1,215 research outputs found
Nonrandom Mixing Models of HIV Transmission
Models of HIV transmission and the AIDS epidemic generally assume random mixing among those infected with HIV and those who
are not. For sexually transmitted HIV, this implies that individuals select sex partners without regard to attributes such as familiarity,
attractiveness, or risk of infection. This paper formulates a model for examining the impact of nonrandom mixing on HIV transmission. We
present threshold conditions that determine when HIV epidemics can occur within the framework of this model. Nonrandom mixing is
introduced by assuming that sexually active individuals select sex partners to minimize the risk of infection. In addition to variability in risky
sex rates, some versions of our model allow for error (or noise) in information exchanged between prospective partners. We investigate
several models including random partner selection (or proportionate mixing), segregation of the population by risky sex rates, a probabilistic
combination of segregation and random selection induced by imperfect information (or preferred mixing), and a model of costly search with
perfect information. We develop examples which show that nonrandom mixing can lead to epidemics that are more severe or less severe than
random mixing. For reasonable parameter choices describing the AIDS epidemic, however, the results suggest that random mixing models
overstate the number of HIV infections that will occur
Scanning-probe spectroscopy of semiconductor donor molecules
Semiconductor devices continue to press into the nanoscale regime, and new
applications have emerged for which the quantum properties of dopant atoms act
as the functional part of the device, underscoring the necessity to probe the
quantum structure of small numbers of dopant atoms in semiconductors[1-3].
Although dopant properties are well-understood with respect to bulk
semiconductors, new questions arise in nanosystems. For example, the quantum
energy levels of dopants will be affected by the proximity of nanometer-scale
electrodes. Moreover, because shallow donors and acceptors are analogous to
hydrogen atoms, experiments on small numbers of dopants have the potential to
be a testing ground for fundamental questions of atomic and molecular physics,
such as the maximum negative ionization of a molecule with a given number of
positive ions[4,5]. Electron tunneling spectroscopy through isolated dopants
has been observed in transport studies[6,7]. In addition, Geim and coworkers
identified resonances due to two closely spaced donors, effectively forming
donor molecules[8]. Here we present capacitance spectroscopy measurements of
silicon donors in a gallium-arsenide heterostructure using a scanning probe
technique[9,10]. In contrast to the work of Geim et al., our data show
discernible peaks attributed to successive electrons entering the molecules.
Hence this work represents the first addition spectrum measurement of dopant
molecules. More generally, to the best of our knowledge, this study is the
first example of single-electron capacitance spectroscopy performed directly
with a scanning probe tip[9].Comment: In press, Nature Physics. Original manuscript posted here; 16 pages,
3 figures, 5 supplementary figure
Flavor in Minimal Conformal Technicolor
We construct a complete, realistic, and natural UV completion of minimal
conformal technicolor that explains the origin of quark and lepton masses and
mixing angles. As in "bosonic technicolor", we embed conformal technicolor in a
supersymmetric theory, with supersymmetry broken at a high scale. The exchange
of heavy scalar doublets generates higher-dimension interactions between
technifermions and quarks and leptons that give rise to quark and lepton masses
at the TeV scale. Obtaining a sufficiently large top quark mass requires strong
dynamics at the supersymmetry breaking scale in both the top and technicolor
sectors. This is natural if the theory above the supersymmetry breaking also
has strong conformal dynamics. We present two models in which the strong top
dynamics is realized in different ways. In both models, constraints from
flavor-changing effects can be easily satisfied. The effective theory below the
supersymmetry breaking scale is minimal conformal technicolor with an
additional light technicolor gaugino. We argue that this light gaugino is a
general consequence of conformal technicolor embedded into a supersymmetric
theory. If the gaugino has mass below the TeV scale it will give rise to an
additional pseudo Nambu-Goldstone boson that is observable at the LHC.Comment: 37 pages; references adde
Current measurement by real-time counting of single electrons
The fact that electrical current is carried by individual charges has been
known for over 100 years, yet this discreteness has not been directly observed
so far. Almost all current measurements involve measuring the voltage drop
across a resistor, using Ohm's law, in which the discrete nature of charge does
not come into play. However, by sending a direct current through a
microelectronic circuit with a chain of islands connected by small tunnel
junctions, the individual electrons can be observed one by one. The quantum
mechanical tunnelling of single charges in this one-dimensional array is time
correlated, and consequently the detected signal has the average frequency
f=I/e, where I is the current and e is the electron charge. Here we report a
direct observation of these time-correlated single-electron tunnelling
oscillations, and show electron counting in the range 5 fA-1 pA. This
represents a fundamentally new way to measure extremely small currents, without
offset or drift. Moreover, our current measurement, which is based on electron
counting, is self-calibrated, as the measured frequency is related to the
current only by a natural constant.Comment: 9 pages, 4 figures; v2: minor revisions, 2 refs added, words added to
title, typos correcte
Socioeconomic Position, Not Race, Is Linked to Death After Cardiac Surgery
BackgroundâHealth disparities have been associated with the prevalence of cardiovascular disease. In cardiac surgery,
association has been found between race, sex, and poorer prognosis after surgery. However, there is a complex interplay
between race, sex, and socioeconomic position (SEP). In our investigation we sought to identify which of these was the
driver of risk-adjusted survival.
