114 research outputs found
World-sheet duality for D-branes with travelling waves
We study D-branes with plane waves of arbitrary profiles as examples of
time-dependent backgrounds in string theory. We show how to reproduce the
quantum mechanical (one-to-one) open-string S-matrix starting from the
closed-string boundary state for the D-branes, thereby establishing the channel
duality of this calculation. The required Wick rotation to a Lorentzian
worldsheet singles out as 'prefered' time coordinate the open-string light-cone
time.Comment: 17 pages, Latex file, uses JHEP3.cls, two figures. Added references
and corrected two typo
Open strings in relativistic ion traps
Electromagnetic plane waves provide examples of time-dependent open string
backgrounds free of corrections. The solvable case of open strings in
a quadrupolar wave front, analogous to pp-waves for closed strings, is
discussed. In light-cone gauge, it leads to non-conformal boundary conditions
similar to those induced by tachyon condensates. A maximum electric gradient is
found, at which macroscopic strings with vanishing tension are pair-produced --
a non-relativistic analogue of the Born-Infeld critical electric field. Kinetic
instabilities of quadrupolar electric fields are cured by standard atomic
physics techniques, and do not interfere with the former dynamic instability. A
new example of non-conformal open-closed duality is found. Propagation of open
strings in time-dependent wave fronts is discussed.Comment: 43 pages, 11 figures, Latex2e, JHEP3.cls style; v2: one-loop
amplitude corrected, open-closed duality proved, refs added, miscellaneous
improvements, see historical note in fil
Delivering a sustainable trauma management training programme tailored for low-resource settings in East, Central and Southern African countries using a cascading course model
Background: Injuries cause five million deaths and 279 Disability Adjusted Life Years (DALYS) each year worldwide. The COSECSA Oxford Orthopaedic Link (COOL) is a multi-country partnership programme that has delivered training in trauma management to nine sub-Saharan countries across a wide-cadre of health-workers using a model of "primary" courses delivered by UK instructors, followed by "cascading" courses led by local faculty. This study examines the impact on knowledge and clinical confidence among health-workers, and compares the performance of "cascading" and "primary" courses delivered in low-resource settings. Methods: Data was collated from 1030 candidates (119 Clinical Officers, 540 Doctors, 260 Nurses and 111 Medical Students) trained over 28 courses (9 "primary" and 19 "cascading" courses) in nine sub-Saharan countries between 2012 and 2013. Knowledge and clinical confidence of candidates were assessed using pre- and post-course MCQs and confidence matrix rating of clinical scenarios. Changes were measured in relation to co-variants of gender, job roles and primary versus cascading courses. Multivariate regression modelling and cost analysis was performed to examine the impact of primary versus cascading courses on candidates' performance. Findings: There was a significant improvement in knowledge (58% to 77%, p < 0.05) and clinical confidence (68% to 90%, p < 0.05) post-course. "Non-doctors" demonstrated a greater improvement in knowledge (22%) and confidence (24%) following the course (p < 0.05). The degree of improvement of MCQ scores differed significantly, with the cascading courses (21%) outperforming primary courses (15%) (p < 0.002). This is further supported by multivariate regression modelling where cascading courses are a strong predictor for improvement in MCQ scores (Coef = 4.83, p < 0.05). Interpretation: Trauma management training of health-workers plays a pivotal role in tackling the ever-growing trauma burden in Africa. Our study suggests cascading PTC courses may be an effective model in delivering trauma training in low-resource settings, however further studies are required to determine its efficacy in improving clinical competence and retention of knowledge and skills in the long term
Noncommutative solitons on Kahler manifolds
We construct a new class of scalar noncommutative multi-solitons on an
arbitrary Kahler manifold by using Berezin's geometric approach to quantization
and its generalization to deformation quantization. We analyze the stability
condition which arises from the leading 1/hbar correction to the soliton energy
and for homogeneous Kahler manifolds obtain that the stable solitons are given
in terms of generalized coherent states. We apply this general formalism to a
number of examples, which include the sphere, hyperbolic plane, torus and
general symmetric bounded domains. As a general feature we notice that on
homogeneous manifolds of positive curvature, solitons tend to attract each
other, while if the curvature is negative they will repel each other.
