4,261 research outputs found
Analytic Confidence Level Calculations using the Likelihood Ratio and Fourier Transform
The interpretation of new particle search results involves a confidence level
calculation on either the discovery hypothesis or the background-only ("null")
hypothesis. A typical approach uses toy Monte Carlo experiments to build an
expected experiment estimator distribution against which an observed
experiment's estimator may be compared. In this note, a new approach is
presented which calculates analytically the experiment estimator distribution
via a Fourier transform, using the likelihood ratio as an ordering estimator.
The analytic approach enjoys an enormous speed advantage over the toy Monte
Carlo method, making it possible to quickly and precisely calculate confidence
level results.Comment: 11 pages, 2 figure
Constraining models of gravity with Planck and WiggleZ power spectrum data
In order to explain cosmic acceleration without invoking "dark" physics, we
consider modified gravity models, which replace the standard
Einstein-Hilbert action in General Relativity with a higher derivative theory.
We use data from the WiggleZ Dark Energy survey to probe the formation of
structure on large scales which can place tight constraints on these models. We
combine the large-scale structure data with measurements of the cosmic
microwave background from the Planck surveyor. After parameterising the
modification of the action using the Compton wavelength parameter , we
constrain this parameter using ISiTGR, assuming an initial non-informative log
prior probability distribution of this cross-over scale. We find that the
addition of the WiggleZ power spectrum provides the tightest constraints to
date on by an order of magnitude, giving at
95% confidence limit. Finally, we test whether the effect of adding the lensing
amplitude and the sum of the neutrino mass is able
to reconcile current tensions present in these parameters, but find
gravity an inadequate explanation.Comment: 21 pages, 7 figures, matches version published in JCAP. The modified
version of ISiTGR used to produce the results in this paper is available at
http://isit.g
Giant topological insulator gap in graphene with 5d adatoms
Two-dimensional topological insulators (2D TIs) have been proposed as
platforms for many intriguing applications, ranging from spintronics to
topological quantum information processing. Realizing this potential will
likely be facilitated by the discovery of new, easily manufactured materials in
this class. With this goal in mind we introduce a new framework for engineering
a 2D TI by hybridizing graphene with impurity bands arising from heavy adatoms
possessing partially filled d-shells, in particular osmium and iridium. First
principles calculations predict that the gaps generated by this means exceed
0.2 eV over a broad range of adatom coverage; moreover, tuning of the Fermi
level is not required to enter the TI state. The mechanism at work is expected
to be rather general and may open the door to designing new TI phases in many
materials.Comment: 7 pages, 8 figure
Optimal Auctions for Correlated Buyers with Sampling
Cr\'emer and McLean [1985] showed that, when buyers' valuations are drawn
from a correlated distribution, an auction with full knowledge on the
distribution can extract the full social surplus. We study whether this
phenomenon persists when the auctioneer has only incomplete knowledge of the
distribution, represented by a finite family of candidate distributions, and
has sample access to the real distribution. We show that the naive approach
which uses samples to distinguish candidate distributions may fail, whereas an
extended version of the Cr\'emer-McLean auction simultaneously extracts full
social surplus under each candidate distribution. With an algebraic argument,
we give a tight bound on the number of samples needed by this auction, which is
the difference between the number of candidate distributions and the dimension
of the linear space they span
Engineering a robust quantum spin Hall state in graphene via adatom deposition
The 2007 discovery of quantized conductance in HgTe quantum wells delivered
the field of topological insulators (TIs) its first experimental confirmation.
While many three-dimensional TIs have since been identified, HgTe remains the
only known two-dimensional system in this class. Difficulty fabricating HgTe
quantum wells has, moreover, hampered their widespread use. With the goal of
breaking this logjam we provide a blueprint for stabilizing a robust TI state
in a more readily available two-dimensional material---graphene. Using symmetry
arguments, density functional theory, and tight-binding simulations, we predict
that graphene endowed with certain heavy adatoms realizes a TI with substantial
band gap. For indium and thallium, our most promising adatom candidates, a
modest 6% coverage produces an estimated gap near 80K and 240K, respectively,
which should be detectable in transport or spectroscopic measurements.
Engineering such a robust topological phase in graphene could pave the way for
a new generation of devices for spintronics, ultra-low-dissipation electronics
and quantum information processing.Comment: 14 pages, 7 figure
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