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MIT Open Access Articles

Mostrando recursos 1 - 20 de 13,935

1. On invariance in hierarchical models - Bouvrie, Jacob Vincent; Rosasco, Lorenzo Andrea; Poggio, Tomaso A.
A goal of central importance in the study of hierarchical models for object recognition -- and indeed the visual cortex -- is that of understanding quantitatively the trade-off between invariance and selectivity, and how invariance and discrimination properties contribute towards providing an improved representation useful for learning from data. In this work we provide a general group-theoretic framework for characterizing and understanding invariance in a family of hierarchical models. We show that by taking an algebraic perspective, one can provide a concise set of conditions which must be met to establish invariance, as well as a constructive prescription for meeting...

2. Search for the associated production of the Higgs boson with a top-quark pair - CMS Collaboration; Apyan, Aram; Barbieri, Richard Alexander; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, Valentina; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Yen-Jie; Levin, A.; Luckey, P. David; Ma, T.; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stoeckli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Zanetti, Marco; Zhukova, Victoria
A search for the standard model Higgs boson produced in association with a top-quark pair (tt¯H) is presented, using data samples corresponding to integrated luminosities of up to 5.1 fb−1 and 19.7 fb−1 collected in pp collisions at center-of-mass energies of 7 TeV and 8 TeV respectively. The search is based on the following signatures of the Higgs boson decay: H → hadrons, H → photons, and H → leptons. The results are characterized by an observed tt¯H signal strength relative to the standard model cross section, μ=σ/σ SM,under the assumption that the Higgs boson decays as expected in the...

3. Measurement of WZ and ZZ production in pp collisions at s√ = 8 TeV in final states with b-tagged jets - CMS Collaboration; Apyan, Aram; Barbieri, Richard Alexander; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, Valentina; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Yen-Jie; Levin, A.; Luckey, P. David; Ma, T.; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stoeckli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Wyslouch, Boleslaw; Yang, M.; Yoon, Andrew; Zanetti, Marco; Zhukova, Victoria
Measurements are reported of the WZ and ZZ production cross sections in proton-proton collisions at s√=8 TeV in final states where one Z boson decays to b-tagged jets. The other gauge boson, either W or Z, is detected through its leptonic decay (either W→eν , μν or Z→e+e− , μ+μ− , or νν¯ ). The results are based on data corresponding to an integrated luminosity of 18.9 fb −1 collected with the CMS detector at the Large Hadron Collider. The measured cross sections, σ(pp→WZ)=30.7±9.3(stat.)±7.1(syst.)±4.1(th.)±1.0(lum.)pb and σ(pp→ZZ)=6.5±1.7(stat.)±1.0(syst.)±0.9(th.)±0.2(lum.)pb, are consistent with next-to-leading order quantum chromodynamics calculations.

4. Observation of the B[0 over s] → J/ψK[0 over S]K ± π ∓ decay - LHCb collaboration; Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, M.
Decays of the form B (s) 0  → J/ψK S 0 h + h (′) −(h (′) = K, π) are searched for in proton-proton collision data corresponding to an integrated luminosity of 1.0 fb−1 recorded with the LHCb detector. The first observation of the B s 0 → J/ψK S 0 K ± π ∓ decay is reported, with significance in excess of 10 standard deviations. The B 0 → J/ψK S 0 K + K − decay is also observed for the first time. The branching fraction of B 0 → J/ψK S 0 π + π − is determined, to significantly better precision than previous measurements,...

5. Study of the kinematic dependences of Λ [0 over b] production in pp collisions and a measurement of the Λ [0 over b]  → Λ [+ over c] π[superscript −] branching fraction - LHCb collaboration; Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, M.
The kinematic dependences of the relative production rates, fΛ[0 over b]/fd , of Λ [0 over b] baryons and B 0 mesons are measured using Λ [0 over b] → Λ [+ over c] π − and B¯¯0 →D+π− decays. The measurements use proton-proton collision data, corresponding to an integrated luminosity of 1 fb−1 at a centre-of-mass energy of 7 TeV, recorded in the forward region with the LHCb experiment. The relative production rates are observed to depend on the transverse momentum, p T, and pseudorapidity, η, of the beauty hadron, in the studied kinematic region 1.5 < p T < 40...

6. Measurement of CP asymmetries in the decays B 0 → K *0 μ + μ − and B + → K + μ + μ − - Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, M.; LHCb collaboration
The direct CP asymmetries of the decays B 0 → K *0 μ + μ − and B + → K + μ + μ − are measured using pp collision data corresponding to an integrated luminosity of 3.0 fb−1 collected with the LHCb detector. The respective control modes B 0 → J/ψK *0 and B + → J/ψK + are used to account for detection and production asymmetries. The measurements are made in several intervals of μ + μ − invariant mass squared, with the ϕ(1020) and charmonium resonance regions excluded. Under the hypothesis of zero CP asymmetry in...

