680 research outputs found
Applying Rule Ensembles to the Search for Super-Symmetry at the Large Hadron Collider
In this note we give an example application of a recently presented
predictive learning method called Rule Ensembles. The application we present is
the search for super-symmetric particles at the Large Hadron Collider. In
particular, we consider the problem of separating the background coming from
top quark production from the signal of super-symmetric particles. The method
is based on an expansion of base learners, each learner being a rule, i.e. a
combination of cuts in the variable space describing signal and background.
These rules are generated from an ensemble of decision trees. One of the
results of the method is a set of rules (cuts) ordered according to their
importance, which gives useful tools for diagnosis of the model. We also
compare the method to a number of other multivariate methods, in particular
Artificial Neural Networks, the likelihood method and the recently presented
boosted decision tree method. We find better performance of Rule Ensembles in
all cases. For example for a given significance the amount of data needed to
claim SUSY discovery could be reduced by 15 % using Rule Ensembles as compared
to using a likelihood method.Comment: 24 pages, 7 figures, replaced to match version accepted for
publication in JHE
Decoherence of a Superposition of Macroscopic Current States in a SQUID
We show that fundamental conservation laws mandate parameter-free mechanisms
of decoherence of quantum oscillations of the superconducting current between
opposite directions in a SQUID -- emission of phonons and photons at the
oscillation frequency. The corresponding rates are computed and compared with
experimental findings. The decohering effects of external mechanical and
magnetic noise are investigated
Close encounters of a rotating star with planets in parabolic orbits of varying inclination and the formation of Hot Jupiters
(abbreviated) We extend the theory of close encounters of a planet on a
parabolic orbit with a star to include the effects of tides induced on the
central rotating star. Orbits with arbitrary inclination to the stellar
rotation axis are considered. We obtain results both from an analytic treatment
and numerical one that are in satisfactory agreement. These results are applied
to the initial phase of the tidal circularisation problem. We find that both
tides induced in the star and planet can lead to a significant decrease of the
orbital semi-major axis for orbits having periastron distances smaller than 5-6
stellar radii (corresponding to periods days after the
circularisation has been completed) with tides in the star being much stronger
for retrograde orbits compared to prograde orbits. We use the simple Skumanich
law for the stellar rotation with its rotational period equal to one month at
the age of 5Gyr. The strength of tidal interactions is characterised by
circularisation time scale, defined as a time scale of evolution of
the planet's semi-major axis due to tides considered as a function of orbital
period after the process of tidal circularisation has been completed.
We find that the ratio of the initial circularisation time scales corresponding
to prograde and retrograde orbits is of order 1.5-2 for a planet of one Jupiter
mass and four days. It grows with the mass of the planet, being
of order five for a five Jupiter mass planet with the same . Thus, the
effect of stellar rotation may provide a bias in the formation of planetary
systems having planets on close orbits around their host stars, as a
consequence of planet-planet scattering, favouring systems with retrograde
orbits. The results may also be applied to the problem of tidal capture of
stars in young stellar clusters.Comment: to be published in Celestial Mechanics and Dynamical Astronom
Computing Volume Bounds of Inclusions by EIT Measurements
The size estimates approach for Electrical Impedance Tomography (EIT) allows
for estimating the size (area or volume) of an unknown inclusion in an
electrical conductor by means of one pair of boundary measurements of voltage
and current. In this paper we show by numerical simulations how to obtain such
bounds for practical application of the method. The computations are carried
out both in a 2D and a 3D setting.Comment: 20 pages with figure
Single and double qubit gates by manipulating degeneracy
A novel mechanism is proposed for single and double qubit state manipulations
in quantum computation with four-fold degenerate energy levels. The principle
is based on starting with a four fold degeneracy, lifting it stepwise
adiabatically by a set of control parameters and performing the quantum gate
operations on non-degenerate states. A particular realization of the proposed
mechanism is suggested by using inductively coupled rf-squid loops in the
macroscopic quantum tunnelling regime where the energy eigen levels are
directly connected with the measurable flux states. The one qubit and two qubit
controlled operations are demonstrated explicitly. The appearance of the flux
states also allows precise read-in and read-out operations by the measurement
of flux.Comment: 6 pages + 5 figures (separately included
Adiabatic Landau-Zener-St\"uckelberg transition with or without dissipation in low spin molecular system V15
The spin one half molecular system V15 shows no barrier against spin
reversal. This makes possible direct phonon activation between the two levels.
