4,567 research outputs found
Studies of Stability and Robustness for Artificial Neural Networks and Boosted Decision Trees
In this paper, we compare the performance, stability and robustness of
Artificial Neural Networks (ANN) and Boosted Decision Trees (BDT) using
MiniBooNE Monte Carlo samples. These methods attempt to classify events given a
number of identification variables. The BDT algorithm has been discussed by us
in previous publications. Testing is done in this paper by smearing and
shifting the input variables of testing samples. Based on these studies, BDT
has better particle identification performance than ANN. The degradation of the
classifications obtained by shifting or smearing variables of testing results
is smaller for BDT than for ANN.Comment: 23 pages, 13 figure
Spiral Disk Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers
Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion
of carbon--oxygen white dwarfs, and serve as standardizable cosmological
candles. However, despite their importance, the nature of the progenitor
systems that give rise to SNe Ia has not been hitherto elucidated.
Observational evidence favors the double-degenerate channel in which merging
white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies
exist between observations and theory, and to date, there has been no
self-consistent merger model that yields a SNe Ia. Here we show that a spiral
mode instability in the accretion disk formed during a binary white dwarf
merger leads to a detonation on a dynamical timescale. This mechanism sheds
light on how white dwarf mergers may frequently yield SNe Ia.Comment: Final version (as in ApJL) with minor edit
Boosted Decision Trees as an Alternative to Artificial Neural Networks for Particle Identification
The efficacy of particle identification is compared using artificial neutral
networks and boosted decision trees. The comparison is performed in the context
of the MiniBooNE, an experiment at Fermilab searching for neutrino
oscillations. Based on studies of Monte Carlo samples of simulated data,
particle identification with boosting algorithms has better performance than
that with artificial neural networks for the MiniBooNE experiment. Although the
tests in this paper were for one experiment, it is expected that boosting
algorithms will find wide application in physics.Comment: 6 pages, 5 figures; Accepted for publication in Nucl. Inst. & Meth.
The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs
Type Ia supernovae (SNe Ia) play a crucial role as standardizable
cosmological candles, though the nature of their progenitors is a subject of
active investigation. Recent observational and theoretical work has pointed to
merging white dwarf binaries, referred to as the double-degenerate channel, as
the possible progenitor systems for some SNe Ia. Additionally, recent
theoretical work suggests that mergers which fail to detonate may produce
magnetized, rapidly-rotating white dwarfs. In this paper, we present the first
multidimensional simulations of the post-merger evolution of white dwarf
binaries to include the effect of the magnetic field. In these systems, the two
white dwarfs complete a final merger on a dynamical timescale, and are tidally
disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot
corona and a thick, differentially-rotating disk. The disk is strongly
susceptible to the magnetorotational instability (MRI), and we demonstrate that
this leads to the rapid growth of an initially dynamically weak magnetic field
in the disk, the spin-down of the white dwarf merger, and to the subsequent
central ignition of the white dwarf merger. Additionally, these magnetized
models exhibit new features not present in prior hydrodynamic studies of white
dwarf mergers, including the development of MRI turbulence in the hot disk,
magnetized outflows carrying a significant fraction of the disk mass, and the
magnetization of the white dwarf merger to field strengths
G. We discuss the impact of our findings on the origins, circumstellar media,
and observed properties of SNe Ia and magnetized white dwarfs.Comment: Accepted ApJ version published on 8/20/13, with significant
additional text added discussing the nature of the magnetized outflows, and
possible CSM observational features relevant to NaID detection
Studies of Boosted Decision Trees for MiniBooNE Particle Identification
Boosted decision trees are applied to particle identification in the
MiniBooNE experiment operated at Fermi National Accelerator Laboratory
(Fermilab) for neutrino oscillations. Numerous attempts are made to tune the
boosted decision trees, to compare performance of various boosting algorithms,
and to select input variables for optimal performance.Comment: 28 pages, 22 figures, submitted to Nucl. Inst & Meth.
