## Recursos de colección

#### Caltech Authors (140.879 recursos)

Repository of works by Caltech published authors.

Type = Report or Paper

1. #### Randomized Single-View Algorithms for Low-Rank Matrix Approximation

Tropp, Joel A.; Yurtsever, Alp; Udell, Madeleine; Cevher, Volkan
This paper develops a suite of algorithms for constructing low-rank approximations of an input matrix from a random linear image of the matrix, called a sketch. These methods can preserve structural properties of the input matrix, such as positive-semidefiniteness, and they can produce approximations with a user-specified rank. The algorithms are simple, accurate, numerically stable, and provably correct. Moreover, each method is accompanied by an informative error bound that allows users to select parameters a priori to achieve a given approximation quality. These claims are supported by computer experiments.

2. #### The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9

Bahramian, Arash; Heinke, Craig O.; Tudor, Vlad; Miller-Jones, James C. A.; Bogdanov, Slavko; Maccarone, Thomas J.; Knigge, Christian; Sivakoff, Gregory R.; Chomiuk, Laura; Strader, Jay; Garcia, Javier A.; Kallman, Timothy
47 Tuc X9 is a low mass X-ray binary (LMXB) in the globular cluster 47 Tucanae, and was previously thought to be a cataclysmic variable. However, Miller-Jones et al. (2015) recently identified a radio counterpart to X9 (inferring a radio/X-ray luminosity ratio consistent with black hole LMXBs), and suggested that the donor star might be a white dwarf. We report simultaneous observations of X9 performed by Chandra, Nustar and Australia Telescope Compact Array. We find a clear 28.18$\pm$0.02 min periodic modulation in the Chandra data, which we identify as the orbital period, confirming this system as an ultracompact X-ray binary. Our X-ray spectral fitting provides evidence for photoionized gas...

3. #### Exploring the nearly degenerate stop region with sbottom decays

An, Haipeng; Gu, Jiayin; Wang, Lian-Tao
A light stop with mass almost degenerate with the lightest neutralino has important connections with both naturalness and dark matter relic abundance. This region is also very hard to probe at colliders. In this paper, we demonstrate the potential of searching for such stop particles at the LHC from sbottom decays, focusing on two channels with final states 2ℓ + E^(miss)_T and 1bℓ + E^(miss)_T. We found that, if the lightest sbottom has mass around or below 1TeV and has a significant branching ratio to decay to stop and W (b → tW), a stop almost degenerate with neutralino can...

4. #### Vacuum Requirements for the Cascade Synchrotron

Peterson, Vincent Z.
This report is an examination of the principal characteristics of a vacuum system for the proposed 300 Bev cascade synchrotron 1). In par- ticular, we wish to see whether the small aperture and large circumference of the Main Ring present vacuum difficulties. Gas scattering losses, eddy currents in the vacuum chamber wall, and cost of a 5-mile vacuum system are of special interest. Attention has been directed primarily toward the Main Ring. The 10 Gev Booster Ring 1) has vacuum requirements similar to existing accelerators, and it is presumed that duplication or modification of such vacuum systems will be practical. Our...

5. #### Why Boltzmann Brains Are Bad

Carroll, Sean M.
Some modern cosmological models predict the appearance of Boltzmann Brains: observers who randomly fluctuate out of a thermal bath rather than naturally evolving from a low-entropy Big Bang. A theory in which most observers are of the Boltzmann Brain type is generally thought to be unacceptable, although opinions differ. I argue that such theories are indeed unacceptable: the real problem is with fluctuations into observers who are locally identical to ordinary observers, and their existence cannot be swept under the rug by a choice of probability distributions over observers. The issue is not that the existence of such observers is ruled out by data, but that the theories...

6. #### Bootstrapping the Spectral Function: On the Uniqueness of Liouville and the Universality of BTZ

Collier, Scott; Kravchuk, Petr; Lin, Ying-Hsuan; Yin, Xi
We introduce spectral functions that capture the distribution of OPE coefficients and density of states in two-dimensional conformal field theories, and show that nontrivial upper and lower bounds on the spectral function can be obtained from semidefinite programming. We find substantial numerical evidence indicating that OPEs involving only scalar Virasoro primaries in a c>1 CFT are necessarily governed by the structure constants of Liouville theory. Combining this with analytic results in modular bootstrap, we conjecture that Liouville theory is the unique unitary c>1 CFT whose primaries have bounded spins. We also use the spectral function method to study modular constraints on CFT spectra, and discuss some implications of our...

7. #### Axion detection via Topological Casimir Effect

Cao, ChunJun; Zhitnitsky, Ariel R.
We propose a new table-top experimental configuration for the direct detection of dark matter axions with mass in the (10^(−6) eV − 10^(−2)eV) range using non-perturbative effects in a system with non-trivial spatial topology. Different from most experimental setups found in literature on direct dark matter axion detection, which relies on ˙θ or ∇~ θ, we found that our system is in principle sensitive to a static θ ≥ 10^(−14) and can also be used to set limit on the fundamental constant θ_(QED) which becomes the fundamental observable parameter of the Maxwell system if some conditions are met. Connection with Witten effect when the induced electric charge...