Methods and ResultsâFrom January 1, 1995, and December 30, 2005, 23 330 patients (15 156 white men, 6932 white
women, 678 black men, and 564 black women) underwent isolated coronary artery bypass grafting, valve, or combined
coronary artery bypass grafting and valve procedures. Median follow-up was 5.8 years (25th and 75th percentiles: 3 and
8.6 years). Effect of race, sex, and SEP on all-cause mortality was examined with 2-phase Cox model and generalized
propensity score technique. As expected, blacks and women had lower SEP as compared with whites and men for all
6 SEP indicators. Patients with lower SEP had more atherosclerotic disease burden, more comorbidity, and were more
symptomatic. Lower SEP was associated with a risk-adjusted dose-dependent reduction in survival after surgery (men,
P 0.0001; women, P 0.0079), but black race, once adjusted for SEP, was not.
ConclusionsâOur large investigation demonstrates that disparities in SEP are present and significantly affect health
outcomes. Although race per se was not the driver for reduced survival, patients of low SEP were predominantly
represented by blacks and women. Socioeconomically disadvantaged patients had significantly higher risk-adjusted
mortality after surgery. Further investigation and targeted intervention should focus specifically on patients of low SEP,
their health behaviors, and secondary prevention efforts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/79037/1/Koch_et_al.pd
Electrical detection of 31P spin quantum states
In recent years, a variety of solid-state qubits has been realized, including
quantum dots, superconducting tunnel junctions and point defects. Due to its
potential compatibility with existing microelectronics, the proposal by Kane
based on phosphorus donors in Si has also been pursued intensively. A key issue
of this concept is the readout of the P quantum state. While electrical
measurements of magnetic resonance have been performed on single spins, the
statistical nature of these experiments based on random telegraph noise
measurements has impeded the readout of single spin states. In this letter, we
demonstrate the measurement of the spin state of P donor electrons in silicon
and the observation of Rabi flops by purely electric means, accomplished by
coherent manipulation of spin-dependent charge carrier recombination between
the P donor and paramagnetic localized states at the Si/SiO2 interface via
pulsed electrically detected magnetic resonance. The electron spin information
is shown to be coupled through the hyperfine interaction with the P nucleus,
which demonstrates the feasibility of a recombination-based readout of nuclear
spins
Minimal Conformal Technicolor and Precision Electroweak Tests
We study the minimal model of conformal technicolor, an SU(2) gauge theory
near a strongly coupled conformal fixed point, with conformal symmetry softly
broken by technifermion mass terms. Conformal symmetry breaking triggers chiral
symmetry breaking in the pattern SU(4) -> Sp(4), which gives rise to a
pseudo-Nambu-Goldstone boson that can act as a composite Higgs boson. The top
quark is elementary, and the top and electroweak gauge loop contributions to
the Higgs mass are cut off entirely by Higgs compositeness. In particular, the
model requires no top partners and no "little Higgs" mechanism. A nontrivial
vacuum alignment results from the interplay of the top loop and technifermion
mass terms. The composite Higgs mass is completely determined by the top loop,
in the sense that m_h/m_t is independent of the vacuum alignment and is
computable by a strong-coupling calculation. There is an additional composite
pseudoscalar A with mass larger than m_h and suppressed direct production at
LHC. We discuss the electroweak fit in this model in detail. Corrections to Z
-> bb and the T parameter from the top sector are suppressed by the enhanced
Sp(4) custodial symmetry. Even assuming that the strong contribution to the S
parameter is positive and usuppressed, a good electroweak fit can be obtained
for v/f ~ 0.25, where v and f are the electroweak and chiral symmetry breaking
scales respectively. This requires fine tuning at the 10% level.Comment: 34 pages, 4 figures; v2: updated precision electroweak fi
Developing and implementing an integrated delirium prevention system of care:a theory driven, participatory research study
