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Search Results: 1 - 10 of 554475 matches for " A. K. Opper "
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Charge Symmetry Breaking and QCD
Gerald A Miller,Allena K. Opper,Edward J. Stephenson
Physics , 2006, DOI: 10.1146/annurev.nucl.56.080805.140446
Abstract: Charge symmetry breaking (CSB) in the strong interaction occurs because of the difference between the masses of the up and down quarks. The use of effective field theories allows us to follow this influence of confined quarks in hadronic and nuclear systems. The progress in observing and understanding CSB is reviewed with particular attention to the recent successful observations of CSB in measurements involving the production of a single neutral pion and to the related theoretical progress.
Parameter estimation and inference for stochastic reaction-diffusion systems: application to morphogenesis in D. melanogaster
Michael A Dewar, Visakan Kadirkamanathan, Manfred Opper, Guido Sanguinetti
BMC Systems Biology , 2010, DOI: 10.1186/1752-0509-4-21
Abstract: We present a Bayesian inference approach to solve both the parameter and state estimation problem for stochastic reaction-diffusion systems. This allows a determination of the full posterior distribution of the parameters (expected values and uncertainty). We benchmark the method by illustrating it on a simple synthetic experiment. We then test the method on real data about the diffusion of the morphogen Bicoid in Drosophila melanogaster. The results show how the precision with which parameters can be inferred varies dramatically, indicating that the ability to infer full posterior distributions on the parameters can have important experimental design consequences.The results obtained demonstrate the feasibility and potential advantages of applying a Bayesian approach to parameter estimation in stochastic reaction-diffusion systems. In particular, the ability to estimate credibility intervals associated with parameter estimates can be precious for experimental design. Further work, however, will be needed to ensure the method can scale up to larger problems.Reaction-diffusion systems play a fundamental role in modelling spatio-temporal dynamics in systems biology. Originally introduced by Turing [1] over 50 years ago to provide a microscopic explanation of morphogenesis, they have been extensively used to explain pattern and organ formation in animals and plants [2,3], as well as other spatio-temporal processes such as quorum sensing in bacterial biofilms [4]. The deterministic reaction-diffusion system is given by a system of partial-differential equationswhere Δ represents the Laplacian operator (second derivative in the spatial directions). Here, c is a vector of concentrations of chemical species, D is a diagonal matrix of diffusion coefficients and f encodes the reaction terms between different species.An example of a systems biology application of this type of models is the formation of morphogen gradients during development. In the simplest case, c represents
Charge symmetry breaking in n p --> d pi^0
A. K. Opper,E. Korkmaz,D. A. Hutcheon,R. Abegg,C. A. Davis,R. W. Finlay,P. W. Green,L. G. Greeniaus,D. V. Jordan,J. A. Niskanen,G. V. O'Rielly,T. A. Porcelli,S. D. Reitzner,P. L. Walden,S. Yen
Physics , 2003, DOI: 10.1103/PhysRevLett.91.212302
Abstract: The forward--backward asymmetry in n p --> d pi^0, which must be zero in the center-of-mass system if charge symmetry is respected, has been measured to be [17.2 +/- 8 (stat) +/- 5.5 (sys)] * 10^{-4}, at an incident neutron energy of 279.5 MeV. This charge symmetry breaking observable was extracted by fitting the data with GEANT-based simulations and is compared to recent chiral effective field theory calculations, with implications regarding the value of the u d quark mass difference.
