314 research outputs found
M-Theory and Two-Dimensional Effective Dynamics
We calculate from M-theory the two-dimensional low energy effective dynamics
of various brane configurations. In the first part we study configurations that
have a dual description in type IIA string theory as two-dimensional (4,0)
Yang-Mills theories with gauge group SU(N_1)xSU(N_2) and chiral fermions in the
bi-fundamental representation. In the second part we derive related equations
of motion which describe the low energy internal dynamics of a supersymmetric
black hole in four-dimensional N=1 supergravity, obtained as an M-fivebrane
wrapped on a complex four-cycle.Comment: Two references added. To appear in Nucl. Phys.
Monopole Dynamics from the M-Fivebrane
We study the BPS states of the M-fivebrane which correspond to monopoles of
N=2 SU(2) gauge theory. Far away from the centres of the monopoles these states
may be viewed as solitons in the Seiberg-Witten effective action. It is argued
that these solutions are smooth and some properties of their moduli space are
discussed.Comment: 30 pages phyzzx. Paragraph added to the introduction and some minor
corrections. To appear in Nucl. Phys.
D-Branes in the Green-Schwarz Formalism
We give a basic account of supersymmetric open strings and D-branes using the
Green-Schwarz formalism, obtaining a manifestly spacetime supersymmetric
description of their spectrum. In addition we discuss a mechanism whereby some
of the D-brane states are projected out and which can lead to chiral quantum
field theories on the brane.Comment: 13 pages phyzzx. Reference added, to appear in Phys. Lett.
Gauge Fields and M-Fivebrane Dynamics
In this paper we obtain both the vector and scalar equations of motion of an
M-fivebrane in the presence of threebrane solitons. The resulting equations of
motion are precisely those obtained from the Seiberg-Witten low energy
effective action for N=2 Yang-Mills, including all quantum corrections. This
analysis extends the work of a previous paper which derived the scalar
equations of motion but not in detail the vector equations. We also discuss
some features of an infinite number of higher derivative terms predicted by M
theory.Comment: 27 pages, Phyzzx. Minor corrections and a reference added. To appear
in NP
Coset Symmetries in Dimensionally Reduced Bosonic String Theory
We discuss the dimensional reduction of various effective actions,
particularly that of the closed Bosonic string and pure gravity, to two and
three dimensions. The result for the closed Bosonic string leads to coset
symmetries which are in agreement with those recently predicted and argued to
be present in a new unreduced formulation of this theory. We also show that
part of the Geroch group appears in the unreduced duality symmetric formulation
of gravity recently proposed. We conjecture that this formulation can be
extended to a non-linear realisation based on a Kac-Moody algebra which we
identify. We also briefly discuss the proposed action of Bosonic M-theory.Comment: Reference adde
MO1 A Guide to Adjusting Survival Time Estimates to Account for Treatment Switching in Randomised Controlled Trials
Assessing methods for dealing with treatment crossover in clinical trials: A follow-up simulation study
Background: Treatment switching commonly occurs in clinical trials of novel interventions, particularly in the advanced or metastatic cancer setting, which causes important problems for health technology assessment. Previous research has demonstrated which adjustment methods are suitable in specific scenarios, but scenarios considered have been limited.
Objectives: We aimed to assess statistical approaches for adjusting survival estimates in the presence of treatment switching in order to determine which methods are most appropriate in a new range of realistic scenarios, building upon previous research. In particular we consider smaller sample sizes, reduced switching proportions, increased levels of censoring, and alternative data generating models.
Methods: We conducted a simulation study to assess the bias, mean squared error and coverage associated with alternative switching adjustment methods across a wide range of realistic scenarios.
