65,582 research outputs found
Chiral Vortons and Cosmological Constraints on Particle Physics
We investigate the cosmological consequences of particle physics theories
that admit stable loops of current-carrying string - vortons. In particular, we
consider chiral theories where a single fermion zero mode is excited in the
string core, such as those arising in supersymmetric theories with a D-term.
The resulting vortons formed in such theories are expected to be more stable
than their non-chiral cousins. General symmetry breaking schemes are considered
in which strings formed at one symmetry breaking scale become current-carrying
at a subsequent phase transition. The vorton abundance is estimated and
constraints placed on the underlying particle physics theories from
cosmological observations. Our constraints on the chiral theory are
considerably more stringent than the previous estimates for more general
theories.Comment: minor corrections made. This version will appear in PR
Type Ia supernova diversity: Standardizing the candles
Future use of type Ia supernovae for cosmology aims not only to determine the
equation of state of dark energy, but also to constrain possible variations in
its value. To achieve this goal, supernovae need to become better calibrated
standard candles - not only to improve the precision of the measurement, but
more importantly to gain better control over systematic uncertainties in order
to ensure the accuracy of the result.
Here we report on a project to quantify the diversity in type Ia supernovae,
and to look for trends and/or sub-types that can be used to improve their
calibration as standard candles. We implement a version of principal component
analysis on type Ia supernova spectra. Although the quantity of data is not
sufficient to draw any firm conclusions we show that this method holds promise
for, at the very least, effectively separating peculiar supernovae. Whether it
can be further used to improve the calibration of normal type Ia's remains a
project for future study.Comment: Conference Proceedings. Cefalu 2006, The multicoloured landscape of
compact objects and their explosive origins. Six pages, three figure
Qualified Immunity and Excessive Force: A Greater or Lesser Role for Juries?
The two-pronged qualified immunity analysis, which is often the deciding point in any Fourth Amendment use of force case, continues to be a difficult issue dictated by abstract rules. First, courts need to further the law by analyzing the facts to determine whether a constitutional right existed and was violated. Second, in determining whether the right was been clearly established, the courts must find just the right middle ground between too broad an analysis – such as merely citing Graham or its several factors – and an analysis that is too narrow – such as requiring the exact fact pattern in a previous case –something the 10th Circuit has recently battled with in several of its recent cases, including two reversed and remanded by the Supreme Court for further consideration. A middle ground, if adequately defined by the appellate courts to enable its consistent application by the lower courts – can exist. It allows for a pragmatic approach that leads courts to rely on established principles derived in previous cases – e.g., using force against unarmed, unresisting citizens violates their rights – without requiring the claimant to search for and discover prior cases with exact facts on point (such as the difference between using a gun or taser in the aforementioned example). The 10th Circuit’s “sliding scale” approach mirrored the intent of the Graham factors, which appears now to have been rejected, at least in deadly force cases, by the Supreme Court. Putting a premium on furthering the law through judicial determinations of constitutional violations benefit law enforcement and citizens alike – and will serve to guide the lower courts by the continued development of clearly established law in the Fourth Amendment jurisprudence
Radiative Transfer in Star Formation: Testing FLD and Hybrid Methods
We perform a comparison between two radiative transfer algorithms commonly
employed in hydrodynamical calculations of star formation: grey flux limited
diffusion and the hybrid scheme, in addition we compare these algorithms to
results from the Monte-Carlo radiative transfer code MOCASSIN. In disc like
density structures the hybrid scheme performs significantly better than the FLD
method in the optically thin regions, with comparable results in optically
thick regions. In the case of a forming high mass star we find the FLD method
significantly underestimates the radiation pressure by a factor of ~100.Comment: 4 Pages; to appear in the proceedings of 'The Labyrinth of Star
Formation', Crete, 18-22 June 201
What makes a 'good group'? Exploring the characteristics and performance of undergraduate student groups
Group work forms the foundation for much of student learning within higher education, and has many educational, social and professional benefits. This study aimed to explore the determinants of success or failure for undergraduate student teams and to define a ‘good group’ through considering three aspects of group success: the task, the individuals, and the team. We employed a mixed methodology, combining demographic data with qualitative observations and task and peer evaluation scores. We determined associations between group dynamic and behaviour, demographic composition, member personalities and attitudes towards one another, and task success. We also employed a cluster analysis to create a model outlining the attributes of a good small group learning team in veterinary education. This model highlights that student groups differ in measures of their effectiveness as teams, independent of their task performance. On the basis of this, we suggest that groups who achieve high marks in tasks cannot be assumed to have acquired team working skills, and therefore if these are important as a learning outcome, they must be assessed directly alongside the task output
Dynamical Stability of Witten Rings
The dynamical stability of cosmic rings, or vortons, is investigated for the
particular equation of state given by the Witten bosonic model. It is found
that there exists a finite range of the state parameter for which the vorton
states are actually stable against dynamical perturbations. Inclusion of the
electromagnetic self action into the equation of state slightly shrinks the
stability region but otherwise yields no qualitative difference. If the Witten
bosonic model represents a good approximation for more realistic string models,
then the cosmological vorton excess problem can only be solved by assuming
either that strings are formed at low energy scales or that some quantum
instability may develop at a sufficient rate.Comment: 11 pages, LaTeX-ReVTeX (v.3), 2 figures available upon request, DAMTP
R-94/1
Dynamical correlation functions for an impenetrable Bose gas with Neumann or Dirichlet boundary conditions
We study the time and temperature dependent correlation functions for an
impenetrable Bose gas with Neumann or Dirichlet boundary conditions . We derive the Fredholm
determinant formulae for the correlation functions, by means of the Bethe
Ansatz. For the special case , we express correlation functions with
Neumann boundary conditions , in terms of solutions of nonlinear partial differential equations
which were introduced in \cite{kojima:Sl} as a generalization of the nonlinear
Schr\"odinger equations. We generalize the Fredholm minor determinant formulae
of ground state correlation functions in \cite{kojima:K}, to the Fredholm determinant formulae for the time
and temperature dependent correlation functions
, ,
Filament wound metal lined propellant tanks for future Earth-to-orbit transports
For future Earth-to-orbit transport vehicles, reusability and lighter weights are sought for the main propellant tanks. To achieve this, a filament wound tank with a metal liner and an intermediate layer of foam-filled honeycomb is proposed. A hydrogen tank is used as an example. To accommodate mismatches in the expansion of liner and overwrap a design is proposed wherin the liner is configured so that the extension of the liner under pressure matches the expected contraction of the same liner due to the presence of a cryogen. In operation, the liner is pressurized at a rate such that the pressure strain matches the contraction due to decrease in temperature. As an alternate approach, compressive pre-stress is placed in the liner such that it will not separate from the overwrap. A finite element program is used to show stresses in the liner and overwrap for various tank pressures for the pre-stressed liner concept. A fracture mechanics analysis is made of the liners to determine tank life. The tank concept shown has a similar weight to the Shuttle external hydrogen tank, but the filament wound tank is expected to be reusable. Integration of the propellant tanks into a future transport vehicle is discussed
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