By tuning the field sweeping rate and the thermal coupling between sample and
thermal reservoir we have control over the phonon-bottleneck phenomena
previously reported in this system. We demonstrate adiabatic motion of molecule
spins in time dependent magnetic fields and with different thermal coupling to
the cryostat bath. We also discuss the origin of the zero-field tunneling
splitting for a half-integer spin.Comment: to appear in Phys. Rev. B - Rapid Communication
Macroscopic resonant tunneling of magnetic flux
We have developed a quantitative theory of resonant tunneling of magnetic
flux between discrete macroscopically distinct quantum states in SQUID systems.
The theory is based on the standard density-matrix approach. Its new elements
include the discussion of the two different relaxation mechanisms that exist
for the double-well potential, and description of the ``photon-assisted''
tunneling driven by external rf radiation. It is shown that in the case of
coherent flux dynamics, rf radiation should lead to splitting of the peaks of
resonant flux tunneling, indicating that the resonant tunneling is a convenient
tool for studying macroscopic quantum coherence of flux.Comment: 11 pages, 8 figure
Stochastic Lagrangian Particle Approach to Fractal Navier-Stokes Equations
In this article we study the fractal Navier-Stokes equations by using
stochastic Lagrangian particle path approach in Constantin and Iyer
\cite{Co-Iy}. More precisely, a stochastic representation for the fractal
Navier-Stokes equations is given in terms of stochastic differential equations
driven by L\'evy processes. Basing on this representation, a self-contained
proof for the existence of local unique solution for the fractal Navier-Stokes
equation with initial data in \mW^{1,p} is provided, and in the case of two
dimensions or large viscosity, the existence of global solution is also
obtained. In order to obtain the global existence in any dimensions for large
viscosity, the gradient estimates for L\'evy processes with time dependent and
discontinuous drifts is proved.Comment: 19 page
Josephson-phase qubit without tunneling
We show that a complete set of one-bit gates can be realized by coupling the
two logical states of a phase qubit to a third level (at higher energy) using
microwave pulses. Thus, one can achieve coherent control without invoking any
tunneling between the qubit levels. We propose two implementations, using
rf-SQUIDs and d-wave Josephson junctions.Comment: REVTeX4, 4pp., 6 EPS figure files; N.B.: "Alec" is my first, and
"Maassen van den Brink" my family name. v2: gate universality fleshed out,
small fix in d-wave decoherence para, discussion expanded, two Refs. added.
v3: some more Refs., a molecular example, and a few minor fixes; final, to
appear in PRB Rapid
Measurement of coherent charge transfer in an adiabatic Cooper pair pump
We study adiabatic charge transfer in a superconducting Cooper pair pump,
focusing on the influence of current measurement on coherence. We investigate
the limit where the Josephson coupling energy between the various parts
of the system is small compared to the Coulomb charging energy . In this
case the charge transferred in a pumping cycle , the charge of one
Cooper pair: the main contribution is due to incoherent Cooper pair tunneling.
We are particularly interested in the quantum correction to , which is due
to coherent tunneling of pairs across the pump and which depends on the
superconducting phase difference between the electrodes: . A measurement of tends to destroy the phase
coherence. We first study an arbitrary measuring circuit and then specific
examples and show that coherent Cooper pair transfer can in principle be
detected using an inductively shunted ammeter
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