437 research outputs found
Branched Coverings and Interacting Matrix Strings in Two Dimensions
We construct the lattice gauge theory of the group G_N, the semidirect
product of the permutation group S_N with U(1)^N, on an arbitrary Riemann
surface. This theory describes the branched coverings of a two-dimensional
target surface by strings carrying a U(1) gauge field on the world sheet. These
are the non-supersymmetric Matrix Strings that arise in the unitary gauge
quantization of a generalized two-dimensional Yang-Mills theory. By classifying
the irreducible representations of G_N, we give the most general formulation of
the lattice gauge theory of G_N, which includes arbitrary branching points on
the world sheet and describes the splitting and joining of strings.Comment: LaTeX2e, 25 pages, 4 figure
Exact solution of discrete two-dimensional R gravity
We exactly solve a special matrix model of dually weighted planar graphs describing pure two-dimensional quantum gravity with a R^2 interaction in order to study the intermediate regimes between the gravitating and flat metric. The flat space is modeled by a regular square lattice, while localized curvature is being introduced through defects of the lattice. No ``flattening'' phase transition is found with respect to the R^2 coupling: the infrared behaviour of the system is that of pure gravity for any finite R^2 coupling. In the limit of infinite coupling, we are able to extract a scaling function interpolating between pure gravity and a phase of a dilute gas of curvature defects on a flat background. We introduce and explain some novel techniques concerning our method of large N character expansions and the calculation of Schur characters on big Young tableaux
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Ultra-processed food consumption in Barbados: evidence from a nationally representative, cross-sectional study.
Our objective was to describe, for the first time in an English-speaking Caribbean country, the contribution of ultra-processed foods (UPFs) to nutrients linked to non-communicable disease. Using a cross-sectional study design, dietary data were collected from two non-consecutive 24-h dietary recalls. Recorded food items were then classified according to their degree of processing by the NOVA system. The present study took place in Barbados (2012-13). A representative population-based sample of 364 adult Barbadians (161 males and 203 females) aged 25-64 years participated in the study. UPFs represented 40⋅5 % (838 kcal/d; 95 % CI 791, 885) of mean energy intake. Sugar-sweetened beverages made the largest contribution to energy within the UPF category. Younger persons (25-44 years) consumed a significantly higher proportion of calories from UPF (NOVA group 4) compared with older persons (45-64 years). The mean energy shares of UPF ranged from 22⋅0 to 58⋅9 % for those in the lowest tertile to highest tertile. Within each tertile, the energy contribution was significantly higher in the younger age group (25-44 years) compared with the older (45-64 years). One-quarter of persons consume ≥50 % of their daily calories from UPF, this being significantly higher in younger persons. The ultra-processed diet fraction contained about six times the mean of free sugars and about 0⋅8 times the dietary fibre of the non-ultra-processed fraction (NOVA groups 1-3). Targeted interventions to decrease the consumption of UPF especially in younger persons is thus of high priority to improve the diet quality of Barbadians.This work was supported by the Ministry of Health and Wellness of the Government of Barbados
Matrix Theory Interpretation of DLCQ String Worldsheets
We study the null compactification of type-IIA-string perturbation theory at
finite temperature. We prove a theorem about Riemann surfaces establishing that
the moduli spaces of infinite-momentum-frame superstring worldsheets are
identical to those of branched-cover instantons in the matrix-string model
conjectured to describe M-theory. This means that the identification of string
degrees of freedom in the matrix model proposed by Dijkgraaf, Verlinde and
Verlinde is correct and that its natural generalization produces the moduli
space of Riemann surfaces at all orders in the genus expansion.Comment: 9 pages LaTeX. Changes in style, typos correcte
Fuzzy Spheres in pp Wave Matrix String Theory
The behaviour of matrix string theory in the background of a type IIA pp wave
at small string coupling, g_s << 1, is determined by the combination M g_s
where M is a dimensionless parameter proportional to the strength of the
Ramond-Ramond background. For M g_s << 1, the matrix string theory is
conventional; only the degrees of freedom in the Cartan subalgebra contribute,
and the theory reduces to copies of the perturbative string. For M g_s >> 1,
the theory admits degenerate vacua representing fundamental strings blown up
into fuzzy spheres with nonzero lightcone momenta. We determine the spectrum of
small fluctuations around these vacua. Around such a vacuum all N-squared
degrees of freedom are excited with comparable energies. The spectrum of masses
has a spacing which is independent of the radius of the fuzzy sphere, in
agreement with expected behaviour of continuum giant gravitons. Furthermore,
for fuzzy spheres characterized by reducible representations of SU(2) and
vanishing Wilson lines, the boundary conditions on the field are characterized
by a set of continuous angles which shows that generically the blown up strings
do not ``close''.Comment: 45 pages REVTeX 4 and AMSLaTeX. 1 figure. v2: references added.
