaptive technique, showing that its vector modes lead, when
input into the vorticity evolution equation, to the same vorticity power
spectrum obtained from the Delaunay method. We incorporate orbit crossing
corrections to the evolution of large scale density and velocity fields in
perturbation theory by using the measured stress tensor modes. We find that at
large scales (k~0.1 h/Mpc) vector modes have very little effect in the density
power spectrum, while scalar modes (velocity dispersion) can induce percent
level corrections at z=0, particularly in the velocity divergence power
spectrum. In addition, we show that the velocity power spectrum is smaller than
predicted by linear theory until well into the nonlinear regime, with little
contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new
appendix. Accepted by PR
Structure of Be probed via secondary beam reactions
The low-lying level structure of the unbound neutron-rich nucleus Be
has been investigated via breakup on a carbon target of secondary beams of
B at 35 MeV/nucleon. The coincident detection of the beam velocity
Be fragments and neutrons permitted the invariant mass of the
Be+ and Be++ systems to be reconstructed. In the case of
the breakup of B, a very narrow structure at threshold was observed in
the Be+ channel. Contrary to earlier stable beam fragmentation
studies which identified this as a strongly interacting -wave virtual state
in Be, analysis here of the Be++ events demonstrated that
this was an artifact resulting from the sequential-decay of the
Be(2) state. Single-proton removal from B was found to
populate a broad low-lying structure some 0.70 MeV above the neutron-decay
threshold in addition to a less prominent feature at around 2.4 MeV. Based on
the selectivity of the reaction and a comparison with (0-3)
shell-model calculations, the low-lying structure is concluded to most probably
arise from closely spaced J=1/2 and 5/2 resonances
(E=0.400.03 and 0.85 MeV), whilst the broad
higher-lying feature is a second 5/2 level (E=2.350.14 MeV). Taken
in conjunction with earlier studies, it would appear that the lowest 1/2
and 1/2 levels lie relatively close together below 1 MeV.Comment: 14 pages, 13 figures, 2 tables. Accepted for publication in Physical
Review
Cosmological Consequences of Nearly Conformal Dynamics at the TeV scale
Nearly conformal dynamics at the TeV scale as motivated by the hierarchy
problem can be characterized by a stage of significant supercooling at the
electroweak epoch. This has important cosmological consequences. In particular,
a common assumption about the history of the universe is that the reheating
temperature is high, at least high enough to assume that TeV-mass particles
were once in thermal equilibrium. However, as we discuss in this paper, this
assumption is not well justified in some models of strong dynamics at the TeV
scale. We then need to reexamine how to achieve baryogenesis in these theories
as well as reconsider how the dark matter abundance is inherited. We argue that
baryonic and dark matter abundances can be explained naturally in these setups
where reheating takes place by bubble collisions at the end of the strongly
first-order phase transition characterizing conformal symmetry breaking, even
if the reheating temperature is below the electroweak scale GeV. We
also discuss inflation as well as gravity wave smoking gun signatures of this
class of models.Comment: 22 pages, 7 figure
Bounding wide composite vector resonances at the LHC
In composite Higgs models (CHMs), electroweak precision data generically push
colourless composite vector resonances to a regime where they dominantly decay
into pairs of light top partners. This greatly attenuates their traces in
canonical collider searches, tailored for narrow resonances promptly decaying
into Standard Model final states. By reinterpreting the CMS same-sign dilepton
(SS2) analysis at the Large Hadron Collider (LHC), originally designed to
search for top partners with electric charge , we demonstrate its
significant coverage over this kinematical regime. We also show the reach of
the 13 TeV run of the LHC, with various integrated luminosity options, for a
possible upgrade of the SS2 search. The top sector of CHMs is found to be
more fine-tuned in the presence of colourless composite resonances in the few
TeV range.Comment: 9 pages, 5 figures. Minor corrections for publication in JHE
Extrapolation of neutron-rich isotope cross-sections from projectile fragmentation
Using the measured fragmentation cross sections produced from the 48Ca and
64Ni beams at 140 MeV per nucleon on 9Be and 181Ta targets, we find that the
cross sections of unmeasured neutron rich nuclei can be extrapolated using a
systematic trend involving the average binding energy. The extrapolated
cross-sections will be very useful in planning experiments with neutron rich
isotopes produced from projectile fragmentation. The proposed method is general
and could be applied to other fragmentation systems including those used in
other radioactive ion beam facilities.Comment: accepted for publication in Europhysics Letter
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