13,470 research outputs found
A Natural Supersymmetric Model with MeV Dark Matter
It has previously been proposed that annihilating dark matter particles with
MeV-scale masses could be responsible for the flux of 511 keV photons observed
from the region of the Galactic Bulge. The conventional wisdom, however, is
that it is very challenging to construct a viable particle physics model
containing MeV dark matter. In this letter, we challenge this conclusion by
describing a simple and natural supersymmetric model in which the lightest
supersymmetric particle naturally has a MeV-scale mass and the other
phenomenological properties required to generate the 511 keV emission. In
particular, the small ( ) effective couplings between dark
matter and the Standard Model fermions required in this scenario naturally lead
to radiative corrections that generate MeV-scale masses for both the dark
matter candidate and the mediator particle.Comment: 4 pages, 1 figure. v2: Small modification to discussion of spectru
Extended MSSM Neutralinos as the Source of the PAMELA Positron Excess
We consider a scenario within the Minimal Supersymmetric Standard Model
extended by a singlet chiral superfield, in which neutralino dark matter
annihilates to light singlet-like Higgs bosons, which proceed to decay to
either electron-positron or muon-antimuon pairs. Unlike neutralino
annihilations in the MSSM, this model can provide a good fit to the PAMELA
cosmic ray positron fraction excess. Furthermore, the singlet-like scalar Higgs
can induce a large Sommerfeld enhancement and provide an annihilation rate
sufficient to accommodate the observed positron excess
Evaluation of distributed gas cooling of pressurized PAFC for utility power generation
A proof-of-concept test for a gas-cooled pressurized phosphoric acid fuel cell is described. After initial feasibility studies in short stacks, two 10 kW stacks are tested. Progress includes: (1) completion of design of the test stations with a recirculating gas cooling loop; (2) atmospheric testing of the baseline stack
Cosmogenic photons as a test of ultra-high energy cosmic ray composition
Although recent measurements of the shower profiles of ultra-high energy
cosmic rays suggest that they are largely initiated by heavy nuclei, such
conclusions rely on hadronic interaction models which have large uncertainties.
We investigate an alternative test of cosmic ray composition which is based on
the observation of ultra-high energy photons produced through cosmic ray
interactions with diffuse low energy photon backgrounds during intergalactic
propagation. We show that if the ultra-high energy cosmic rays are dominated by
heavy nuclei, the flux of these photons is suppressed by approximately an order
of magnitude relative to the proton-dominated case. Future observations by the
Pierre Auger Observatory may be able to use this observable to constrain the
composition of the primaries, thus providing an important cross-check of
hadronic interaction models.Comment: 4 pages, 2 figure
The PAMELA and ATIC Excesses From a Nearby Clump of Neutralino Dark Matter
In this letter, we suggest that a nearby clump of 600-1000 GeV neutralinos
may be responsible for the excesses recently observed in the cosmic ray
positron and electron spectra by the PAMELA and ATIC experiments. Although
neutralino dark matter annihilating throughout the halo of the Milky Way is
predicted to produce a softer spectrum than is observed, and violate
constraints from cosmic ray antiproton measurements, a large nearby (within 1-2
kiloparsecs of the Solar System) clump of annihilating neutralinos can lead to
a spectrum which is consistent with PAMELA and ATIC, while also producing an
acceptable antiproton flux. Furthermore, the presence of a large dark matter
clump can potentially accommodate the very large annihilation rate required to
produce the PAMELA and ATIC signals.Comment: 4 pages, 2 figure
How and Where Should I Ride This Thing? āRules Of The Roadā for Personal Transportation Devices
In recent years, āPersonal Transportation Devicesā (PTDs) have exploded onto streets and sidewalks. These small devices transport individual persons at slow speeds and are either human-powered or motorized. Examples include electric (kick) scooters, skateboards, e-skateboards, roller blades, and Segways. One key to successfully integrating PTDs into community streets will be the implementation of consistent and suitable regulations over user behavior: ārules of the roadā for PTD riders. To help local officials identify appropriate rules for rider behavior, this report documents and analyzes existing PTD regulations across 176 jurisdictions and then presents recommendations for a set of state-level ārules of the roadā designed to balance safety and freedom of movement for all road users, including PTD riders.To identify the current state of PTD rules of the road, we documented and analyzed the existing regulations at three levels of government: all 50 states and 5 U.S. territories, 101 cities, and 20 college campuses. This review found that PTD users operate in a murky regulatory environment, with rules often poorly defined, contradictory, or altogether absent.Results of this analysis, a literature review, and interviews with 21 stakeholders, were used to craft a model state-level regulatory code that aims to introduce consistent and well-grounded regulation of PTDs. The general philosophy underpinning the model legislation is that PTD rules should protect public safety, permit PTD use as a convenient travel option, be easy to understand and remember, allow for new devices without new regulations, and be based on facts about PTD use and users. Working from these principles, core recommended elements of the recommended PTD regulations are as follows: states should set comprehensive regulations for PTD riders (with local gov-ernments given flexibility to limit certain uses when necessitated by local conditions); PTDs should be regulated as a class, not device-by-device; and PTD users should be permitted to ride on both streets and sidewalks, subject to rules that protect safety and free movement for all travelers
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