4,471 research outputs found
Minimal gauge origin of baryon triality and flavorful signatures at the LHC
Baryon triality (B3) is a Z3 discrete symmetry that can protect the proton
from decay. Although its realization does not require supersymmetry, it is
particularly appealing in the supersymmetry as an alternative to the popular
R-parity. We discuss the issues in gauging B3, and present the minimal
supersymmetric model with B3 as the remnant discrete symmetry of a TeV scale
U(1) gauge symmetry. A flavor-dependent U(1) charge is necessary to achieve
this, and it results in very distinguishable and flavorful predictions for the
LHC experiments. We find a complementarity between a 2-lepton sneutrino
resonance and a 4-lepton Z' resonance in the supersymmetry search. In addition,
we introduce baryon tetrality (B4), which would play an equivalent role if
there are four fermion generations.Comment: Version to appear in PL
New opportunities of freeform gratings using diamond machining
With the recent development of new ultra fine aluminium alloys and progress in the field of directly machined freeform surfaces, diamond machined freeform gratings could play an important part in future spectrographs or integral field units, particularly at SWIR and LWIR wavelengths where the improved thermal performance of metal optics at cryogenic temperatures is well established. Freeform diamond machined gratings can offer a cost-effective, compact, and flexible alternative to gratings fabricated by other methods such as ion beam etching or complement these technologies. In this paper, both the advantages and limitations of 5 axis diamond machined freeform gratings are presented and potential applications are discussed
Navigating the Return Trip from the Moon Using Earth-Based Ground Tracking and GPS
NASA s Constellation Program is planning a human return to the Moon late in the next decade. From a navigation perspective, one of the most critical phases of a lunar mission is the series of burns performed to leave lunar orbit, insert onto a trans-Earth trajectory, and target a precise re-entry corridor in the Earth s atmosphere. A study was conducted to examine sensitivity of the navigation performance during this phase of the mission to the type and availability of tracking data from Earth-based ground stations, and the sensitivity to key error sources. This study also investigated whether GPS measurements could be used to augment Earth-based tracking data, and how far from the Earth GPS measurements would be useful. The ability to track and utilize weak GPS signals transmitted across the limb of the Earth is highly dependent on the configuration and sensitivity of the GPS receiver being used. For this study three GPS configurations were considered: a "standard" GPS receiver with zero dB antenna gain, a "weak signal" GPS receiver with zero dB antenna gain, and a "weak signal" GPS receiver with an Earth-pointing direction antenna (providing 10 dB additional gain). The analysis indicates that with proper selection and configuration of the GPS receiver on the Orion spacecraft, GPS can potentially improve navigation performance during the critical final phases of flight prior to Earth atmospheric entry interface, and may reduce reliance on two-way range tracking from Earth-based ground stations
Perpendicular-current Studies of Electron Transport Across Metal/Metal Interfaces
We review what we have learned about the scattering of electrons by the
interfaces between two different metals (M1/M2) in the
current-perpendicular-to-plane (CPP) geometry. In this geometry, the intrinsic
quantity is the specific resistance, AR, the product of the area through which
the CPP current flows times the CPP resistance. We describe results for both
non-magnetic/non-magnetic (N1/N2) and ferromagnetic/non-magnetic (F/N) pairs.