8. #### Confined Dense Circumstellar Material Surrounding a Regular Type II Supernova: The Unique Flash-Spectroscopy Event - SN 2013fs

Yaron, O.; Perley, D. A.; Gal-Yam, A.; Groh, J. H.; Horesh, A.; Ofek, E. O.; Kulkarni, S. R.; Sollerman, J.; Fransson, C.; Rubin, A.; Szabo, P.; Sapir, N.; Taddia, F.; Cenko, S. B.; Valenti, S.; Arcavi, I.; Howell, D. A.; Kasliwal, M. M.; Vreeswijk, P. M.; Khazov, D.; Fox, O. D.; Cao, Y.; Gnat, O.; Kelly, P. L.; Nugent, P. E.; Filippenko, A. V.; Laher, R. R.; Wozniak, P. R.; Lee, W. H.; Rebbapragada, U. D.; Maguire, K.; Sullivan, M.; Soumagnac, M. T.
With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, that sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF13dqy = SN 2013fs, a mere ~3 hr after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ~6 hr post-explosion) spectra, map the distribution of material in the immediate environment (≾ 10^(15) cm) of the...

9. #### Active Faulting in Northern Turkey

Allen, Clarence R.
The North Anatolian fault zone of Turkey has become widely publicized in recent years because of the remarkable series of earthquakes that began along it in 1939 -- most of which have been associated with dextral surface displacements that have successively delineated the fault trace from east to west (Ketin and Roesli, 1953; Ambraseys and Zátopek, 1968). It is not so generally recognized that even prior to 1939 the fault zone could easily have been recognized on the basis of abundant and through-going features of Quaternary displacements, and that the North Anatolian fault is almost completely analogous to the better-known active transcurrent faults of the circum-Pacific region, such as...

10. #### A Distributed Amplifier Using Transistors

A distributed amplifier with a stable gain of 10, risetime of 2.5 nanoseconds for 125 ohm load impedance is described. The maximum output voltage is 3.2 volts with negative polarity. The amplifier consists of 2 stages of 6 Philco 2N1742 transistors each and an emitter follower using a 2N1500. Design formulae are derived and detailed performance of a specific amplifier is given.

11. #### Effects of magnet non-linearities on betatron oscillation frequencies for the 300 bev proton synchrotron

Matthews, Jon
The purpose of this note is to study the effects on betatron frequencies -- in particular, their variation with particle energy — due to non-linear magnet fields. The idea is to determine whether non- linearities can be introduced deliberately in such a way that ν, the number of betatron wavelengths in the circumference, is independent of particle energy for energies near the equilibrium energy. The result is that non-linearities of a magnitude such that the field index n varies by about 1 per cent per cm will suffice.

12. #### Automated construction of molecular active spaces from atomic valence orbitals

Sayfutyarova, Elvira R.; Sun, Qiming; Chan, Garnet K.-L.; Knizia, Gerald
We introduce the atomic valence active space (AVAS), a simple and well-defined automated technique for constructing active orbital spaces for use in multi-configuration and multi-reference (MR) electronic structure calculations. Concretely, the technique constructs active molecular orbitals capable of describing all relevant electronic configurations emerging from a targeted set of atomic valence orbitals (e.g., the metal d orbitals in a complex). This is achieved via a linear transformation of the occupied and unoccupied orbital spaces from an easily obtainable single-reference wavefunction (such as from a Hartree-Fock or Kohn-Sham calculations) based on projectors to targeted atomic valence orbitals. We discuss the background,...

13. #### The Python-based Simulations of Chemistry Framework (PySCF)

Sun, Qiming; Berkelbach, Timothy C.; Blunt, Nick S.; Booth, George H.; Guo, Sheng; Li, Zhendong; Liu, Junzi; McClain, James; Sharma, Sandeep; Wouters, Sebastian; Chan, Garnet Kin-Lic
PySCF is a general-purpose electronic structure platform designed from the ground up to emphasize code simplicity, both to aid new method development, as well as for flexibility in computational workflow. The package provides a wide range of tools to support simulations of finite size systems, extended systems with periodic boundary conditions, low dimensional periodic systems, and custom Hamiltonians, using mean-field and post-mean-field methods with standard Gaussian basis functions. To ensure easy of extensibility, PySCF uses the Python language to implement almost all its features, while computationally critical paths are implemented with heavily optimized C routines. Using this combined Python/C implementation,...

14. #### Twisted gauge theories in 3D Walker-Wang models

Wang, Zitao; Chen, Xie
Three dimensional gauge theories with a discrete gauge group can emerge from spin models as a gapped topological phase with fractional point excitations (gauge charge) and loop excitations (gauge flux). It is known that 3D gauge theories can be "twisted", in the sense that the gauge flux loops can have nontrivial braiding statistics among themselves and such twisted gauge theories are realized in models discovered by Dijkgraaf and Witten. A different framework to systematically construct three dimensional topological phases was proposed by Walker and Wang and a series of examples have been studied. Can the Walker Wang construction be used...

