3,378 research outputs found
The competition of hydrogen-like and isotropic interactions on polymer collapse
We investigate a lattice model of polymers where the nearest-neighbour
monomer-monomer interaction strengths differ according to whether the local
configurations have so-called ``hydrogen-like'' formations or not. If the
interaction strengths are all the same then the classical -point
collapse transition occurs on lowering the temperature, and the polymer enters
the isotropic liquid-drop phase known as the collapsed globule. On the other
hand, strongly favouring the hydrogen-like interactions give rise to an
anisotropic folded (solid-like) phase on lowering the temperature. We use Monte
Carlo simulations up to a length of 256 to map out the phase diagram in the
plane of parameters and determine the order of the associated phase
transitions. We discuss the connections to semi-flexible polymers and other
polymer models. Importantly, we demonstrate that for a range of energy
parameters two phase transitions occur on lowering the temperature, the second
being a transition from the globule state to the crystal state. We argue from
our data that this globule-to-crystal transition is continuous in two
dimensions in accord with field-theory arguments concerning Hamiltonian walks,
but is first order in three dimensions
Layering transitions for adsorbing polymers in poor solvents
An infinite hierarchy of layering transitions exists for model polymers in
solution under poor solvent or low temperatures and near an attractive surface.
A flat histogram stochastic growth algorithm known as FlatPERM has been used on
a self- and surface interacting self-avoiding walk model for lengths up to 256.
The associated phases exist as stable equilibria for large though not infinite
length polymers and break the conjectured Surface Attached Globule phase into a
series of phases where a polymer exists in specified layer close to a surface.
We provide a scaling theory for these phases and the first-order transitions
between them.Comment: 4 pages, 4 figure
CP Studies of the Higgs Sector
The CP structure of the Higgs sector will be of great interest to future
colliders. The measurement of the CP properties of candidate Higgs particles
will be essential in order to distinguish models of electroweak symmetry
breaking, and to discover or place limits on CP-violation in the Higgs sector.
In this report we briefly summarize various methods of determining the CP
properties of Higgs bosons at different colliders and identify areas where more
study is required. We also provide an example of a synergy between the LHC, an
e+e- Linear Collider and a Photon Collider, for the examination of CP-violation
in a Two-Higgs-Doublet-Model.Comment: A contribution to the LHC / LC Study Group document; 9 pages, 2
figure
Advanced Communications Technology Satellite (ACTS): Design and On-Orbit Performance Measurements
The Advanced Communications Technology Satellite (ACTS), developed and built by Lockheed Martin Astro Space for the NASA Lewis Research Center, was launched in September 1993 on the Shuttle STS 51 mission. ACTS is a digital experimental communications test bed that incorporates gigahertz bandwidth transponders operating at Ka band, hopping spot beams, on-board storage and switching, and dynamic rain fade compensation. This paper describes the ACTS enabling technologies, the design of the communications payload, the constraints imposed on the spacecraft bus, and the measurements conducted to verify the performance of the system in orbit
Optimal ratio between phase basis and bit basis in QKD
In the original BB84 protocol, the bit basis and the phase basis are used
with equal probability. Lo et al (J. of Cryptology, 18, 133-165 (2005))
proposed to modify the ratio between the two bases by increasing the final key
generation rate. However, the optimum ratio has not been derived. In this
letter, in order to examine this problem, the ratio between the two bases is
optimized for exponential constraints given Eve's information
distinguishability and the final error probability
Pulling absorbing and collapsing polymers from a surface
A self-interacting polymer with one end attached to a sticky surface has been
studied by means of a flat-histogram stochastic growth algorithm known as
FlatPERM. We examined the four-dimensional parameter space of the number of
monomers up to 91, self-attraction, surface attraction and force applied to an
end of the polymer. Using this powerful algorithm the \emph{complete} parameter
space of interactions and force has been considered. Recently it has been
conjectured that a hierarchy of states appears at low temperature/poor solvent
conditions where a polymer exists in a finite number of layers close to a
surface. We find re-entrant behaviour from a stretched phase into these
layering phases when an appropriate force is applied to the polymer. We also
find that, contrary to what may be expected, the polymer desorbs from the
surface when a sufficiently strong critical force is applied and does
\emph{not} transcend through either a series of de-layering transitions or
monomer-by-monomer transitions.Comment: 4 pages, 4 figure
Leptonic universality breaking in Upsilon decays as a probe of new physics
In this work we examine the possible existence of new physics beyond the
standard model which could modify the branching fractions of the leptonic
(mainly tauonic) decays of bottomonium vector resonances below the
threshold. The decay width is factorized as the product of two pieces: a) the
probability of an intermediate pseudoscalar color-singlet state
(coupling to the dominant Fock state of the Upsilon via a magnetic dipole
transition) and a soft (undetected) photon; b) the annihilation width of the
pair into two leptons, mediated by a non-standard CP-odd Higgs boson
of mass about 10 GeV, introducing a quadratic dependence on the lepton mass in
the partial width. The process would be unwittingly ascribed to the
leptonic channel thereby (slightly) breaking lepton universality. A possible
mixing of the pseudoscalar Higgs and bottomonium resonances is also considered.
Finally, several experimental signatures to check out the validity of the
conjecture are discussed.Comment: LaTeX, 22 pages, 2 EPS figure
Advanced Communications Technology Satellite (ACTS): Design and on-orbit performance measurements
The Advanced Communications Technology Satellite (ACTS), developed and built by Lockheed Martin Astro space for the NASA Lewis Research Center, was launched in September 1993 on the shuttle STS 51 mission. ACTS is a digital experimental communications test bed that incorporates gigahertz bandwidth transponders operating at Ka band, hopping spot beams, on-board storage and switching, and dynamic rain fade compensation. This paper describes the ACTS enabling technologies, the design of the communications payload, the constraints imposed on the spacecraft bus, and the measurements conducted to verify the performance of the system in orbit
CP violation through particle mixing and the H-A lineshape
We consider the possibility of looking for CP-mixing effects in two-Higgs
doublet models (and particularly in the MSSM) by studying the lineshape of the
CP-even (H) and CP-odd (A) neutral scalars. In most cases H and A come quite
degenerate in mass, and their s-channel production would lead to nearly
overlapping resonances. CP-violating effects may connect these two Higgs
bosons, giving origin to one-loop particle mixing, which, due to their mass
proximity, can be resonantly enhanced. The corresponding transition amplitude
contains then CP-even and CP-odd components; besides the signal of
intereference between both amplitudes, leading to a CP-odd asymmetry, we
propose to look for the mixing probability itself, a quantity which, although
CP-even, can originate only from a CP-odd amplitude. We show that, in general,
the effect of such a mixing probability cannot be mimicked by (or be
re-absorbed into) a simple redefinition of the H and A masses in the context of
a CP-conserving model. Specifically, the effects of the CP-mixing are such
that, either the mass-splitting of the H and A bosons cannot be accounted for
in the absence of CP-mixing, and/or the detailed energy dependence of the
produced lineshape is clearly different from the one obtained by redefining the
masses, but not allowing any mixing. This analysis suggests that the detailed
study of the lineshape of this Higgs system may provide valuable information on
the CP nature of the underlying theory.Comment: 16 pages, 13 figures; v2: added one reference; v3: radiative
corrections taken into account, agreement now with CP-SuperH, conclusions
unchanged. v3 matches the paper version accepted for publication in JHE
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