7. Light-quark and gluon jet discrimination in pp collisions at √s = 7 TeV with the ATLAS detector - Taylor, Frank E.; ATLAS Collaboration
A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb −1 of proton–proton collision data at s√=7 TeV collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of ≈ 10–20 %. In data, light-quark jets can be tagged with an efficiency of ≈50% while achieving...

8. Measurement of χ[subscript c1] and χ[subscript c2] Production with = 7 TeV Pp Collisions at ATLAS - ATLAS Collaboration; Taylor, Frank E.
The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are measured in pp collisions at s√ = 7 TeV with the ATLAS detector at the LHC using 4.5 fb−1 of integrated luminosity. The χ c states are reconstructed through the radiative decay χ c → J/ψγ (with J/ψ → μ + μ −) where photons are reconstructed from γ → e + e − conversions. The production rate of the χ c2 state relative to the χ c1 state is measured for prompt and non-prompt χ c as a function of J/ψ transverse momentum....

9. mediation: R package for causal mediation analysis - Tingley, Dustin; Yamamoto, Teppei; Hirose, Kentaro; Keele, Luke; Imai, Kosuke
In this paper, we describe the R package mediation for conducting causal mediation analysis in applied empirical research. In many scientific disciplines, the goal of researchers is not only estimating causal effects of a treatment but also understanding the process in which the treatment causally affects the outcome. Causal mediation analysis is frequently used to assess potential causal mechanisms. The mediation package implements a comprehensive suite of statistical tools for conducting such an analysis. The package is organized into two distinct approaches. Using the model-based approach, researchers can estimate causal mediation effects and conduct sensitivity analysis under the standard research...

10. The maximal length of a k-separator permutation - Gunby, Benjamin P.
A permutation σ ∈ S[subscript n] is a k-separator if all of its patterns of length k are distinct. Let F(k) denote the maximal length of a k-separator. Hegarty (2013) showed that k + ⌊√2k − 1⌋ − 1 ≤ F(k) ≤ k + ⌊√2k − 3⌋, and conjectured that F(k) = k + ⌊√2k − 1⌋ − 1. This paper will strengthen the upper bound to prove the conjecture for all sufficiently large k (in particular, for all k ≥ 320801).

11. Bounding sequence extremal functions with formations - Geneson, Jesse; Prasad, Rohil; Tidor, Jonathan
An (r,s)-formation is a concatenation of s permutations of r letters. If u is a sequence with r distinct letters, then let Ex(u,n) be the maximum length of any r-sparse sequence with n distinct letters which has no subsequence isomorphic to u. For every sequence u define fw(u), the formation width of u, to be the minimum s for which there exists r such that there is a subsequence isomorphic to u in every (r,s)-formation. We use fw(u) to prove upper bounds on Ex(u,n) for sequences u such that u contains an alternation with the same formation width as u. We...

12. Measurement of prompt J/ψ pair production in pp collisions at s √s = 7 Tev - Apyan, Aram; Barbieri, Richard Alexander; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, Valentina; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Yen-Jie; Levin, A.; Luckey, P. David; Ma, Teng; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stoeckli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Zanetti, Marco; Zhukova, Victoria
Production of prompt J/ψ meson pairs in proton-proton collisions at s√ = 7 TeV is measured with the CMS experiment at the LHC in a data sample corresponding to an integrated luminosity of about 4.7 fb[superscript −1]. The two J/ψ mesons are fully reconstructed via their decays into μ + μ − pairs. This observation provides for the first time access to the high-transverse-momentum region of J/ψ pair production where model predictions are not yet established. The total and differential cross sections are measured in a phase space defined by the individual J/ψ transverse momentum (p[subscript T][superscript J/ψ] ) and...

13. Artificial Tribotactic Microscopic Walkers: Walking Based on Friction Gradients - Steimel, Joshua P.; Alexander-Katz, Alfredo; Aragones Gomez, Juan Luis
Friction, the resistive force between two surfaces sliding past each other, is at the core of a wide diversity of locomotion schemes. While such schemes are well described for homogeneous environments, locomotion based on friction in inhomogeneous environments has not received much attention. Here we introduce and demonstrate the concept of tribotaxis, a motion that is guided by gradients in the friction coefficient. Our system is composed of microwalkers that undergo an effective frictional interaction with biological receptors on the substrate, which is regulated by the density of such receptors. When actuated stochastically, microwalkers migrate to regions of higher friction,...

14. Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead–lead collisions at √sNN=2.76 = 2.76 TeV with the ATLAS detector - Taylor, Frank E.
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at sNN√=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v[subscript 2], was measured in the pseudorapidity range |η|≤2.5 with the event-plane method. In order to include tracks with very low transverse momentum p[subscript T] , thus reducing the uncertainty in v[subscript 2] integrated over p[subscript T] , a 1 μb[superscript −1] data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v[subscript 2] is compared to other measurements...

