2,282 research outputs found
Wall Crossing from Dirac Zeromodes
We explore the physics of two-body decay of BPS states using semiclassical
analysis to construct explicit solutions that illustrate the main features of
wall crossing, for both ordinary and framed BPS states. In particular we
recover the primitive wall-crossing formula from an asymptotic analysis of
certain Dirac-type operators on monopole moduli spaces. Along the way we give
an asymptotic metric for the moduli space of singular monopoles, analogous to
the Gibbons-Manton and Lee-Weinberg-Yi metrics for the moduli space of smooth
monopoles, and we find evidence for the existence of stable non-BPS
boundstates. Our discussion applies to four-dimensional N = 2 super-Yang-Mills
theories with general gauge group and general 't Hooft defects.Comment: 32 pages plus appendices; 1 figure. v2: references added, typos
fixed. v3: references added, published versio
Spectral networks
We introduce new geometric objects called spectral networks. Spectral
networks are networks of trajectories on Riemann surfaces obeying certain local
rules. Spectral networks arise naturally in four-dimensional N=2 theories
coupled to surface defects, particularly the theories of class S. In these
theories spectral networks provide a useful tool for the computation of BPS
degeneracies: the network directly determines the degeneracies of solitons
living on the surface defect, which in turn determine the degeneracies for
particles living in the 4d bulk. Spectral networks also lead to a new map
between flat GL(K,C) connections on a two-dimensional surface C and flat
abelian connections on an appropriate branched cover Sigma of C. This
construction produces natural coordinate systems on moduli spaces of flat
GL(K,C) connections on C, which we conjecture are cluster coordinate systems.Comment: 87 pages, 48 figures; v2: typos, correction to general rule for signs
of BPS count
Sample preference for colorectal cancer screening tests: blood or stool?
Objective: Despite the high prevalence of CRC and the proven benefits of faecal sampling tests, participation rates in CRC screening are suboptimal. Literature has identified a number of barriers to participation, including faecal aversion. Emerging test technologies suggest blood-based molecular markers might provide an alternative, more acceptable option, for CRC screening tests. We aim to determine preference for blood compared to faeces as the sample for the screening test. Methods: A survey was mailed to 956 South Australians aged 50 to 74 years. Data were collected on sample preference, demographic variables, and ratings of screening test convenience and comfort. Results: The survey yielded a 43% response rate. The majority of participants preferred to provide a blood sample (78% v 22%, p < 0.001). Women were more likely to prefer blood than men (82% vs 74%, p = 0.05). Sample experience influenced preferences, with a significantly higher preference for faeces among participants with experience in faecal sampling (27% vs 17% with no experience, p < 0.05). Participants who preferred to provide a faecal sample rated it significantly more convenient (p < 0.001), more comfortable (p < 0.001), and more acceptable (p < 0.001) than those who preferred blood sampling. Conclusions: Survey participants overwhelmingly indicate a preference for the idea of a blood sample over a faecal sample for CRC screening. Preference was influenced by gender, experience with sampling method and the individualâs perception of sampling convenience, sampling comfort and sample acceptability. Our results suggest population participation rates are likely to improve with blood-based screening tests.Joanne M. Osborne, Carlene Wilson, Vivienne Moore, Tess Gregory, Ingrid Flight, Graeme P. Youn
Kondo Effect in Electromigrated Gold Break Junctions
We present gate-dependent transport measurements of Kondo impurities in bare
gold break junctions, generated with high yield using an electromigration
process that is actively controlled. Thirty percent of measured devices show
zero-bias conductance peaks. Temperature dependence suggests Kondo temperatures
\~7K. The peak splitting in magnetic field is consistent with theoretical
predictions for g=2, though in many devices the splitting is offset from 2guB
by a fixed energy. The Kondo resonances observed here may be due to
atomic-scale metallic grains formed during electromigration.Comment: 5 pages, 3 figure
Wall-Crossing in Coupled 2d-4d Systems
We introduce a new wall-crossing formula which combines and generalizes the
Cecotti-Vafa and Kontsevich-Soibelman formulas for supersymmetric 2d and 4d
systems respectively. This 2d-4d wall-crossing formula governs the
wall-crossing of BPS states in an N=2 supersymmetric 4d gauge theory coupled to
a supersymmetric surface defect. When the theory and defect are compactified on
a circle, we get a 3d theory with a supersymmetric line operator, corresponding
to a hyperholomorphic connection on a vector bundle over a hyperkahler space.
The 2d-4d wall-crossing formula can be interpreted as a smoothness condition
for this hyperholomorphic connection. We explain how the 2d-4d BPS spectrum can
be determined for 4d theories of class S, that is, for those theories obtained
by compactifying the six-dimensional (0,2) theory with a partial topological
twist on a punctured Riemann surface C. For such theories there are canonical
surface defects. We illustrate with several examples in the case of A_1
theories of class S. Finally, we indicate how our results can be used to
produce solutions to the A_1 Hitchin equations on the Riemann surface C.Comment: 170 pages, 45 figure
Exact and Asymptotic Degeneracies of Small Black Holes
We examine the recently proposed relations between black hole entropy and the
topological string in the context of type II/heterotic string dual models. We
consider the degeneracies of perturbative heterotic BPS states. In several
examples with N=4 and N=2 supersymmetry, we show that the macroscopic
degeneracy of small black holes agrees to all orders with the microscopic
degeneracy, but misses non-perturbative corrections which are computable on the
heterotic side. Using these examples we refine the previous proposals and
comment on their domain of validity as well as on the relevance of helicity
supertraces.Comment: 35pp. harvmac b-mode; v2 is substantially rewritten and includes new
results; v3 contains further clarifications, and some new results; v3: final
version to match published versio
T-Duality, and the K-Theoretic Partition Function of TypeIIA Superstring Theory
We study the partition function of type IIA string theory on 10-manifolds of
the form T^2 x X where X is 8-dimensional, compact, and spin. We pay particular
attention to the effects of the topological phases in the supergravity action
implied by the K-theoretic formulation of RR fields, and we use these to check
the T-duality invariance of the partition function. We find that the partition
function is only T-duality invariant when we take into account the T-duality
anomalies in the RR sector, the fermionic path integral (including 4-fermi
interaction terms), and 1-loop corrections including worldsheet instantons. We
comment on applications of our computation to speculations about the role of
the Romans mass in M-theory. We also discuss some issues which arise when one
attempts to extend these considerations to checking the full U-duality
invariance of the theory.Comment: 73 pages, harvmac, b-mod
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