5,426 research outputs found
Orbital debris removal with solar concentrators
In 1993 solar concentrators were first proposed to deflect asteroids away from a collision course with the Earth. The original concept was expanded by the authors, and proved to be effective and technologically feasible. One way to deflect the asteroid is to produce a slow decay of its orbit by inducing a thrust via concentrated solar light. Two mechanisms have been investigated: the sublimation of the surface of the asteroid to generate a jet of gas and the induced thrust by light pressure and enhanced Yarkovsky effect. If the concentrators are reduced in size, a similar concept can be adopted to remove orbital debris and inert satellites. In this paper, we present an orbital debris removal system based on concentrated solar light. We will show how enhanced solar pressure, generated by a formation of solar concentrators, can be used to accelerate the decay of small inert objects orbiting the Earth. A set of modified proximal motion equations is proposed to describe the relative dynamics of the solar concentrators with respect to the target piece of debris. The paper will provide an analysis of the cost of the optimal control of the concentrators during the deflection of the debris and a system engineering analysis. In particular, we will show that the concentrator acts as an active solar sail while not deflecting, and as a hybrid solar sail (i.e. the orbit is maintained with an auxiliary low-thrust engine) while deflecting the debris. The results will show that objects with even a small area-to-mass ratio (down to 0.01) can be brought from an 800 km to a 200 km altitude orbit in few hundred days of constant operation. The paper will discuss also the possibility to vaporize some small size targets with high power solar pumped laser
Light stops, blind spots, and isospin violation in the MSSM
In the framework of the MSSM, we examine several simplified models where only
a few superpartners are light. This allows us to study WIMP--nucleus scattering
in terms of a handful of MSSM parameters and thereby scrutinize their impact on
dark matter direct-detection experiments. Focusing on spin-independent
WIMP--nucleon scattering, we derive simplified, analytic expressions for the
Wilson coefficients associated with Higgs and squark exchange. We utilize these
results to study the complementarity of constraints due to direct-detection,
flavor, and collider experiments. We also identify parameter configurations
that produce (almost) vanishing cross sections. In the proximity of these
so-called blind spots, we find that the amount of isospin violation may be much
larger than typically expected in the MSSM. This feature is a generic property
of parameter regions where cross sections are suppressed, and highlights the
importance of a careful analysis of the nucleon matrix elements and the
associated hadronic uncertainties. This becomes especially relevant once the
increased sensitivity of future direct-detection experiments corners the MSSM
into these regions of parameter space.Comment: 38 pages, 15 figures. v2: expanded text in Sec. 3 concerning relic
density and (g-2)_mu constraints, clarified text on isospin violation. Fig. 1
is new, minor changes to Figs. 3,4,10. References added, journal versio
Dark Matter: Connecting LHC searches to direct detection
In these proceedings we review the interplay between LHC searches for dark
matter and direct detection experiments. For this purpose we consider two prime
examples: the effective field theory (EFT) approach and the minimal
supersymmetric standard model (MSSM). In the EFT scenario we show that for
operators which do not enter directly direct detection at tree-level, but only
via loop effects, LHC searches give complementary constraints. In the MSSM stop
and Higgs exchange contribute to the direct detection amplitude. Therefore, LHC
searches for supersymmetric particles and heavy Higgses place constraints on
the same parameter space as direct detection.Comment: 5 pages, 3 figures, proceedings for Moriond Gravitation 201
Critical branching processes in digital memcomputing machines
Memcomputing is a novel computing paradigm that employs time non-locality
(memory) to solve combinatorial optimization problems. It can be realized in
practice by means of non-linear dynamical systems whose point attractors
represent the solutions of the original problem. It has been previously shown
that during the solution search digital memcomputing machines go through a
transient phase of avalanches (instantons) that promote dynamical long-range
order. By employing mean-field arguments we predict that the distribution of
the avalanche sizes follows a Borel distribution typical of critical branching
processes with exponent . We corroborate this analysis by solving
various random 3-SAT instances of the Boolean satisfiability problem. The
numerical results indicate a power-law distribution with exponent , in very good agreement with the mean-field analysis. This indicates
that memcomputing machines self-tune to a critical state in which avalanches
are characterized by a branching process, and that this state persists across
the majority of their evolution.Comment: 5 pages, 3 figure
Effect of electron-phonon scattering on shot noise in nanoscale junctions
We investigate the effect of electron-phonon inelastic scattering on shot
noise in nanoscale junctions in the regime of quasi-ballistic transport. We
predict that when the local temperature of the junction is larger than its
lowest vibrational mode energy , the inelastic contribution to shot noise
(conductance) increases (decreases) with bias as (). The
corresponding Fano factor thus increases as . We also show that the
inelastic contribution to the Fano factor saturates with increasing thermal
current exchanged between the junction and the bulk electrodes to a value
which, for , is independent of bias. A measurement of shot noise may
thus provide information about the local temperature and heat dissipation in
nanoscale conductors.Comment: 4 pages, 2 figure
- …