6,249 research outputs found
ASTROS: A multidisciplinary automated structural design tool
ASTROS (Automated Structural Optimization System) is a finite-element-based multidisciplinary structural optimization procedure developed under Air Force sponsorship to perform automated preliminary structural design. The design task is the determination of the structural sizes that provide an optimal structure while satisfying numerous constraints from many disciplines. In addition to its automated design features, ASTROS provides a general transient and frequency response capability, as well as a special feature to perform a transient analysis of a vehicle subjected to a nuclear blast. The motivation for the development of a single multidisciplinary design tool is that such a tool can provide improved structural designs in less time than is currently needed. The role of such a tool is even more apparent as modern materials come into widespread use. Balancing conflicting requirements for the structure's strength and stiffness while exploiting the benefits of material anisotropy is perhaps an impossible task without assistance from an automated design tool. Finally, the use of a single tool can bring the design task into better focus among design team members, thereby improving their insight into the overall task
Evolution in the Volumetric Type Ia Supernova Rate from the Supernova Legacy Survey
We present a measurement of the volumetric Type Ia supernova (SN Ia) rate (SNR_Ia) as a function of redshift for the first four years of data from the Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS). This analysis includes 286 spectroscopically confirmed and more than 400 additional photometrically identified SNe Ia within the redshift range 0.1 ≤ z ≤ 1.1. The volumetric SNR_Ia evolution is consistent with a rise to z ~ 1.0 that follows a power law of the form (1+z)^α, with α = 2.11 ± 0.28. This evolutionary trend in the SNLS rates is slightly shallower than that of the cosmic star formation history (SFH) over the same redshift range. We combine the SNLS rate measurements with those from other surveys that complement the SNLS redshift range, and fit various simple SN Ia delay-time distribution (DTD) models to the combined data. A simple power-law model for the DTD (i.e., ∝ t^(–β)) yields values from β = 0.98 ± 0.05 to β = 1.15 ± 0.08 depending on the parameterization of the cosmic SFH. A two-component model, where SNR_Ia is dependent on stellar mass (M_stellar) and star formation rate (SFR) as SNR_(Ia)(z) = A × M_(stellar)(z) + B × SFR(z), yields the coefficients A = (1.9 ± 0.1) × 10^(–1)4 SNe yr^(–1) M^(–1)_☉ and B = (3.3 ± 0.2) × 10^(–4) SNe yr^(–1) (M_☉ yr^(–1))^(–1). More general two-component models also fit the data well, but single Gaussian or exponential DTDs provide significantly poorer matches. Finally, we split the SNLS sample into two populations by the light-curve width (stretch), and show that the general behavior in the rates of faster-declining SNe Ia (0.8 ≤ s < 1.0) is similar, within our measurement errors, to that of the slower objects (1.0 ≤ s < 1.3) out to z ~ 0.8
Applications of structural optimization methods to fixed-wing aircraft and spacecraft in the 1980s
This report is the summary of a technical survey on the applications of structural optimization in the U.S. aerospace industry through the 1980s. Since applications to rotary wing aircraft will be covered by other literature, applications to fixed-wing aircraft and spacecraft were considered. It became clear that very significant progress has been made during this decade, indicating this technology is about to become one of the practical tools in computer aided structural design
Toward Multi-Differential Cross Sections: Measuring Two Angularities on a Single Jet
The analytic study of differential cross sections in QCD has typically
focused on individual observables, such as mass or thrust, to great success.