15. #### SMC X-3: the closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field

Tsygankov, S. S.; Doroshenko, V.; Lutovinov, A. A.; Mushtukov, A. A.; Poutanen, J.
Magnetic field of accreting neutron stars determines their overall behaviour including the maximum possible luminosity. Some models require an above-average magnetic field strength (> 10^13 G) in order to explain super-Eddington mass accretion rate in the recently discovered class of pulsating ultraluminous X-ray sources (ULX). The peak luminosity of SMC X-3 during its major outburst in 2016-2017 reached ~2.5x10^39 erg/s comparable to that in ULXs thus making this source the nearest ULX-pulsar. SMC X-3 belongs to the class of transient X-ray pulsars with Be optical companions, and exhibited a giant outburst in July 2016 - February 2017. The source has been observed during the entire outburst with the Swift/XRT...

16. #### Novel computational method for predicting polytherapy switching strategies to overcome tumor heterogeneity and evolution

Jonsson, Vanessa D.; Blakely, Collin M.; Lin, Luping; Asthana, Saurabh; Olivas, Victor; Gubens, Matthew A.; Matni, Nikolai; Bastian, Boris C.; Taylor, Barry S.; Doyle, John C.; Bivona, Trever G.
The success of targeted cancer therapy is limited by drug resistance that can result from tumor genetic heterogeneity. The current approach to address resistance typically involves initiating a new treatment after clinical/radiographic disease progression, ultimately resulting in futility in most patients. Towards a potential alternative solution, we developed a novel computational framework that uses human cancer profiling data to systematically identify dynamic, pre-emptive, and sometimes non-intuitive treatment strategies that can better control tumors in real-time. By studying lung adenocarcinoma clinical specimens and preclinical models, our computational analyses revealed that the best anti-cancer strategies addressed existing resistant subpopulations as they emerged...

17. #### Transcriptomic Description of an Endogenous Female State in C. elegans

Angeles-Albores, David; Leighton, Daniel H. W.; Tsou, Tiffany; Khaw, Tiffany H.; Antoshechkin, Igor; Sternberg, Paul W.
Understanding genome and gene function in a whole organism requires us to fully comprehend the life cycle and the physiology of the organism in question. Although C. elegans is traditionally though of as a hermaphrodite, XX animals exhaust their sperm and become (endogenous) females after 3 days of egg-laying. The molecular physiology of this state has not been studied as intensely as other parts of the life cycle, in spite of documented changes in behavior and metabolism that occur at this stage. To study the female state of C. elegans, we designed an experiment to measure the transcriptomes of 1st...

18. #### A Bayesian approach to inferring chemical signal timing and amplitude in a temporal logic gate using the cell population distributional response

Baetica, Ania A.; Catanach, Thomas A.; Hsiao, Victoria; Murray, Richard M.; Beck, James L.
Stochastic gene expression poses an important challenge for engineering robust behaviors in a heterogeneous cell population. Cells address this challenge by operating on distributions of cellular responses generated by noisy processes. Similarly, a previously published temporal logic gate considers the distribution of responses across a cell population under chemical inducer pulsing events. The design uses a system of two integrases to engineer an E. coli strain with four DNA states that records the temporal order of two chemical signal events. The heterogeneous cell population response was used to infer the timing and duration of the two chemical signals for a...

19. #### Embedding theory for excited states with inclusion of self-consistent environment screening

Lischner, Johannes; Zgid, Dominika; Chan, Garnet Kin-Lic; Arias, T. A.
We present a general embedding theory of electronic excitations of a relatively small, localized system in contact with an extended, chemically complex environment. We demonstrate how to include the screening response of the environment into highly accurate electronic structure calculation of the localized system by means of an effective interaction between the electrons, which contains only screening processes occurring in the environment. For the common case of a localized system which constitutes an inhomogeneity in an otherwise homogeneous system, such as a defect in a crystal, we show how matrix elements of the environment-screened interaction can be calculated from density-functional...

20. #### An architecture for efficient multimodal gravitational wave parameter estimation with linear surrogate models

O'Shaughnessy, Richard; Blackman, Jonathan; Field, Scott E.
The recent direct detection of gravitational waves has further emphasized the need for fast, low-cost, and accurate methods to infer the parameters of gravitational wave sources. The performance of these calculations is limited by the cost of evaluating the likelihood function, due to expense in data handling and waveform generation. Building on recently developed surrogate models and a novel parameter estimation pipeline, we show how to quickly generate the likelihood function as a simple, analytic closed-form expression. Using a simple variant of a production-scale parameter estimation code, we demonstrate our method using surrogate models of effective-one-body and numerical relativity waveforms. Our study is the first time these models have...

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