Background: Delirium is a common complication for older people in hospital. Evidence suggests that delirium incidence in hospital may be reduced by about a third through a multi-component intervention targeted at known modifiable risk factors. We describe the research design and conceptual framework underpinning it that informed the development of a novel delirium prevention system of care for acute hospital wards. Particular focus of the study was on developing an implementation process aimed at embedding practice change within routine care delivery. Methods: We adopted a participatory action research approach involving staff, volunteers, and patient and carer representatives in three northern NHS Trusts in England. We employed Normalization Process Theory to explore knowledge and ward practices on delirium and delirium prevention. We established a Development Team in each Trust comprising senior and frontline staff from selected wards, and others with a potential role or interest in delirium prevention. Data collection included facilitated workshops, relevant documents/records, qualitative one-to-one interviews and focus groups with multiple stakeholders and observation of ward practices. We used grounded theory strategies in analysing and synthesising data. Results: Awareness of delirium was variable among staff with no attention on delirium prevention at any level; delirium prevention was typically neither understood nor perceived as meaningful. The busy, chaotic and challenging ward life rhythm focused primarily on diagnostics, clinical observations and treatment. Ward practices pertinent to delirium prevention were undertaken inconsistently. Staff welcomed the possibility of volunteers being engaged in delirium prevention work, but existing systems for volunteer support were viewed as a barrier. Our evolving conception of an integrated model of delirium prevention presented major implementation challenges flowing from minimal understanding of delirium prevention and securing engagement of volunteers alongside practice change. The resulting Prevention of Delirium (POD) Programme combines a multi-component delirium prevention and implementation process, incorporating systems and mechanisms to introduce and embed delirium prevention into routine ward practices. Conclusions: Although our substantive interest was in delirium prevention, the conceptual and methodological strategies pursued have implications for implementing and sustaining practice and service improvements more broadly
The cost of changing physical activity behaviour: Evidence from a "physical activity pathway" in the primary care setting
Copyright @ 2011 Boehler et al.BACKGROUND: The âPhysical Activity Care Pathwayâ (a Pilot for the âLetâs Get Movingâ policy) is a systematic approach to integrating physical activity promotion into the primary care setting. It combines several methods reported to support behavioural change, including brief interventions, motivational interviewing, goal setting, providing written resources, and follow-up support. This paper compares costs falling on the UK National Health Service (NHS) of implementing the care pathway using two different recruitment strategies and provides initial insights into the cost of changing physical activity behaviour. METHODS: A combination of a time driven variant of activity based costing, audit data through EMIS and a survey of practice managers provided patient-level cost data for 411 screened individuals. Self reported physical activity data of 70 people completing the care pathway at three month was compared with baseline using a regression based âdifference in differencesâ approach. Deterministic and probabilistic sensitivity analyses in combination with hypothesis testing were used to judge how robust findings are to key assumptions and to assess the uncertainty around estimates of the cost of changing physical activity behaviour. RESULTS: It cost ÂŁ53 (SD 7.8) per patient completing the PACP in opportunistic centres and ÂŁ191 (SD 39) at disease register sites. The completer rate was higher in disease register centres (27.3% vs. 16.2%) and the difference in differences in time spent on physical activity was 81.32 (SE 17.16) minutes/week in patients completing the PACP; so that the incremental cost of converting one sedentary adult to an âactive stateâ of 150 minutes of moderate intensity physical activity per week amounts to ÂŁ 886.50 in disease register practices, compared to opportunistic screening. CONCLUSIONS: Disease register screening is more costly than opportunistic patient recruitment. However, additional costs come with a higher completion rate and better outcomes in terms of behavioural change in patients completing the care pathway. Further research is needed to rigorously evaluate intervention efficiency and to assess the link between behavioural change and changes in quality adjusted life years (QALYs).This article is available through the Brunel Open Access Publishing Fund
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