1,025 research outputs found
NLO QCD predictions for + jets production with Sherpa
We present precise predictions for prompt photon production in association
with a boson and jets. They are obtained within the Sherpa framework as a
consistently merged inclusive sample. Leptonic decays of the boson are
fully included in the calculation with all offshell effects. Virtual matrix
elements are provided by OpenLoops and parton shower effects are simulated with
a dipole parton shower. Thanks to the NLO QCD corrections included not only for
inclusive production but also for the + 1-jet process we
find significantly reduced systematic uncertainties and very good agreement
with experimental measurements at TeV. Predictions at
TeV are displayed including a study of theoretical uncertainties.
In view of an application of these simulations within LHC experiments, we
discuss in detail the necessary combination with a simulation of the + jets
final state. In addition to a corresponding prescription we introduce
recommended cross checks to avoid common pitfalls during the overlap removal
between the two samples.Comment: 20 pages, 15 Figure
Human Origins and the Search for “Missing Links”
The study of human evolution is filled with exciting discoveries, contentious disputes, and immense promise. Johannes Krause reviews John Reader's book on the history of paleoanthropology
Coupling different discretizations for fluid structure interaction in a monolithic approach
In this thesis we present a monolithic coupling approach for the simulation of phenomena involving interacting fluid and structure using different discretizations for the subproblems. For many applications in fluid dynamics, the Finite Volume method is the first choice in simulation science. Likewise, for the simulation of structural mechanics the Finite Element method is one of the most, if not the most, popular discretization method. However, despite the advantages of these discretizations in their respective application domains, monolithic coupling schemes have so far been restricted to a single discretization for both subproblems. We present a fluid structure coupling scheme based on a mixed Finite Volume/Finite Element method that combines the benefits of these discretizations. An important challenge in coupling fluid and structure is the transfer of forces and velocities at the fluidstructure interface in a stable and efficient way. In our approach this is achieved by means of a fully implicit formulation, i.e., the transfer of forces and displacements is carried out in a common set of equations for fluid and structure. We assemble the two different discretizations for the fluid and structure subproblems as well as the coupling conditions for forces and displacements into a single large algebraic system. Since we simulate real world problems, as a consequence of the complexity of the considered geometries, we end up with algebraic systems with a large number of degrees of freedom. This necessitates the use of parallel solution techniques. Our work covers the design and implementation of the proposed heterogeneous monolithic coupling approach as well as the efficient solution of the arising large nonlinear systems on distributed memory supercomputers. We apply Newton’s method to linearize the fully implicit coupled nonlinear fluid structure interaction problem. The resulting linear system is solved with a Krylov subspace correction method. For the preconditioning of the iterative solver we propose the use of multilevel methods. Specifically, we study a multigrid as well as a two-level restricted additive Schwarz method. We illustrate the performance of our method on a benchmark example and compare the afore mentioned different preconditioning strategies for the parallel solution of the monolithic coupled system
Information Directed Sampling and Bandits with Heteroscedastic Noise
In the stochastic bandit problem, the goal is to maximize an unknown function
via a sequence of noisy evaluations. Typically, the observation noise is
assumed to be independent of the evaluation point and to satisfy a tail bound
uniformly on the domain; a restrictive assumption for many applications. In
this work, we consider bandits with heteroscedastic noise, where we explicitly
allow the noise distribution to depend on the evaluation point. We show that
this leads to new trade-offs for information and regret, which are not taken
into account by existing approaches like upper confidence bound algorithms
(UCB) or Thompson Sampling. To address these shortcomings, we introduce a
frequentist regret analysis framework, that is similar to the Bayesian
framework of Russo and Van Roy (2014), and we prove a new high-probability
regret bound for general, possibly randomized policies, which depends on a
quantity we refer to as regret-information ratio. From this bound, we define a
frequentist version of Information Directed Sampling (IDS) to minimize the
regret-information ratio over all possible action sampling distributions. This
further relies on concentration inequalities for online least squares
regression in separable Hilbert spaces, which we generalize to the case of
heteroscedastic noise. We then formulate several variants of IDS for linear and
reproducing kernel Hilbert space response functions, yielding novel algorithms
for Bayesian optimization. We also prove frequentist regret bounds, which in
the homoscedastic case recover known bounds for UCB, but can be much better
when the noise is heteroscedastic. Empirically, we demonstrate in a linear
setting with heteroscedastic noise, that some of our methods can outperform UCB
and Thompson Sampling, while staying competitive when the noise is
homoscedastic.Comment: Figure 1a,2a update
Bias-Robust Bayesian Optimization via Dueling Bandits
We consider Bayesian optimization in settings where observations can be
adversarially biased, for example by an uncontrolled hidden confounder. Our
first contribution is a reduction of the confounded setting to the dueling
bandit model. Then we propose a novel approach for dueling bandits based on
information-directed sampling (IDS). Thereby, we obtain the first efficient
kernelized algorithm for dueling bandits that comes with cumulative regret
guarantees. Our analysis further generalizes a previously proposed
semi-parametric linear bandit model to non-linear reward functions, and
uncovers interesting links to doubly-robust estimation
Stochastic Bandits with Context Distributions
We introduce a stochastic contextual bandit model where at each time step the
environment chooses a distribution over a context set and samples the context
from this distribution. The learner observes only the context distribution
while the exact context realization remains hidden. This allows for a broad
range of applications where the context is stochastic or when the learner needs
to predict the context. We leverage the UCB algorithm to this setting and show
that it achieves an order-optimal high-probability bound on the cumulative
regret for linear and kernelized reward functions. Our results strictly
generalize previous work in the sense that both our model and the algorithm
reduce to the standard setting when the environment chooses only Dirac delta
distributions and therefore provides the exact context to the learner. We
further analyze a variant where the learner observes the realized context after
choosing the action. Finally, we demonstrate the proposed method on synthetic
and real-world datasets.Comment: Accepted at NeurIPS 201
Linear Partial Monitoring for Sequential Decision-Making: Algorithms, Regret Bounds and Applications
Partial monitoring is an expressive framework for sequential decision-making
with an abundance of applications, including graph-structured and dueling
bandits, dynamic pricing and transductive feedback models. We survey and extend
recent results on the linear formulation of partial monitoring that naturally
generalizes the standard linear bandit setting. The main result is that a
single algorithm, information-directed sampling (IDS), is (nearly) worst-case
rate optimal in all finite-action games. We present a simple and unified
analysis of stochastic partial monitoring, and further extend the model to the
contextual and kernelized setting
Probabilistic load flow for uncertainty based grid operation
Traditional algorithms used in grid operation and planning only evaluate one deterministic state. Uncertainties introduced by the increasing utilization of renewable energy sources have to be dealt with when determining the operational state of a grid. From this perspective the probability of certain operational states and of possible bottlenecks is important information to support the grid operator or planner in their daily work. From this special need the field of application for Probabilistic Load Flow methods evolved. Uncertain influences like power plant outages, deviations from the forecasted injected wind power and load have to be considered by their corresponding probability. With the help of probability density functions an integrated consideration of the partly stochastic behaviour of power plants und loads is possible
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