7,273 research outputs found

    W=0 Pairing in (N,N)(N,N) Carbon Nanotubes away from Half Filling

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    We use the Hubbard Hamiltonian HH on the honeycomb lattice to represent the valence bands of carbon single-wall (N,N)(N,N) nanotubes. A detailed symmetry analysis shows that the model allows W=0 pairs which we define as two-body singlet eigenstates of HH with vanishing on-site repulsion. By means of a non-perturbative canonical transformation we calculate the effective interaction between the electrons of a W=0 pair added to the interacting ground state. We show that the dressed W=0 pair is a bound state for resonable parameter values away from half filling. Exact diagonalization results for the (1,1) nanotube confirm the expectations. For (N,N)(N,N) nanotubes of length ll, the binding energy of the pair depends strongly on the filling and decreases towards a small but nonzero value as l→∞l \to \infty. We observe the existence of an optimal doping when the number of electrons per C atom is in the range 1.2÷\div1.3, and the binding energy is of the order of 0.1 ÷\div 1 meV.Comment: 16 pages, 6 figure

    Screening Experiments for Simulation: A Review

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    This article reviews so-called screening in simulation; i.e., it examines the search for the really important factors in experiments with simulation models that have very many factors (or inputs). The article focuses on a most efficient and effec- tive screening method, namely Sequential Bifurcation. It ends with a discussion of possible topics for future research, and forty references for further study.Screening;Metamodel;Response Surface;Design

    Certifying and removing disparate impact

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    What does it mean for an algorithm to be biased? In U.S. law, unintentional bias is encoded via disparate impact, which occurs when a selection process has widely different outcomes for different groups, even as it appears to be neutral. This legal determination hinges on a definition of a protected class (ethnicity, gender, religious practice) and an explicit description of the process. When the process is implemented using computers, determining disparate impact (and hence bias) is harder. It might not be possible to disclose the process. In addition, even if the process is open, it might be hard to elucidate in a legal setting how the algorithm makes its decisions. Instead of requiring access to the algorithm, we propose making inferences based on the data the algorithm uses. We make four contributions to this problem. First, we link the legal notion of disparate impact to a measure of classification accuracy that while known, has received relatively little attention. Second, we propose a test for disparate impact based on analyzing the information leakage of the protected class from the other data attributes. Third, we describe methods by which data might be made unbiased. Finally, we present empirical evidence supporting the effectiveness of our test for disparate impact and our approach for both masking bias and preserving relevant information in the data. Interestingly, our approach resembles some actual selection practices that have recently received legal scrutiny.Comment: Extended version of paper accepted at 2015 ACM SIGKDD Conference on Knowledge Discovery and Data Minin

    Search for technipions in exclusive production of diphotons with large invariant masses at the LHC

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    We focus on exclusive production of neutral technipion π~0\tilde \pi^0 in pppp collisions at the LHC, i.e. on pp→ppπ~0p p \to p p \tilde \pi^0 reaction. The dependence of the cross section on parameters of recently proposed vector-like Technicolor model is studied. Characteristic features of the differential distributions are discussed. For not too large technipion masses the diphoton decay channel has the dominant branching fraction. This is also the main reason for an enhanced production of neutral technipions in γγ\gamma\gamma-fusion reaction. We discuss potential backgrounds of the QCD and QED origin to the pp→pp(π~0→γγ)p p \to p p (\tilde{\pi}^0 \to \gamma \gamma) process at large invariant γγ\gamma\gamma masses. We conclude that compared to inclusive case the signal-to-background ratio in the considered exclusive reaction is vary favorable which thereby could serve as a good probe for Technicolor dynamics searches at the LHC.Comment: 20 pages, 12 figures, 1 tabl

    Simulating Three-Dimensional Hydrodynamics on a Cellular-Automata Machine

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    We demonstrate how three-dimensional fluid flow simulations can be carried out on the Cellular Automata Machine 8 (CAM-8), a special-purpose computer for cellular-automata computations. The principal algorithmic innovation is the use of a lattice-gas model with a 16-bit collision operator that is specially adapted to the machine architecture. It is shown how the collision rules can be optimized to obtain a low viscosity of the fluid. Predictions of the viscosity based on a Boltzmann approximation agree well with measurements of the viscosity made on CAM-8. Several test simulations of flows in simple geometries -- channels, pipes, and a cubic array of spheres -- are carried out. Measurements of average flux in these geometries compare well with theoretical predictions.Comment: 19 pages, REVTeX and epsf macros require

    The spontaneous Z2\mathbb{Z}_2 breaking Twin Higgs

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    The Twin Higgs model seeks to address the little hierarchy problem by making the Higgs a pseudo-Goldstone of a global SU(4)SU(4) symmetry that is spontaneously broken to SU(3)SU(3). Gauge and Yukawa couplings, which explicitly break SU(4)SU(4), enjoy a discrete Z2\mathbb{Z}_2 symmetry that accidentally maintains SU(4)SU(4) at the quadratic level and therefore keeps the Higgs light. Contrary to most beyond the Standard Model theories, the quadratically divergent corrections to the Higgs mass are cancelled by a mirror sector, which is uncharged under the Standard Model groups. However, the Twin Higgs with an exact Z2\mathbb{Z}_2 symmetry leads to equal vevs in the Standard Model and mirror sectors, which is phenomenologically unviable. An explicit Z2\mathbb{Z}_2 breaking potential must then be introduced and tuned against the SU(4)SU(4) breaking terms to produce a hierarchy of vevs between the two sectors. This leads to a moderate but non-negligible tuning. We propose a model to alleviate this tuning, without the need for an explicit Z2\mathbb{Z}_2 breaking sector. The model consists of two SU(4)SU(4) fundamental Higgses, one whose vacuum preserves Z2\mathbb{Z}_2 and one whose vacuum breaks it. As the interactions between the two Higgses are turned on, the Z2\mathbb{Z}_2 breaking is transmitted from the broken to the unbroken sector and a small hierarchy of vevs is naturally produced. The presence of an effective tadpole and feedback between the two Higgses lead to a sizable improvement of the tuning. The resulting Higgs boson is naturally very Standard Model like.Comment: 17 pages, 6 figures, references update
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