104,145 research outputs found
QBDT, a new boosting decision tree method with systematic uncertainties into training for High Energy Physics
A new boosting decision tree (BDT) method, QBDT, is proposed for the
classification problem in the field of high energy physics (HEP). In many HEP
researches, great efforts are made to increase the signal significance with the
presence of huge background and various systematical uncertainties. Why not
develop a BDT method targeting the significance directly? Indeed, the
significance plays a central role in this new method. It is used to split a
node in building a tree and to be also the weight contributing to the BDT
score. As the systematical uncertainties can be easily included in the
significance calculation, this method is able to learn about reducing the
effect of the systematical uncertainties via training. Taking the search of the
rare radiative Higgs decay in proton-proton collisions as example, QBDT and the popular Gradient BDT (GradBDT)
method are compared. QBDT is found to reduce the correlation between the signal
strength and systematical uncertainty sources and thus to give a better
significance. The contribution to the signal strength uncertainty from the
systematical uncertainty sources using the new method is 50-85~\% of that using
the GradBDT method.Comment: 29 pages, accepted for publication in NIMA, algorithm available at
https://github.com/xialigang/QBD
Standing sausage modes in coronal loops with plasma flow
Magnetohydrodynamic waves are important for diagnosing the physical
parameters of coronal plasmas. Field-aligned flows appear frequently in coronal
loops.We examine the effects of transverse density and plasma flow structuring
on standing sausage modes trapped in coronal loops, and examine their
observational implications. We model coronal loops as straight cold cylinders
with plasma flow embedded in a static corona. An eigen-value problem governing
propagating sausage waves is formulated, its solutions used to construct
standing modes. Two transverse profiles are distinguished, one being the
generalized Epstein distribution (profile E) and the other (N) proposed
recently in Nakariakov et al.(2012). A parameter study is performed on the
dependence of the maximum period and cutoff length-to-radius
ratio in the trapped regime on the density parameters
( and profile steepness ) and flow parameters (magnitude
and profile steepness ). For either profile, introducing a flow
reduces relative to the static case. depends
sensitively on for profile N but is insensitive to for profile E. By
far the most important effect a flow introduces is to reduce the capability for
loops to trap standing sausage modes: may be
substantially reduced in the case with flow relative to the static one. If the
density distribution can be described by profile N, then measuring the sausage
mode period can help deduce the density profile steepness. However, this
practice is not feasible if profile E better describes the density
distribution. Furthermore, even field-aligned flows with magnitudes
substantially smaller than the ambient Alfv\'en speed can make coronal loops
considerably less likely to support trapped standing sausage modes.Comment: 11 pages, 9 figures, to appear in Astronomy & Astrophysic
Hypothesis Testing of Matrix Graph Model with Application to Brain Connectivity Analysis
Brain connectivity analysis is now at the foreground of neuroscience
research. A connectivity network is characterized by a graph, where nodes
represent neural elements such as neurons and brain regions, and links
represent statistical dependences that are often encoded in terms of partial
correlations. Such a graph is inferred from matrix-valued neuroimaging data
such as electroencephalography and functional magnetic resonance imaging. There
have been a good number of successful proposals for sparse precision matrix
estimation under normal or matrix normal distribution; however, this family of
solutions do not offer a statistical significance quantification for the
estimated links. In this article, we adopt a matrix normal distribution
framework and formulate the brain connectivity analysis as a precision matrix
hypothesis testing problem. Based on the separable spatial-temporal dependence
structure, we develop oracle and data-driven procedures to test the global
hypothesis that all spatial locations are conditionally independent, which are
shown to be particularly powerful against the sparse alternatives. In addition,
simultaneous tests for identifying conditional dependent spatial locations with
false discovery rate control are proposed in both oracle and data-driven
settings. Theoretical results show that the data-driven procedures perform
asymptotically as well as the oracle procedures and enjoy certain optimality
properties. The empirical finite-sample performance of the proposed tests is
studied via simulations, and the new tests are applied on a real
electroencephalography data analysis
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