175 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
Topological geon black holes in Einstein-Yang-Mills theory
We construct topological geon quotients of two families of
Einstein-Yang-Mills black holes. For Kuenzle's static, spherically symmetric
SU(n) black holes with n>2, a geon quotient exists but generically requires
promoting charge conjugation into a gauge symmetry. For Kleihaus and Kunz's
static, axially symmetric SU(2) black holes a geon quotient exists without
gauging charge conjugation, and the parity of the gauge field winding number
determines whether the geon gauge bundle is trivial. The geon's gauge bundle
structure is expected to have an imprint in the Hawking-Unruh effect for
quantum fields that couple to the background gauge field.Comment: 27 pages. v3: Presentation expanded. Minor corrections and addition
Introduction to Isolated Horizons in Numerical Relativity
We present a coordinate-independent method for extracting mass (M) and
angular momentum (J) of a black hole in numerical simulations. This method,
based on the isolated horizon framework, is applicable both at late times when
the black hole has reached equilibrium, and at early times when the black holes
are widely separated. We show how J and M can be determined in numerical
simulations in terms of only those quantities which are intrinsic to the
apparent horizon. We also present a numerical method for finding the rotational
symmetry vector field (required to calculate J) on the horizon.Comment: 14 pages, revtex4, 7 figures. Final PRD versio
Evolution of density perturbations in a realistic universe
Prompted by the recent more precise determination of the basic cosmological
parameters and growing evidence that the matter-energy content of the universe
is now dominated by dark energy and dark matter we present the general solution
of the equation that describes the evolution of density perturbations in the
linear approximation. It turns out that as in the standard CDM model the
density perturbations grow very slowly during the radiation dominated epoch and
their amplitude increases by a factor of about 4000 in the matter and later
dark energy dominated epoch of expansion of the universe.Comment: 19 pages, 4 figure
Spatial Degrees of Freedom in Everett Quantum Mechanics
Stapp claims that, when spatial degrees of freedom are taken into account,
Everett quantum mechanics is ambiguous due to a "core basis problem." To
examine an aspect of this claim I generalize the ideal measurement model to
include translational degrees of freedom for both the measured system and the
measuring apparatus. Analysis of this generalized model using the Everett
interpretation in the Heisenberg picture shows that it makes unambiguous
predictions for the possible results of measurements and their respective
probabilities. The presence of translational degrees of freedom for the
measuring apparatus affects the probabilities of measurement outcomes in the
same way that a mixed state for the measured system would. Examination of a
measurement scenario involving several observers illustrates the consistency of
the model with perceived spatial localization of the measuring apparatus.Comment: 34 pp., no figs. Introduction, discussion revised. Material
tangential to main point remove
Circular orbits of corotating binary black holes: comparison between analytical and numerical results
We compare recent numerical results, obtained within a ``helical Killing
vector'' (HKV) approach, on circular orbits of corotating binary black holes to
the analytical predictions made by the effective one body (EOB) method (which
has been recently extended to the case of spinning bodies). On the scale of the
differences between the results obtained by different numerical methods, we
find good agreement between numerical data and analytical predictions for
several invariant functions describing the dynamical properties of circular
orbits. This agreement is robust against the post-Newtonian accuracy used for
the analytical estimates, as well as under choices of resummation method for
the EOB ``effective potential'', and gets better as one uses a higher
post-Newtonian accuracy. These findings open the way to a significant
``merging'' of analytical and numerical methods, i.e. to matching an EOB-based
analytical description of the (early and late) inspiral, up to the beginning of
the plunge, to a numerical description of the plunge and merger. We illustrate
also the ``flexibility'' of the EOB approach, i.e. the possibility of
determining some ``best fit'' values for the analytical parameters by
comparison with numerical data.Comment: Minor revisions, accepted for publication in Phys. Rev. D, 19 pages,
6 figure
Vacuum Stability in Heterotic M-Theory
The problem of the stabilization of moduli is discussed within the context of
compactified strongly coupled heterotic string theory. It is shown that all
geometric, vector bundle and five-brane moduli are completely fixed, within a
phenomenologically acceptable range, by non-perturbative physics. This result
requires, in addition to the full space of moduli, non-vanishing Neveu-Schwarz
flux, gaugino condensation with threshold corrections and the explicit form of
the Pfaffians in string instanton superpotentials. The stable vacuum presented
here has a negative cosmological constant. The possibility of ``lifting'' this
to a metastable vacuum with positive cosmological constant is briefly
discussed.Comment: 39 pages, minor correction
Do we live in the universe successively dominated by matter and antimatter?
We wonder if a cyclic universe may be dominated alternatively by matter and
antimatter. Such a scenario demands a mechanism for transformation of matter to
antimatter (or antimatter to matter) during the final stage of a big crunch. By
giving an example, we have shown that in principle such a mechanism is
possible. Our mechanism is based on a hypothetical repulsion between matter and
antimatter, existing at least deep inside the horizon of a black hole. When
universe is reduced to a supermassive black hole of a small size, a very strong
field of the conjectured force might create (through a Schwinger type
mechanism) particle-antiparticle pairs from the quantum vacuum. The amount of
antimatter created from the vacuum is equal to the decrease of mass of the
black hole and violently repelled from it. When the size of the black hole is
sufficiently small, the creation of antimatter may become so fast, that matter
of our Universe might be transformed to antimatter in a fraction of second.
Such a fast conversion of matter into antimatter may look as a Big Bang. Our
mechanism prevents a singularity; a new cycle might start with an initial size
more than 30 orders of magnitude greater than the Planck length, suggesting
that there is no need for inflationary scenario in Cosmology. In addition,
there is no need to invoke CP violation for explanation of matter-antimatter
asymmetry. Simply, our present day Universe is dominated by matter, because the
previous universe was dominated by antimatter
- …