Applications of these results are discussed.Comment: 26 pages, 3 figures, harvmac; references adde
GNAQ and GNA11 mutations and downstream YAP activation in choroidal nevi
Background: Mutations in GNAQ/11 genes are considered an early event in the development of uveal melanoma that may derive from a pre-existing nevus. The Hippo pathway, by way of YAP activation, rather than MAP kinase, has a role in the oncogenic capacity of GNAQ/11 mutations.Methods: We investigated 16 nevi from 13 human eyes for driver GNAQ/11 mutations using droplet digital PCR and determined whether nevi are clonal by quantifying mutant nevus cell fractions. Immunohistochemistry was performed on 15 nevi to analyse YAP activation.Results: For 15 out of 16 nevi, a GNAQ/11 mutation was detected in the nevus cells albeit at a low frequency with a median of 13%. Nuclear YAP, a transcriptional co-activator in the Hippo tumour-suppressor pathway, was detected in 14/15 nevi.Conclusions: Our analysis suggests that a mutation in GNAQ/11 occurs in a subset of choroidal nevus cells. We hypothesise that GNAQ/11 mutant-driven extracellular mitogenic signalling involving YAP activation leads to accumulation of wild-type nevus cells
Non-commutative Branes from M-theory
The analysis of the worldvolume effective actions of the M-theory
Kaluza-Klein monopole and 9-brane suggests that it should be possible to
describe non-abelian configurations of M2-branes or M5-branes if the M2-branes
are transverse to the eleventh direction and the M5-branes are wrapped on it.
This is determined by the fact that the Kaluza-Klein monopole and the M9-brane
are constrained to move in particular isometric spacetimes. We show that the
same kind of situation is implied by the analysis of the brane descent
relations in M-theory. We compute some of the non-commutative couplings of the
worldvolume effective actions of these non-abelian systems of M2 and M5 branes
and show that they indicate the existence of configurations corresponding to N
branes expanding into a higher dimensional M-brane. The reduction to Type II
brings up new descriptions of coincident D-branes at strong coupling. We show
that these systems have the right non-commutative charges to describe certain
expanded configurations playing a role in the framework of the AdS/CFT
correspondence. Finally, we discuss the realization of non-commutative brane
configurations as topological solitons in non-abelian brane-antibrane systems.Comment: 31 pages, latex file, references adde
Time Evolution in Open String Theory
We discuss a general iterative procedure for constructing time dependent
solutions in open string theory describing rolling of a generic tachyon field
away from its maximum. These solutions are characterized by two parameters
labelling the initial position and velocity of the tachyon field, but one of
these parameters can be removed by using time translation symmetry. The Wick
rotated version of the resulting one parameter family of inequivalent solutions
describes a one parameter family of boundary conformal field theories, each
member of which is related to the boundary conformal field theory describing
the original D-brane system by a nearly marginal deformation. We apply this
technique to construct a time dependent solution on a D-brane in bosonic string
theory which can be interpreted as the creation of a lower dimensional brane
during the decay of an unstable D-brane.Comment: LaTeX file, 35 pages, 2 figures, discussion on multiple rolling
tachyons expanded to include the general case, references added; v4: typos
correcte
Reproducibility in the absence of selective reporting : An illustration from large-scale brain asymmetry research
Altres ajuts: Max Planck Society (Germany).The problem of poor reproducibility of scientific findings has received much attention over recent years, in a variety of fields including psychology and neuroscience. The problem has been partly attributed to publication bias and unwanted practices such as p-hacking. Low statistical power in individual studies is also understood to be an important factor. In a recent multisite collaborative study, we mapped brain anatomical left-right asymmetries for regional measures of surface area and cortical thickness, in 99 MRI datasets from around the world, for a total of over 17,000 participants. In the present study, we revisited these hemispheric effects from the perspective of reproducibility. Within each dataset, we considered that an effect had been reproduced when it matched the meta-analytic effect from the 98 other datasets, in terms of effect direction and significance threshold. In this sense, the results within each dataset were viewed as coming from separate studies in an "ideal publishing environment," that is, free from selective reporting and p hacking. We found an average reproducibility rate of 63.2% (SD = 22.9%, min = 22.2%, max = 97.0%). As expected, reproducibility was higher for larger effects and in larger datasets. Reproducibility was not obviously related to the age of participants, scanner field strength, FreeSurfer software version, cortical regional measurement reliability, or regional size. These findings constitute an empirical illustration of reproducibility in the absence of publication bias or p hacking, when assessing realistic biological effects in heterogeneous neuroscience data, and given typically-used sample sizes
All-sky search for long-duration gravitational wave transients with initial LIGO
We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10-500 s in a frequency band of 40-1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10-5 and 9.4×10-4 Mpc-3 yr-1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves. © 2016 American Physical Society
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