One-armed spiral instability in double-degenerate post-merger accretion disks
This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.Increasing observational and theoretical evidence points to binary white dwarf mergers as the origin of some if not most normal Type Ia supernovae (SNe Ia). In this paper, we discuss the post-merger evolution of binary white dwarf (WD) mergers, and their relevance to the double-degenerate channel of SNe Ia. We present 3D simulations of carbon-oxygen (C/O) WD binary systems undergoing unstable mass transfer, varying both the total mass and the mass ratio. We demonstrate that these systems generally give rise to a one-armed gravitational spiral instability. The spiral density modes transport mass and angular momentum in the disk even in the absence of a magnetic field, and are most pronounced for secondary-to-primary mass ratios larger than 0.6. We further analyze carbon burning in these systems to assess the possibility of detonation. Unlike the case of a 1.1 + 1.0M C/O WD binary, we find that WD binary systems with lower mass and smaller mass ratios do not detonate as SNe Ia up to ∼ 8−22 outer dynamical times. Two additional models do however undergo net heating, and their secular increase in temperature could possibly result in a detonation on timescales longer than those considered hereWe thank James Guillochon, Daan Van Rossum, Chris Byrohl, and Pranav Dave for useful discussions. We also would like to thank the anonymous reviewer for their useful comments and insights. The work of EG-B, GA-S and PL-A was partially funded by MINECO AYA2014-59084-P grant and by the AGAUR. The software used in this work was in part developed by the DOE NNSA-ASC OASCR Flash Center at the University of Chicago. This work used the Extreme Science and Engineering discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. Simulations at UMass Dartmouth were performed on a computer cluster supported by NSF grant CNS-0959382 and AFOSR DURIP grant FA9550-10-1-0354. RTF thanks the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the Kavli Institute for Theoretical Physics, supported in part by the national Science Foundation under grant NSF PHY11-25915, for visiting support during which this work was completed. This research has made use of resources from NASA’s Astrophysics Data System and the yt astrophysics analysis software suite (Turk et al. 2011)
Spiral instability can drive thermonuclear explosions in binary white dwarf mergers
This is the final version of the article. Available from American Astronomical Society via the DOI in this record.Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon–oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.We thank James Guillochon, Lars Bildsten, Matthew Wise, and Gunnar Martin Lellep for useful discussions and Matthias Aegenheyster for his contributions to the FLASH analysis codes. E.G.B. acknowledges support from MCINN grant AYA2011–23102, and from the European Union FEDER fund. The software used in this work was in part developed by the DOE NNSA-ASC OASCR Flash Center at the University of Chicago. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. Simulations at UMass Dartmouth were performed on a computer cluster supported by NSF grant CNS-0959382 and AFOSR DURIP grant FA9550-10-1-0354. This research has made use of NASA's Astrophysics Data System and the yt astrophysics analysis software suite Turk et al. (2011). R.T.F. is grateful to have had the opportunity to complete this paper during a visit to the Kavli Institute for Theoretical Physics, which is supported in part by the National Science Foundation under grant No. NSF PHY11-25915
Experimental determination of the effective strong coupling constant
We present a first attempt to experimentally extract an effective strong
coupling constant that we define to be a low Q2 extension of a previous
definition by S. Brodsky et al. following an initial work of G. Grunberg. Using
Jefferson Lab data and sum rules, we establish its Q2-behavior over the
complete Q2-range. The result is compared to effective coupling constants
inferred from different processes and to calculations based on Schwinger-Dyson
equations, hadron spectroscopy or lattice QCD. Although the connection between
the experimentally extracted effective coupling constants and the calculations
is not established it is interesting to note that their behaviors are similar.Comment: Published in Physics Letters B 650 4 24
Synchrotron x-ray study of lattice vibrations in CdCr2O4
Using inelastic x-ray scattering we have investigated lattice vibrations in a
geometric frustrated system CdCr2O4 that upon cooling undergoes a spin-Peierls
phase transition at TN = 7.8 K from a cubic and paramagnetic to a tetragonal
and Neel state. Phonon modes measured around Brillouin zone boundaries show
energy shifts when the transition occurs. Our analysis shows that the shifting
can be understood as the ordinary effects of the lowering of the crystal
symmetry
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