Figure redrawn using LaTe
Constraints on a Massive Dirac Neutrino Model
We examine constraints on a simple neutrino model in which there are three
massless and three massive Dirac neutrinos and in which the left handed
neutrinos are linear combinations of doublet and singlet neutrinos. We examine
constraints from direct decays into heavy neutrinos, indirect effects on
electroweak parameters, and flavor changing processes. We combine these
constraints to examine the allowed mass range for the heavy neutrinos of each
of the three generations.Comment: latex, 29 pages, 7 figures (not included), MIT-CTP-221
Black Holes and the SYM Phase Diagram
Making combined use of the Matrix and Maldacena conjectures, the relation
between various thermodynamic transitions in super Yang-Mills (SYM) and
supergravity is clarified. The thermodynamic phase diagram of an object in DLCQ
M-theory in four and five non-compact space dimensions is constructed; matrix
strings, matrix black holes, and black -branes are among the various phases.
Critical manifolds are characterized by the principles of correspondence and
longitudinal localization, and a triple point is identified. The microscopic
dynamics of the Matrix string near two of the transitions is studied; we
identify a signature of black hole formation from SYM physics.Comment: 36 pages, latex; 6 eps figure
Capture and enrichment of CD34-positive haematopoietic stem and progenitor cells from blood circulation using P-selectin in an implantable device
Clinical infusion of haematopoietic stem and progenitor cells (HSPCs) is vital for restoration of haematopoietic function in many cancer patients. Previously, we have demonstrated an ability to mimic physiological cell trafficking in order to capture CD34-positive (CD34+) HSPCs using monolayers of the cell adhesion protein P-selectin in flow chambers. The current study aimed to determine if HSPCs could be captured directly from circulating blood in vivo. Vascular shunt prototypes, coated internally with P-selectin, were inserted into the femoral artery of rats. Blood flow through the cell capture device resulted in a wall shear stress of 4–6 dynes/cm2. After 1-h blood perfusion, immunofluorescence microscopy and flow cytometric analysis revealed successful capture of mononuclear cells positive for the HSPC surface marker CD34. Purity of captured CD34+ cells showed sevenfold enrichment over levels found in whole blood, with an average purity of 28%. Robust cell capture and HSPC enrichment were also demonstrated in devices that were implanted in a closed-loop arterio-venous shunt conformation for 2 h. Adherent cells were viable in culture and able to differentiate into burst-forming units. This study demonstrated an ability to mimic the physiological arrest of HSPCs from blood in an implantable device and may represent a practical alternative for adult stem cell capture and enrichment
Extending Sensitivity for Low-Mass Neutral Heavy Lepton Searches
We point out the importance of two-body final states of weak isosinglet
neutral heavy leptons predicted in several models of new physics beyond the
standard model. We concentrate on muon-type neutral heavy leptons
with mass GeV which can be searched for with increased sensitivity at a
new round of neutrino experiments at CERN and Fermilab. Providing explicit
decay rate formulae for the , , , ,
, and final states, we use general scaling features to
estimate sensitivity of searches in current and future experiments,
emphasizing the importance of the decay mode.Comment: 14 pages, 8 figure
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