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Search Results: 1 - 10 of 462959 matches for " Hamilton A "
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The ATLAS Trigger System Commissioning and Performance
A. Hamilton
Physics , 2010,
Abstract: The ATLAS trigger has been used very successfully to collect collision data during 2009 and 2010 LHC running at centre of mass energies of 900 GeV, 2.36 TeV, and 7 TeV. This paper presents the ongoing work to commission the ATLAS trigger with proton collisions, including an overview of the performance of the trigger based on extensive online running. We describe how the trigger has evolved with increasing LHC luminosity and give a brief overview of plans for forthcoming LHC running.
Variations in abundance: genome-wide responses to genetic variation... and vice versa
Bruce A Hamilton
Genome Biology , 2002, DOI: 10.1186/gb-2002-3-10-reviews1029
Abstract: Interpreting the functional significance of genetic polymorphisms in natural populations poses a major challenge. Here I review recent work in yeast, flies, mice and primates that examines the influences of naturally occurring sequence variation, chromosomal order and speciation on genome-wide expression profiles of both RNA and protein. A synthetic view from these experiments would suggest that gene expression is not randomly distributed along chromosomes, that variations in mRNA and protein expression within a single species result from a surprising balance between polymorphisms acting in cis and polymorphisms acting in trans to the regulated gene, and consequently that relatively few adaptive changes could have major impacts in remodeling gene expression patterns over the course of evolution.Sequence polymorphism and genome-wide haplotype maps have begun to catalog the extent and structure of genetic variation present in human populations [1,2] and to a lesser extent inbred strains of model organisms [3,4,5,6]. An emerging theme from such maps is that linked polymorphisms tend to travel through a population together, creating haplotype blocks [2,7], by virtue of either recombination hotspots or population expansions of chromosomes carrying ancestral recombination events. Recent reports have also documented variation in gene expression profiles between individuals and, more importantly, reproducible variations between inbred strains [8]. However, assigning functional significance to individual polymorphisms at the level of either sequence or expression is a different problem. A series of recent papers has taken a complementary approach by examining the sources of variation in expression profiles between strains of model organisms.The availability of nearly complete genome sequences and large sets of gene expression data has led several groups to consider whether gene expression profiles are structured by chromosomal order [9,10,11,12,13]. Individual gene clusters
Towards Optimal Measurement of Power Spectra II: A Basis of Positive, Compact, Statistically Orthogonal Kernels
A. J. S. Hamilton
Physics , 1997, DOI: 10.1093/mnras/289.2.295
Abstract: This is the second of two papers which address the problem of measuring the unredshifted power spectrum of fluctuations from a galaxy survey in optimal fashion. A key quantity is the Fisher matrix, which is the inverse of the covariance matrix of minimum variance estimators of the power spectrum of the survey. It is shown that bases of kernels which give rise to complete sets of statistically orthogonal windowed power spectra are obtained in general from the eigenfunctions of the Fisher matrix scaled by some arbitrary positive definite scaling matrix. Among the many possible bases of kernels, there is a basis, obtained by applying an infinitely steep scaling function, which leads to kernels which are positive and compact in Fourier space. This basis of kernels, along with the associated minimum variance pair weighting derived in the previous paper, would appear to offer a solution to the problem of how to measure the unredshifted power spectrum optimally. Illustrative kernels are presented for the case of the PSCz survey.
Towards Optimal Measurement of Power Spectra I: Minimum Variance Pair Weighting and the Fisher Matrix
A. J. S. Hamilton
Physics , 1997, DOI: 10.1093/mnras/289.2.285
Abstract: This is the first of a pair of papers which address the problem of measuring the unredshifted power spectrum in optimal fashion from a survey of galaxies, with arbitrary geometry, for Gaussian or non-Gaussian fluctuations, in real or redshift space. In this first paper, that pair weighting is derived which formally minimizes the expected variance of the unredshifted power spectrum windowed over some arbitrary kernel. The inverse of the covariance matrix of minimum variance estimators of windowed power spectra is the Fisher information matrix, which plays a central role in establishing optimal estimators. Actually computing the minimum variance pair window and the Fisher matrix in a real survey still presents a formidable numerical problem, so here a perturbation series solution is developed. The properties of the Fisher matrix evaluated according to the approximate method suggested here are investigated in more detail in the second paper.
