6,957 research outputs found
Close Approach during Hard Binary--Binary Scattering
We report on an extensive series of numerical experiments of binary--binary
scattering, analysing the cross--section for close approach during interactions
for a range of hard binary parameters of interest in globular cluster cores. We
consider the implied rate for tidal interactions for different globular
clusters and compare our results with previous, complementary estimates of
stellar collision rates in globular clusters. We find that the collision rate
for binary--binary encounters dominates in low density clusters if the binary
fraction in the cluster is larger than for wide main--sequence binaries.
In dense clusters binary--single interactions dominate the collision rate and
the core binary fraction must be \ltorder 0.1 per decade in semi--major axis
or too many collisions take place compared to observations. The rates are
consistent if binaries with semi--major axes are overabundant in
low density clusters or if breakup and ejection substantially lowers the binary
fraction in denser clusters. Given reasonable assumptions about fractions of
binaries in the cores of low density clusters such as NGC~5053, we cannot
account for all the observed blue stragglers by stellar collisions during
binary encounters, suggesting a substantial fraction may be due to coalescence
of tight primordial binaries.Comment: 13 pages including 13 ps figures. MNRAS in pres
The Optimal Single Copy Measurement for the Hidden Subgroup Problem
The optimization of measurements for the state distinction problem has
recently been applied to the theory of quantum algorithms with considerable
successes, including efficient new quantum algorithms for the non-abelian
hidden subgroup problem. Previous work has identified the optimal single copy
measurement for the hidden subgroup problem over abelian groups as well as for
the non-abelian problem in the setting where the subgroups are restricted to be
all conjugate to each other. Here we describe the optimal single copy
measurement for the hidden subgroup problem when all of the subgroups of the
group are given with equal a priori probability. The optimal measurement is
seen to be a hybrid of the two previously discovered single copy optimal
measurements for the hidden subgroup problem.Comment: 8 pages. Error in main proof fixe
Dark Energy Survey Year 3 results:optimizing the lens sample in a combined galaxy clustering and galaxy-galaxy lensing analysis
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Quantum operations that cannot be implemented using a small mixed environment
To implement any quantum operation (a.k.a. ``superoperator'' or ``CP map'')
on a d-dimensional quantum system, it is enough to apply a suitable overall
unitary transformation to the system and a d^2-dimensional environment which is
initialized in a fixed pure state. It has been suggested that a d-dimensional
environment might be enough if we could initialize the environment in a mixed
state of our choosing. In this note we show with elementary means that certain
explicit quantum operations cannot be realized in this way. Our counterexamples
map some pure states to pure states, giving strong and easily manageable
conditions on the overall unitary transformation. Everything works in the more
general setting of quantum operations from d-dimensional to d'-dimensional
spaces, so we place our counterexamples within this more general framework.Comment: LATEX, 8 page
A tutorial task and tertiary courseware model for collaborative learning communities
RAED provides a computerised infrastructure to support the development and administration of Vicarious Learning in collaborative learning communities spread across multiple universities and workplaces. The system is based on the OASIS middleware for Role-based Access Control. This paper describes the origins of the model and the approach to implementation and outlines some of its benefits to collaborative teachers and learners
The reduced cost of providing a nationally recognised service for familial hypercholesterolaemia
OBJECTIVE: Familial hypercholesterolaemia (FH) affects 1 in 500 people in the UK population and is associated with premature morbidity and mortality from coronary heart disease. In 2008, National Institute for Health and Care Excellence (NICE) recommended genetic testing of potential FH index cases and cascade testing of their relatives. Commissioners have been slow to respond although there is strong evidence of cost and clinical effectiveness. Our study quantifies the recent reduced cost of providing a FH service using generic atorvastatin and compares NICE costing estimates with three suggested alternative models of care (a specialist-led service, a dual model service where general practitioners (GPs) can access specialist advice, and a GP-led service).METHODS: Revision of existing 3?year costing template provided by NICE for FH services, and prediction of costs for running a programme over 10?years. Costs were modelled for the first population-based FH service in England which covers Southampton, Hampshire, Isle of Wight and Portsmouth (SHIP). Population 1.95 million.RESULTS: With expiry of the Lipitor (Pfizer atorvastatin) patent the cost of providing a 10-year FH service in SHIP reduces by 42.5% (£4.88 million on patent vs £2.80 million off patent). Further cost reductions are possible as a result of the reduced cost of DNA testing, more management in general practice, and lower referral rates to specialists. For instance a dual-care model with GP management of patients supported by specialist advice when required, costs £1.89 million.CONCLUSIONS: The three alternative models of care are now <50% of the cost of the original estimates undertaken by NICE
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