15. Protecting Circuits from Computationally Bounded and Noisy Leakage - Faust, Sebastian; Rabin, Tal; Reyzin, Leonid; Tromer, Eran; Vaikuntanathan, Vinod
Physical computational devices leak side-channel information that may, and often does, reveal secret internal states. We present a general transformation that compiles any circuit into a circuit with the same functionality but resilience against well-defined classes of leakage. Our construction requires a small, stateless, and computation-independent leak-proof component that draws random elements from a fixed distribution. In essence, we reduce the problem of shielding arbitrarily complex circuits to the problem of shielding a single, simple component. Our approach is based on modeling the adversary as a powerful observer that inspects the device via a limited measurement apparatus. We allow the...

16. Modular Synthesis of Sketches Using Models - Singh, Rohit; Singh, Rishabh; Xu, Zhilei; Krosnick, Rebecca P.; Solar Lezama, Armando
One problem with the constraint-based approaches to synthesis that have become popular over the last few years is that they only scale to relatively small routines, on the order of a few dozen lines of code. This paper presents a mechanism for modular reasoning that allows us to break larger synthesis problems into small manageable pieces. The approach builds on previous work in the verification community of using high-level specifications and partially interpreted functions (we call them models) in place of more complex pieces of code in order to make the analysis modular. The main contribution of this paper is to...

17. Are lock-free concurrent algorithms practically wait-free? - Alistarh, Dan; Censor-Hillel, Keren; Shavit, Nir N.
Lock-free concurrent algorithms guarantee that some concurrent operation will always make progress in a finite number of steps. Yet programmers prefer to treat concurrent code as if it were wait-free, guaranteeing that all operations always make progress. Unfortunately, designing wait-free algorithms is generally a very complex task, and the resulting algorithms are not always efficient. While obtaining efficient wait-free algorithms has been a long-time goal for the theory community, most non-blocking commercial code is only lock-free. This paper suggests a simple solution to this problem. We show that, for a large class of lock-free algorithms, under scheduling conditions which approximate those...

18. Development of siRNA-probes for studying intracellular trafficking of siRNA nanoparticles - Alabi, Christopher A.; Sahay, Gaurav; Langer, Robert; Anderson, Daniel Griffith
One important barrier facing the delivery of short interfering RNAs (siRNAs) via synthetic nanoparticles is the rate of nanoparticle disassembly. However, our ability to optimize the release kinetics of siRNAs from nanoparticles for maximum efficacy is limited by the lack of methods to track their intracellular disassembly. Towards this end, we describe the design of two different siRNA-based fluorescent probes whose fluorescence emission changes in response to the assembly state of the nanoparticle. The first probe design involves a redox-sensitive fluorescence-quenched probe that fluoresces only when the nanoparticle is disassembled in a reductive environment. The second probe design is based...

19. Modelling human embryoid body cell adhesion to a combinatorial library of polymer surfaces - Epa, V. Chandana; Yang, Jing; Mei, Ying; Hook, Andrew L.; Langer, Robert; Davies, Martyn C.; Alexander, Morgan R.; Winkler, David A.; Anderson, Daniel Griffith
Designing materials to control biology is an intense focus of biomaterials and regenerative medicine research. Discovering and designing materials with appropriate biological compatibility or active control of cells and tissues is being increasingly undertaken using high throughput synthesis and assessment methods. We report a relatively simple but powerful machine-learning method of generating models that link microscopic or molecular properties of polymers or other materials to their biological effects. We illustrate the potential of these methods by developing the first robust, predictive, quantitative, and purely computational models of adhesion of human embryonic stem cell embryoid bodies (hEB) to the surfaces of...

20. Combinatorial discovery of polymers resistant to bacterial attachment - Hook, Andrew L; Chang, Chien-Yi; Yang, Jing; Luckett, Jeni; Cockayne, Alan; Atkinson, Steve; Mei, Ying; Bayston, Roger; Irvine, Derek J; Langer, Robert; Williams, Paul; Davies, Martyn C; Alexander, Morgan R; Anderson, Daniel Griffith
Bacterial attachment and subsequent biofilm formation pose key challenges to the optimal performance of medical devices. In this study, we determined the attachment of selected bacterial species to hundreds of polymeric materials in a high-throughput microarray format. Using this method, we identified a group of structurally related materials comprising ester and cyclic hydrocarbon moieties that substantially reduced the attachment of pathogenic bacteria (Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli). Coating silicone with these 'hit' materials achieved up to a 30-fold (96.7%) reduction in the surface area covered by bacteria compared with a commercial silver hydrogel coating in vitro, and the...

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