Redshift Distortions and Omega in IRAS Surveys
A. J. S. Hamilton
Physics , 1995,
Abstract: Redshift space distortions on large scales can be used to measure the linear growth rate parameter $\ff \approx \Omega^{0.6}/b$. I report here measurements of such distortions in the IRAS 2 Jy, 1.2 Jy, and QDOT redshift surveys, finding $\ff = 0.69^{+ .21}_{- .19}$ from a merged QDOT plus 1.2 Jy catalogue. Unfortunately, confidence in this result is undermined by a marked ($4\sigma$) change in the pattern of clustering in QDOT beyond about $80 h^{-1} Mpc$. A similar effect may be present at a mild level in the 1.2 Jy survey. The effect may be caused by systematic variation in the effective flux limit of the IRAS PSC over the sky, with a dispersion of $\sim 0.1$ Jy on scales $\sim 7^{\circ}$. If so, then the value of $\ff$ inferred from redshift distortions in IRAS surveys may be systematically underestimated.
Formulae for Growth Factors In Expanding Universes Containing Matter and a Cosmological Constant
A. J. S. Hamilton
Physics , 2000, DOI: 10.1046/j.1365-8711.2001.04137.x
Abstract: Formulae are presented for the linear growth factor D/a and its logarithmic derivative dlnD/dlna in expanding Friedmann-Robertson-Walker Universes with arbitrary matter and vacuum densities. The formulae permit rapid and stable numerical evaluation. A fortran program is available at http://casa.colorado.edu/~ajsh/growl/ .
Linear Redshift Distortions: A Review
A. J. S. Hamilton
Physics , 1997,
Abstract: Redshift maps of galaxies in the Universe are distorted by the peculiar velocities of galaxies along the line of sight. The amplitude of the distortions on large, linear scales yields a measurement of the linear redshift distortion parameter, which is $\beta \approx \Omega_0^{0.6}/b$ in standard cosmology with cosmological density $\Omega_0$ and light-to-mass bias $b$. All measurements of $\beta$ from linear redshift distortions published up to mid 1997 are reviewed. The average and standard deviation of the reported values is $\beta_{optical} = 0.52 \pm 0.26$ for optically selected galaxies, and $\beta_{IRAS} = 0.77 \pm 0.22$ for IRAS selected galaxies. The implied relative bias is $b_{optical}/b_{IRAS} \approx 1.5$. If optical galaxies are unbiased, then $\Omega_0 = 0.33^{+0.32}_{-0.22}$, while if IRAS galaxies are unbiased, then $\Omega_0 = 0.63^{+0.35}_{-0.27}$.
The Black Hole Particle Accelerator as a Machine to make Baby Universes
A. J. S. Hamilton
Physics , 2013,
Abstract: General relativity predicts that the inner horizon of an astronomically realistic rotating black hole is subject to the mass inflation instability. The inflationary instability acts like a gravity-powered particle accelerator of extraordinary power, accelerating accreted streams of particles along the principal outgoing and ingoing null directions at the inner horizon to collision energies that would, if nothing intervened, typically exceed exponentially the Planck energy. The inflationary instability is fueled by ongoing accretion, and is occurring inevitably in essentially every black hole in our Universe. This extravagant machine, the Black Hole Particle Accelerator, has the hallmarks of a device to make baby universes. Since collisions are most numerous inside supermassive black holes, reproductive efficiency requires our Universe to make supermassive black holes efficiently, as is observed.
Uncorrelated Modes of the Nonlinear Power Spectrum
A. J. S. Hamilton
Physics , 1999, DOI: 10.1046/j.1365-8711.2000.03071.x
Abstract: Nonlinear evolution causes the galaxy power spectrum to become broadly correlated over different wavenumbers. It is shown that prewhitening the power spectrum - transforming the power spectrum in such a way that the noise covariance becomes proportional to the unit matrix - greatly narrows the covariance of power. The eigenfunctions of the covariance of the prewhitened nonlinear power spectrum provide a set of almost uncorrelated nonlinear modes somewhat analogous to the Fourier modes of the power spectrum itself in the linear, Gaussian regime. These almost uncorrelated modes make it possible to construct a near minimum variance estimator and Fisher matrix of the prewhitened nonlinear power spectrum analogous to the Feldman-Kaiser-Peacock estimator of the linear power spectrum. The paper concludes with summary recipes, in gourmet, fine, and fastfood versions, of how to measure the prewhitened nonlinear power spectrum from a galaxy survey in the FKP approximation. An Appendix presents FFTLog, a code for taking the fast Fourier or Hankel transform of a periodic sequence of logarithmically spaced points, which proves useful in some of the manipulations.
Perturbation theory of spherically symmetric self-similar black holes
A. J. S. Hamilton
Physics , 2007,
Abstract: The theory of perturbations of spherically symmetric self-similar black holes is presented, in the Newman-Penrose formalism. It is shown that the wave equations for gravitational, electromagnetic, and scalar waves are separable, though not decoupled. A generalization of the Teukolsky equation is given. Monopole and dipole modes are treated. The Newman-Penrose wave equations governing polar and axial spin-0 perturbations are explored.
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