Here, we present a first study of double differential jet cross sections
considering two recoil-free angularities measured on a single jet. By analyzing
the phase space defined by the two angularities and using methods from
soft-collinear effective theory, we prove that the double differential cross
section factorizes at the boundaries of the phase space. We also show that the
cross section in the bulk of the phase space cannot be factorized using only
soft and collinear modes, excluding the possibility of a global factorization
theorem in soft-collinear effective theory. Nevertheless, we are able to define
a simple interpolation procedure that smoothly connects the factorization
theorem at one boundary to the other. We present an explicit example of this at
next-to-leading logarithmic accuracy and show that the interpolation is unique
up to order in the exponent of the cross section, under reasonable
assumptions. This is evidence that the interpolation is sufficiently robust to
account for all logarithms in the bulk of phase space to the accuracy of the
boundary factorization theorem. We compare our analytic calculation of the
double differential cross section to Monte Carlo simulation and find
qualitative agreement. Because our arguments rely on general structures of the
phase space, we expect that much of our analysis would be relevant for the
study of phenomenologically well-motivated observables, such as
-subjettiness, energy correlation functions, and planar flow.Comment: 43 pages plus appendices, 8 figures. v2 as published in JHEP. minor
typos correcte
Factorization and Resummation for Groomed Multi-Prong Jet Shapes
Observables which distinguish boosted topologies from QCD jets are playing an
increasingly important role at the Large Hadron Collider (LHC). These
observables are often used in conjunction with jet grooming algorithms, which
reduce contamination from both theoretical and experimental sources. In this
paper we derive factorization formulae for groomed multi-prong substructure
observables, focusing in particular on the groomed observable, which is
used to identify boosted hadronic decays of electroweak bosons at the LHC. Our
factorization formulae allow systematically improvable calculations of the
perturbative distribution and the resummation of logarithmically enhanced
terms in all regions of phase space using renormalization group evolution. They
include a novel factorization for the production of a soft subjet in the
presence of a grooming algorithm, in which clustering effects enter directly
into the hard matching. We use these factorization formulae to draw robust
conclusions of experimental relevance regarding the universality of the
distribution in both and collisions. In particular, we show that
the only process dependence is carried by the relative quark vs. gluon jet
fraction in the sample, no non-global logarithms from event-wide correlations
are present in the distribution, hadronization corrections are controlled by
the perturbative mass of the jet, and all global color correlations are
completely removed by grooming, making groomed a theoretically clean QCD
observable even in the LHC environment. We compute all ingredients to one-loop
accuracy, and present numerical results at next-to-leading logarithmic accuracy
for collisions, comparing with parton shower Monte Carlo simulations.
Results for collisions, as relevant for phenomenology at the LHC, are
presented in a companion paper.Comment: 66 pages, 18 figure
Non-Global Logarithms, Factorization, and the Soft Substructure of Jets
An outstanding problem in QCD and jet physics is the factorization and
resummation of logarithms that arise due to phase space constraints, so-called
non-global logarithms (NGLs). In this paper, we show that NGLs can be
factorized and resummed down to an unresolved infrared scale by making
sufficiently many measurements on a jet or other restricted phase space region.
Resummation is accomplished by renormalization group evolution of the objects
in the factorization theorem and anomalous dimensions can be calculated to any
perturbative accuracy and with any number of colors. To connect with the NGLs
of more inclusive measurements, we present a novel perturbative expansion which
is controlled by the volume of the allowed phase space for unresolved
emissions. Arbitrary accuracy can be obtained by making more and more
measurements so to resolve lower and lower scales. We find that even a minimal
number of measurements produces agreement with Monte Carlo methods for
leading-logarithmic resummation of NGLs at the sub-percent level over the full
dynamical range relevant for the Large Hadron Collider. We also discuss other
applications of our factorization theorem to soft jet dynamics and how to
extend to higher-order accuracy.Comment: 46 pages + appendices, 10 figures. v2: added current figures 4 and 5,
as well as corrected several typos in appendices. v3: corrected some typos,
added current figure 9, and added more discussion of fixed-order versus
dressed gluon expansions. v4: fixed an error in numerics of two-dressed
gluon; corrected figure 8, modified comparison to BMS. Conclusions unchanged.
v5: fixed minor typ
Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey
We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = –0.91^(+0.16)_(–0.20)(stat)^(+0.07)_(–0.14)(sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems
- …