319 research outputs found
The process at a Collider
The helicity amplitudes for the process are
studied to 1-loop order in the minimal SUSY (MSSM) model, where is the
CP-odd Higgs particle.
Simple exact analytic formulae are obtained, in terms of the and
Passarino-Veltman functions; in spite of the fact that the loop-diagrams often
involve different particles running along their sides. For a usual mSUGRA set
of parameters, is
expected. If SUSY is realized in Nature, these expressions should be useful for
understanding the Higgs sector.Comment: Misprints in typos corrected, 1 reference added e-mail:
[email protected]
Far-field approximation for hydrodynamic interactions in parallel-wall geometry
A complete analysis is presented for the far-field creeping flow produced by
a multipolar force distribution in a fluid confined between two parallel planar
walls. We show that at distances larger than several wall separations the flow
field assumes the Hele-Shaw form, i.e., it is parallel to the walls and varies
quadratically in the transverse direction. The associated pressure field is a
two-dimensional harmonic function that is characterized by the same multipolar
number m as the original force multipole. Using these results we derive
asymptotic expressions for the Green's matrix that represents Stokes flow in
the wall-bounded fluid in terms of a multipolar spherical basis. This Green's
matrix plays a central role in our recently proposed algorithm [Physica A xx,
{\bf xxx} (2005)] for evaluating many-body hydrodynamic interactions in a
suspension of spherical particles in the parallel-wall geometry. Implementation
of our asymptotic expressions in this algorithm increases its efficiency
substantially because the numerically expensive evaluation of the exact matrix
elements is needed only for the neighboring particles. Our asymptotic analysis
will also be useful in developing hydrodynamic algorithms for wall-bounded
periodic systems and implementing acceleration methods by using corresponding
results for the two-dimensional scalar potential.Comment: 28 pages 5 figure
Adsorption of Self-Assembled Rigid Rods on Two-Dimensional Lattices
Monte Carlo (MC) simulations have been carried out to study the adsorption on
square and triangular lattices of particles with two bonding sites that, by
decreasing temperature or increasing density, polymerize reversibly into chains
with a discrete number of allowed directions and, at the same time, undergo a
continuous isotropic-nematic (IN) transition. The process has been monitored by
following the behavior of the adsorption isotherms for different values of
lateral interaction energy/temperature. The numerical data were compared with
mean-field analytical predictions and exact functions for noninteracting and 1D
systems. The obtained results revealed the existence of three adsorption
regimes in temperature. (1) At high temperatures, above the critical one
characterizing the IN transition at full coverage Tc(\theta=1), the particles
are distributed at random on the surface and the adlayer behaves as a
noninteracting 2D system. (2) At very low temperatures, the asymmetric monomers
adsorb forming chains over almost the entire range of coverage, and the
adsorption process behaves as a 1D problem. (3) In the intermediate regime, the
system exhibits a mixed regime and the filling of the lattice proceeds
according to two different processes. In the first stage, the monomers adsorb
isotropically on the lattice until the IN transition occurs in the system and,
from this point, particles adsorb forming chains so that the adlayer behaves as
a 1D fluid. The two adsorption processes are present in the adsorption
isotherms, and a marked singularity can be observed that separates both
regimes. Thus, the adsorption isotherms appear as sensitive quantities with
respect to the IN phase transition, allowing us (i) to reproduce the phase
diagram of the system for square lattices and (ii) to obtain an accurate
determination of the phase diagram for triangular lattices.Comment: Langmuir, 201
Credimus
We believe that economic design and computational complexity---while already
important to each other---should become even more important to each other with
each passing year. But for that to happen, experts in on the one hand such
areas as social choice, economics, and political science and on the other hand
computational complexity will have to better understand each other's
worldviews.
This article, written by two complexity theorists who also work in
computational social choice theory, focuses on one direction of that process by
presenting a brief overview of how most computational complexity theorists view
the world. Although our immediate motivation is to make the lens through which
complexity theorists see the world be better understood by those in the social
sciences, we also feel that even within computer science it is very important
for nontheoreticians to understand how theoreticians think, just as it is
equally important within computer science for theoreticians to understand how
nontheoreticians think
Existence versus Exploitation: The Opacity of Backbones and Backdoors Under a Weak Assumption
Backdoors and backbones of Boolean formulas are hidden structural properties.
A natural goal, already in part realized, is that solver algorithms seek to
obtain substantially better performance by exploiting these structures.
However, the present paper is not intended to improve the performance of SAT
solvers, but rather is a cautionary paper. In particular, the theme of this
paper is that there is a potential chasm between the existence of such
structures in the Boolean formula and being able to effectively exploit them.
This does not mean that these structures are not useful to solvers. It does
mean that one must be very careful not to assume that it is computationally
easy to go from the existence of a structure to being able to get one's hands
on it and/or being able to exploit the structure.
For example, in this paper we show that, under the assumption that P
NP, there are easily recognizable families of Boolean formulas with strong
backdoors that are easy to find, yet for which it is hard (in fact,
NP-complete) to determine whether the formulas are satisfiable. We also show
that, also under the assumption P NP, there are easily recognizable sets
of Boolean formulas for which it is hard (in fact, NP-complete) to determine
whether they have a large backbone
Semi-automatic Proofs about Object Graphs in Separation Logic
Published correctness proofs of garbage collectors in separationlogic to date depend on extensive manual, interactive formulamanipulations. This paper shows that the approach of symbolicexecution in separation logic, as first developed by Smallfoot,also encompasses reasoning about object graphs given by the reachabilityof objects. This approach yields semi-automatic proofs oftwo central garbage collection algorithms: Schorr-Waite graph marking and Cheney's collector. Our framework is developed as a conservativeextension of Isabelle/HOL. Our verification environment re-uses theSimpl framework for classical Hoare logic
Dielectric function and plasmons in graphene
The electromagnetic response of graphene, expressed by the dielectric
function, and the spectrum of collective excitations are studied as a function
of wave vector and frequency. Our calculation is based on the full band
structure, calculated within the tight-binding approximation. As a result, we
find plasmons whose dispersion is similar to that obtained in the single-valley
approximation by Dirac fermions. In contrast to the latter, however, we find a
stronger damping of the plasmon modes due to inter-band absorption. Our
calculation also reveals effects due to deviations from the linear Dirac
spectrum as we increase the Fermi energy, indicating an anisotropic behavior
with respect to the wave vector of the external electromagnetic field
On the ground state energy scaling in quasi-rung-dimerized spin ladders
On the basis of periodic boundary conditions we study perturbatively a large
N asymptotics (N is the number of rungs) for the ground state energy density
and gas parameter of a spin ladder with slightly destroyed rung-dimerization.
Exactly rung-dimerized spin ladder is treated as the reference model. Explicit
perturbative formulas are obtained for three special classes of spin ladders.Comment: 4 page
Sensitive detection of photoexcited carriers by resonant tunneling through a single quantum dot
We show that the resonant tunnel current through a single energy level of an
individual quantum dot within an ensemble of dots is strongly sensitive to
photoexcited holes that become bound in the close vicinity of the dot. The
presence of these holes lowers the electrostatic energy of the quantum dot
state and switches the current carrying channel from fully open to fully closed
with a high on/off ratio (> 50). The device can be reset by means of a bias
voltage pulse. These properties are of interest for charge sensitive photon
counting devices.Comment: 5 pages, 4 figure
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