The Importance of Adjuvant Agents in Acute Intrathecal Baclofen Withdrawal: Case Report and Review of the Literature  [PDF]
Christopher M. Wang, Susan E. Opper
Open Journal of Anesthesiology (OJAnes) , 2012, DOI: 10.4236/ojanes.2012.24024
Abstract: Objective: Baclofen (β-p-chlorophenyl-GABA) selectively activates the GABAB subtype of γ-aminobutyric acid (GABA) receptors, a group of receptors known to provide inhibitory neurotransmission in the central nervous system. Available for over thirty years in oral form for the treatment of skeletal muscle spasticity, its availability now includes continuous intrathecal infusion via an internally implanted pump. While ideal for long-term attenuation of symptoms, this treatment modality can also become disastrous should the pump empty and withdrawal subsequently ensue. Case Report: A 48-year-old male with a past medical history of T8 spinal cord injury from a motor vehicle crash originally presented with altered mental status. Because of resultant paraplegia and spasticity from his injuries, a neurosurgeon implanted an intrathecal baclofen pump three years prior to presentation with symptomatic relief. Further exploration revealed that he had missed his scheduled pump refill appointment and interrogation of his pump confirmed a completely empty reservoir. The patient endured a protracted hospital course that included rhabdomyolysis, acute renal failure, lactic acidosis, respiratory failure, and systemic inflammatory response syndrome. Treatment included benzodiazepines, dantrolene, aggressive hydration, opiates, and refill of his intrathecal baclofen pump. His mentation and ventilatory status improved with recovery from his critical illness and eventual discharge. Conclusions: Treatment of intrathecal baclofen withdrawal should focus on restoration of previous intrathecal baclofen levels by refill of the intrathecal pump. Adjuvant medications such as benzodiazepines, propofol, cyproheptadine, dantrolene, tizanidine, and opiates may prove crucial in helping with muscle spasticity while these levels are reestablished. A high index of suspicion, leading to timely initiation of proper treatment, may serve as the most important factor in successful recovery from this life-threatening syndrome.
Efficient statistical inference for stochastic reaction processes
Andreas Ruttor,Manfred Opper
Physics , 2009, DOI: 10.1103/PhysRevLett.103.230601
Abstract: We address the problem of estimating unknown model parameters and state variables in stochastic reaction processes when only sparse and noisy measurements are available. Using an asymptotic system size expansion for the backward equation we derive an efficient approximation for this problem. We demonstrate the validity of our approach on model systems and generalize our method to the case when some state variables are not observed.
Retarded Learning: Rigorous Results from Statistical Mechanics
D. Herschkowitz,M. Opper
Physics , 2001, DOI: 10.1103/PhysRevLett.86.2174
Abstract: We study learning of probability distributions characterized by an unknown symmetry direction. Based on an entropic performance measure and the variational method of statistical mechanics we develop exact upper and lower bounds on the scaled critical number of examples below which learning of the direction is impossible. The asymptotic tightness of the bounds suggests an asymptotically optimal method for learning nonsmooth distributions.
Statistical Mechanics of Learning: A Variational Approach for Real Data
D. Malzahn,M. Opper
Physics , 2002, DOI: 10.1103/PhysRevLett.89.108302
Abstract: Using a variational technique, we generalize the statistical physics approach of learning from random examples to make it applicable to real data. We demonstrate the validity and relevance of our method by computing approximate estimators for generalization errors that are based on training data alone.
Tractable approximations for probabilistic models: The adaptive TAP mean field approach
Manfred Opper,Ole Winther
Physics , 2001, DOI: 10.1103/PhysRevLett.86.3695
Abstract: We develop an advanced mean field method for approximating averages in probabilistic data models that is based on the TAP approach of disorder physics. In contrast to conventional TAP, where the knowledge of the distribution of couplings between the random variables is required, our method adapts to the concrete couplings. We demonstrate the validity of our approach, which is sofar restricted to models with non-glassy behaviour, by replica calculations for a wide class of models as well as by simulations for a real data set.
Statistical Mechanics of Learning in the Presence of Outliers
Rainer Dietrich,Manfred Opper
Mathematics , 1998, DOI: 10.1088/0305-4470/31/46/005
Abstract: Using methods of statistical mechanics, we analyse the effect of outliers on the supervised learning of a classification problem. The learning strategy aims at selecting informative examples and discarding outliers. We compare two algorithms which perform the selection either in a soft or a hard way. When the fraction of outliers grows large, the estimation errors undergo a first order phase transition.
Obesity increases operating room times in patients undergoing primary hip arthroplasty: a retrospective cohort analysis
Bassam Kadry,Christopher D. Press,Hassan Alosh,Isaac M. Opper,Joe Orsini,Igor A. Popov,Jay B. Brodsky,Alex Macario
PeerJ , 2015, DOI: 10.7717/peerj.530
Abstract: Background. Obesity impacts utilization of healthcare resources. The goal of this study was to measure the relationship between increasing body mass index (BMI) in patients undergoing total hip arthroplasty (THA) with different components of operating room (OR) time.
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