24 research outputs found
D0 Matrix Mechanics: New Fuzzy Solutions at Large N
We wish to consider in this report the large N limit of a particular matrix
model introduced by Myers describing D-brane physics in the presence of an RR
flux background. At finite N, fuzzy spheres appear naturally as non-trivial
solutions to this matrix model and have been extensively studied. In this
report, we wish to demonstrate several new classes of solutions which appear in
the large N limit, corresponding to the fuzzy cylinder,the fuzzy plane and a
warped fuzzy plane. The latter two solutions arise from a possible "central
extension" to our model that arises after we account for non-trivial issues
involved in the large N limit. As is the case for finite N, these new solutions
are to be interpreted as constituent D0-branes forming D2 bound states
describing new fuzzy geometries.Comment: revised version: references added, derivation of "central extensions"
improved upon. To appear in JHE
Matrix dynamics of fuzzy spheres
We study the dynamics of fuzzy two-spheres in a matrix model which represents
string theory in the presence of RR flux. We analyze the stability of known
static solutions of such a theory which contain commuting matrices and SU(2)
representations. We find that irreducible as well as reducible representations
are stable. Since the latter are of higher energy, this stability poses a
puzzle. We resolve this puzzle by noting that reducible representations have
marginal directions corresponding to non-spherical deformations. We obtain new
static solutions by turning on these marginal deformations. These solutions now
have instability or tachyonic directions. We discuss condensation of these
tachyons which correspond to classical trajectories interpolating from
multiple, small fuzzy spheres to a single, large sphere. We briefly discuss
spatially independent configurations of a D3/D5 system described by the same
matrix model which now possesses a supergravity dual.Comment: 26 pages, 3 figures, uses JHEP.cls; (v2) references adde
D-branes in a Big Bang/Big Crunch Universe: Nappi-Witten Gauged WZW Model
We study D-branes in the Nappi-Witten model, which is a gauged WZW model
based on (SL(2,R) x SU(2)) / (U(1) x U(1)). The model describes a four
dimensional space-time consisting of cosmological regions with big bang/big
crunch singularities and static regions with closed time-like curves. The aim
of this paper is to investigate by D-brane probes whether there are pathologies
associated with the cosmological singularities and the closed time-like curves.
We first classify D-branes in a group theoretical way, and then examine DBI
actions for effective theories on the D-branes. In particular, we show that
D-brane metric from the DBI action does not include singularities, and wave
functions on the D-branes are well behaved even in the presence of closed
time-like curves.Comment: 50 pages, 2 figures, minor change
On the problem of the justification of river rights
This article aims to work out the social conditions that determine whether the
communication of river rights finds success in society. Employing the context of
hydropower development in the Mekong region, the article finds that an essentialist
strategy which claims that river rights have unlimited âmoralâ validity regardless of any
of the decision consequences is unlikely to succeed. Instead, it is proposed that moral
conflicts over river rights may ultimately only be resolvable âunmorallyâ, that is, by
procedural legitimacy â and this is best captured by employing a methodological
framework composed of thematic, social and temporal dimension
Prognostic utility of sestamibi lung uptake does not require adjustment for stress-related variables: A retrospective cohort study
BACKGROUND: Increased (99m)Tc-sestamibi stress lung-to-heart ratio (sLHR) has been shown to predict cardiac outcomes similar to pulmonary uptake of thallium. Peak heart rate and use of pharmacologic stress affect the interpretation of lung thallium uptake. The current study was performed to determine whether (99m)Tc-sestamibi sLHR measurements are affected by stress-related variables, and whether this in turn affects prognostic utility. METHODS: sLHR was determined in 718 patients undergoing (99m)Tc-sestamibi SPECT stress imaging. sLHR was assessed in relation to demographics, hemodynamic variables and outcomes (mean follow up 5.6 ± 1.1 years). RESULTS: Mean sLHR was slightly greater in males than in females (P < 0.01) and also showed a weak negative correlation with age (P < 0.01) and systolic blood pressure (P < 0.01), but was unrelated to stress method or heart rate at the time of injection. In patients undergoing treadmill exercise, sLHR was also positively correlated with peak workload (P < 0.05) but inversely with double product (P < 0.05). The combined explanatory effect of sex, age and hemodynamic variables on sLHR was less than 10%. The risk of acute myocardial infarction (AMI) or death increased by a factor of 1.7â1.8 for each SD increase in unadjusted sLHR, and was unaffected by adjustment for sex, age and hemodynamic variables (hazard ratios 1.6â1.7). The area under the ROC curve for the unadjusted sLHR was 0.65 (95% CI 0.59â0.71, P < 0.0001) and was unchanged for the adjusted sLHR (0.65, 95% CI 0.61â0.72, P < 0.0001). CONCLUSION: Stress-related variables have only a weak effect on measured sLHR. Unadjusted and adjusted sLHR provide equivalent prognostic information for prediction of AMI or death
Update On The Triumf Central Control System
The continuing evolution of the TRIUMF Central Control System is currently subject to three major influences. Most imperative is the need to replace the obsolete Nova computers that still handle much of the routine cyclotron operation. Smooth integration of a number of isolated control systems into the CCS is another focus. The third major force arises from the requirements of developing new projects in a system that has reached serious expansion constraints. The plan for phasing out the Novas, integrating the orphaned systems and allowing expansion of the CCS in a smooth fashion is discussed. This is a conservative approach, relying on the strengths of the present hardware configuration, the substantial financial investment in existing equipment, and the expertise of available personnel, while meeting the present and future requirements. Details of the modified and expanded hardware configuration are described. This review also briefly mentions new support for device access, X-window ..