We focus especially upon cases where M1/M2 are lattice matched (i.e., have the
same crystal structure and the same lattice parameters to within ~ 1%), because
in these cases no-free-parameter calculations of 2AR agree surprisingly well
with measured values. But we also list and briefly discuss cases where M1/M2
are not lattice matched, either having different crystal structures, or lattice
parameters that differ by several percent. The published calculations of 2AR in
these latter cases do not agree so well with measured values.Comment: 6 pages, 2 figures, 2 tables. In Press: Applied Surface Scienc
Large electric-field induced strain in BiFeO3 ceramics
Large bipolar strain of up to 0.36% (peak-to-peak value) was measured in
BiFeO3 ceramics at low frequency (0.1 Hz) and large amplitude (140 kV/cm) of
the driving field. This strain is comparable to that achievable in highly
efficient Pb-based perovskite ceramics, such as Pb(Zr,Ti)O3 and
Pb(Mg,Nb)O3-PbTiO3. The strain showed a strong dependence on the field
frequency and is likely largely associated with domain switching involving
predominantly non-180{\deg} domain walls. In addition, rearrangement of charged
defects by applying electric field of low frequency depins these domain walls,
resulting in a more efficient switching and, consequently, an increased
response
Risk factors for transmission of Ebola or Marburg virus disease: a systematic review and meta-analysis
Background The Ebola virus disease outbreak that started in Western Africa in 2013 was unprecedented because it spread within densely populated urban environments and affected many thousands of people. As a result, previous advice and guidelines need to be critically reviewed, especially with regard to transmission risks in different contexts. Methods Scientific and grey literature were searched for articles about any African filovirus. Articles were screened for information about transmission (prevalence or odds ratios especially). Data were extracted from eligible articles and summarised narratively with partial meta-analysis. Study quality was also evaluated. Results 31 reports were selected from 6552 found in the initial search. Eight papers gave numerical odds for contracting filovirus illness, 23 further articles provided supporting anecdotal observations about how transmission probably occurred for individuals. Many forms of contact (conversation, sharing a meal, sharing a bed, direct or indirect touching) were unlikely to result in disease transmission during incubation or early illness. Amongst household contacts who reported directly touching a case, the attack rate was 32% (95% CI 26-38%). Risk of disease transmission between household members without direct contact was low (1%; 95% CI 0-5%). Caring for a case in the community, especially until death, and participation in traditional funeral rites were strongly associated with acquiring disease, probably due to a high degree of direct physical contact with case or cadaver. Conclusions Transmission of filovirus is unlikely except through close contact, especially during the most severe stages of acute illness. More data are needed about the context, intimacy and timing of contact required to raise the odds of disease transmission. Risk factors specific to urban settings may need to be determined
Biologically active Phytophthora mating hormone prepared by catalytic asymmetric total synthesis
A Phytophthora mating hormone with an array of 1,5-stereogenic centers has been synthesized by using our recently developed methodology of catalytic enantioselective conjugate addition of Grignard reagents. We applied this methodology in a diastereo- and enantioselective iterative route and obtained two of the 16 possible stereoisomers of Phytophthora hormone α1. These synthetic stereoisomers induced the formation of sexual spores (oospores) in A2 mating type strains of three heterothallic Phytophthora species, P. infestans, P. capsici, and P. nicotianae but not in A1 mating type strains. The response was concentration-dependent, and the oospores were viable. These results demonstrate that the biological activity of the synthetic hormone resembles that of the natural hormone α1. Mating hormones are essential components in the sexual life cycle of a variety of organisms. For plant pathogens like Phytophthora, sexual reproduction is important as a source of genetic variation. Moreover, the thick-walled oospores are the most durable propagules that can survive harsh environmental conditions. Sexual reproduction can thus greatly affect disease epidemics. The availability of synthetic compounds mimicking the activity of Phytophthora mating hormone will be instrumental for further unravelling sexual reproduction in this important group of plant pathogens.
Optomechanical zipper cavity lasers: theoretical analysis of tuning range and stability
The design of highly wavelength tunable semiconductor laser structures is
presented. The system is based on a one dimensional photonic crystal cavity
consisting of two patterned, doubly-clamped nanobeams, otherwise known as a
"zipper" cavity. Zipper cavities are highly dispersive with respect to the gap
between nanobeams in which extremely strong radiation pressure forces exist.
Schemes for controlling the zipper cavity wavelength both optically and
electrically are presented. Tuning ranges as high as 75nm are achieved for a
nominal design wavelength of 1.3micron. Sensitivity of the mechanically
compliant laser structure to thermal noise is considered, and it is found that
dynamic back-action of radiation pressure in the form of an optical or
electrical spring can be used to stabilize the laser frequency. Fabrication of
zipper cavity laser structures in GaAs material with embedded self-assembled
InAs quantum dots is presented, along with measurements of photoluminescence
spectroscopy of the zipper cavity modes.Comment: 20 pages, 8 figure
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