12,791 research outputs found
The Red Queen visits Minkowski Space
When Alice went `Through the Looking Glass' [1], she found herself in a
situation where she had to run as fast as she could in order to stay still. In
accordance with the dictum that truth is stranger than fiction, we will see
that it is possible to find a situation in special relativity where running
towards one's target is actually counter-productive. Although the situation is
easily analysed algebraically, the qualitative properties of the analysis are
greatly illuminated by the use of space-time diagrams
Integrating heterogeneous distributed COTS discrete-event simulation packages: An emerging standards-based approach
This paper reports on the progress made toward the emergence of standards to support the integration of heterogeneous discrete-event simulations (DESs) created in specialist support tools called commercial-off-the-shelf (COTS) discrete-event simulation packages (CSPs). The general standard for heterogeneous integration in this area has been developed from research in distributed simulation and is the IEEE 1516 standard The High Level Architecture (HLA). However, the specific needs of heterogeneous CSP integration require that the HLA is augmented by additional complementary standards. These are the suite of CSP interoperability (CSPI) standards being developed under the Simulation Interoperability Standards Organization (SISO-http://www.sisostds.org) by the CSPI Product Development Group (CSPI-PDG). The suite consists of several interoperability reference models (IRMs) that outline different integration needs of CSPI, interoperability frameworks (IFs) that define the HLA-based solution to each IRM, appropriate data exchange representations to specify the data exchanged in an IF, and benchmarks termed CSP emulators (CSPEs). This paper contributes to the development of the Type I IF that is intended to represent the HLA-based solution to the problem outlined by the Type I IRM (asynchronous entity passing) by developing the entity transfer specification (ETS) data exchange representation. The use of the ETS in an illustrative case study implemented using a prototype CSPE is shown. This case study also allows us to highlight the importance of event granularity and lookahead in the performance and development of the Type I IF, and to discuss possible methods to automate the capture of appropriate values of lookahead
Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides
Intervalley scattering by atomic defects in monolayer transition metal
dichalcogenides (TDMs; MX2) presents a serious obstacle for applications
exploiting their unique valley-contrasting properties. Here, we show that the
symmetry of the atomic defects can give rise to an unconventional protection
mechanism against intervalley scattering in monolayer TMDs. The predicted
defect-dependent selection rules for intervalley scattering can be verified via
Fourier transform scanning tunneling spectroscopy (FT-STS), and provide a
unique identification of, e.g., atomic vacancy defects (M vs X). Our findings
put the absence of the intervalley FT-STS peak in recent experiments in a
different perspective.Comment: 7 pages, 4 figures + supplementary. Published versio
The effect of microstructure on the fracture toughness of titanium alloys
The microstructure of the alpha titanium alloy Ti-5Al-2.5Sn and the metastable beta titanium alloy Beta 3 was examined. The material was from normal and extra low interstitial grade plates which were either air-cooled or furnace-cooled from an annealing treatment. Beta 3 was studied in alpha-aged and omega-aged plates which were heat treated to similar strength levels. Tensile and plane strain fracture toughness tests were conducted at room temperature on the alpha-aged material. The microstructure and fracture mechanisms of alloys were studied using optical metallography, electron microscopy, microprobe analyses, and texture pole figures. Future experiments are described
World-line Quantisation of a Reciprocally Invariant System
We present the world-line quantisation of a system invariant under the
symmetries of reciprocal relativity (pseudo-unitary transformations on ``phase
space coordinates" which preserve the Minkowski
metric and the symplectic form, and global shifts in these coordinates,
together with coordinate dependent transformations of an additional compact
phase coordinate, ). The action is that of free motion over the
corresponding Weyl-Heisenberg group. Imposition of the first class constraint,
the generator of local time reparametrisations, on physical states enforces
identification of the world-line cosmological constant with a fixed value of
the quadratic Casimir of the quaplectic symmetry group , the semi-direct product of the pseudo-unitary group with
the Weyl-Heisenberg group (the central extension of the global translation
group, with central extension associated to the phase variable ).
The spacetime spectrum of physical states is identified. Even though for an
appropriate range of values the restriction enforced by the cosmological
constant projects out negative norm states from the physical spectrum, leaving
over spin zero states only, the mass-squared spectrum is continuous over the
entire real line and thus includes a tachyonic branch as well
Have We Observed the Higgs (Imposter)?