Results: Our results generally supported those found in previous research, but the novel scenarios considered meant that we could make conclusions based upon a more robust evidence base. Simple methods such as censoring or excluding patients that switch again resulted in high levels of bias. More complex randomisation-based methods (e.g. Rank Preserving Structural Failure Time Models (RPSFTM)) were unbiased when the “common treatment effect” held. Observational-based methods (e.g. inverse probability of censoring weights (IPCW)) coped better with time-dependent treatment effects but are heavily data reliant, and generally led to higher levels of bias in our simulations. Novel “two stage” methods produced relatively low bias across all simulated scenarios. All methods generally produced higher bias when the simulated sample size was smaller and when the censoring proportion was higher. All methods generally produced lower bias when switching proportions were lower. We find that the size of the treatment effect in terms of an acceleration factor has an important bearing on the levels of bias associated with the adjustment methods.
Conclusions: Randomisation-based methods can accurately adjust for treatment switching when the treatment effect received by patients that switch is the same as that received by patients randomised to the experimental group. When this is not the case observational-based methods or simple twostage methods should be considered, although the IPCW is prone to substantial bias when the proportion of patients that switch is greater than approximately 90%. Simple methods such as censoring or excluding patients that switch should not be used
Relativistic D-brane Scattering is Extremely Inelastic
We study the effects of quantum production of open strings on the
relativistic scattering of D-branes. We find strong corrections to the brane
trajectory from copious production of highly-excited open strings, whose
typical oscillator level is proportional to the square of the rapidity. In the
corrected trajectory, the branes rapidly coincide and remain trapped in a
configuration with enhanced symmetry. This is a purely stringy effect which
makes relativistic brane collisions exceptionally inelastic. We trace this
effect to velocity-dependent corrections to the open-string mass, which render
open strings between relativistic D-branes surprisingly light. We observe that
pair-creation of open strings could play an important role in cosmological
scenarios in which branes approach each other at very high speeds.Comment: 30 pages; added references and a comment about velocity-dependent
masse
Survival extrapolation incorporating general population mortality using excess hazard and cure models: a tutorial
Background
Different parametric survival models can lead to widely discordant extrapolations and decision uncertainty in cost-effectiveness analyses. The use of excess hazard (EH) methods, which incorporate general population mortality data, has the potential to reduce model uncertainty. This review highlights key practical considerations of EH methods for estimating long-term survival.
Methods
Demonstration of methods used a case study of 686 patients from the German Breast Cancer Study Group, followed for a maximum of 7.3 y and divided into low (1/2) and high (3) grade cancers. Seven standard parametric survival models were fit to each group separately. The same 7 distributions were then used in an EH framework, which incorporated general population mortality rates, and fitted both with and without a cure parameter. Survival extrapolations, restricted mean survival time (RMST), and difference in RMST between high and low grades were compared up to 30 years along with Akaike information criterion goodness-of-fit and cure fraction estimates. The sensitivity of the EH models to lifetable misspecification was investigated.
Results
In our case study, variability in survival extrapolations was extensive across the standard models, with 30-y RMST ranging from 7.5 to 14.3 y. Incorporation of general population mortality rates using EH cure methods substantially reduced model uncertainty, whereas EH models without cure had less of an effect. Long-term treatment effects approached the null for most models but at varying rates. Lifetable misspecification had minimal effect on RMST differences.
Conclusions
EH methods may be useful for survival extrapolation, and in cancer, EHs may decrease over time and be easier to extrapolate than all-cause hazards. EH cure models may be helpful when cure is plausible and likely to result in less extrapolation variability
Vibrational Study of 13C-enriched C60 Crystals
The infrared (IR) spectrum of solid C60 exhibits many weak vibrational modes.
Symmetry breaking due to 13C isotopes provides a possible route for optically
activating IR-silent vibrational modes. Experimental spectra and a
semi-empirical theory on natural abundance and 13C-enriched single crystals of
C60 are presented. By comparing the experimental results with the theoretical
results, we exclude this isotopic activation mechanism from the explanation for
weakly active fundamentals in the spectra.Comment: Accepted for Phys. Rev. B, typeset in REVTEX v3.0 in LaTeX.
Postscript file including figures is available at
http://insti.physics.sunysb.edu/~mmartin/papers/c13twocol2.ps File with
figures will be e-mailed by reques
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