We interpret the new particle at the Large Hadron Collider as a CP-even
scalar and investigate its electroweak quantum number. Assuming an unbroken
custodial invariance as suggested by precision electroweak measurements, only
four possibilities are allowed if the scalar decays to pairs of gauge bosons,
as exemplified by a dilaton/radion, a non-dilatonic electroweak singlet scalar,
an electroweak doublet scalar, and electroweak triplet scalars. We show that
current LHC data already strongly disfavor both the dilatonic and non-dilatonic
singlet imposters. On the other hand, a generic Higgs doublet give excellent
fits to the measured event rates of the newly observed scalar resonance, while
the Standard Model Higgs boson gives a slightly worse overall fit due to the
lack signal in the tau tau channel. The triplet imposter exhibits some tension
with the data. The global fit indicates the enhancement in the diphoton channel
could be attributed to an enhanced partial decay width, while the production
rates are consistent with the Standard Model expectations. We emphasize that
more precise measurements of the ratio of event rates in the WW over ZZ
channels, as well as the event rates in b bbar and tau tau channels, are needed
to further distinguish the Higgs doublet from the triplet imposter.Comment: 20 pages, 4 figures; v2: updated with most recent public data as of
August 7. A generic Higgs doublet now gives the best fit to data, while the
triplet imposter exhibits some tensio
The influence of composition, annealing treatment, and texture on the fracture toughness of Ti-5Al-2.5Sn plate at cryogenic temperatures
The plane strain fracture toughness K sub Ic and conventional tensile properties of two commercially produced one-inch thick Ti-5Al-2.5Sn plates were determined at cryogenic temperatures. One plate was extra-low interstitial (ELI) grade, the other normal interstitial. Portions of each plate were mill annealed at 1088 K (1500 F) followed by either air cooling or furnace cooling. The tensile properties, flow curves, and K sub Ic of these plates were determined at 295 K (room temperature), 77 K (liquid nitrogen temperature), and 20 K (liquid hydrogen temperature)
What if the Higgs couplings to W and Z bosons are larger than in the Standard Model?
We derive a general sum rule relating the Higgs coupling to W and Z bosons to
the total cross section of longitudinal gauge boson scattering in I=0,1,2
isospin channels. The Higgs coupling larger than in the Standard Model implies
enhancement of the I=2 cross section. Such an enhancement could arise if the
Higgs sector is extended by an isospin-2 scalar multiplet including a doubly
charged, singly charged, and another neutral Higgs.Comment: 11 pages, no figures. v2: comments and references added. v3: early
QCD references adde
Metal Cooling in Simulations of Cosmic Structure Formation
The addition of metals to any gas can significantly alter its evolution by
increasing the rate of radiative cooling. In star-forming environments,
enhanced cooling can potentially lead to fragmentation and the formation of
low-mass stars, where metal-free gas-clouds have been shown not to fragment.
Adding metal cooling to numerical simulations has traditionally required a
choice between speed and accuracy. We introduce a method that uses the
sophisticated chemical network of the photoionization software, Cloudy, to
include radiative cooling from a complete set of metals up to atomic number 30
(Zn) that can be used with large-scale three-dimensional hydrodynamic
simulations. Our method is valid over an extremely large temperature range (10
K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this
density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our
method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo.
Using cooling rates generated with this method, we study the physical
conditions that led to the transition from Population III to Population II star
formation. While C, O, Fe, and Si have been previously shown to make the
strongest contribution to the cooling in low-metallicity gas, we find that up
to 40% of the metal cooling comes from fine-structure emission by S, when solar
abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of
density and temperature exist where gas is both thermally unstable and has a
cooling time less than its dynamical time. We identify these doubly unstable
regions as the most inducive to fragmentation. At high redshifts, the CMB
inhibits efficient cooling at low temperatures and, thus, reduces the size of
the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure
Linking and causality in globally hyperbolic spacetimes
The linking number is defined if link components are zero homologous.
Our affine linking invariant generalizes to the case of linked
submanifolds with arbitrary homology classes. We apply to the study of
causality in Lorentz manifolds. Let be a spacelike Cauchy surface in a
globally hyperbolic spacetime . The spherical cotangent bundle
is identified with the space of all null geodesics in
Hence the set of null geodesics passing through a point gives an
embedded -sphere in called the sky of Low observed
that if the link is nontrivial, then are causally
related. This motivated the problem (communicated by Penrose) on the Arnold's
1998 problem list to apply link theory to the study of causality. The spheres
are isotopic to fibers of They are nonzero
homologous and is undefined when is closed, while is well defined. Moreover, if is not an
odd-dimensional rational homology sphere. We give a formula for the increment
of \alk under passages through Arnold dangerous tangencies. If is
such that takes values in and is conformal to having all
the timelike sectional curvatures nonnegative, then are causally
related if and only if . We show that in
nonrefocussing are causally unrelated iff can be deformed
to a pair of -fibers of by an isotopy through skies. Low
showed that if (\ss, g) is refocussing, then is compact. We show that the
universal cover of is also compact.Comment: We added: Theorem 11.5 saying that a Cauchy surface in a refocussing
space time has finite pi_1; changed Theorem 7.5 to be in terms of conformal
classes of Lorentz metrics and did a few more changes. 45 pages, 3 figures. A
part of the paper (several results of sections 4,5,6,9,10) is an extension
and development of our work math.GT/0207219 in the context of Lorentzian
geometry. The results of sections 7,8,11,12 and Appendix B are ne
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