282 research outputs found
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
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
Effect of n-3 fatty acids on the antitumour effects of cytotoxic drugs
Background: n-3 fatty acids are increasingly being administered to cancer patients for the treatment of cachexia, and it is thus important to know of any potential interactions with ongoing cytotoxic drug therapy. Materials and methods: For this reason eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were administered to mice bearing the cachexia-inducing MAC16 colon adenocarcinoma, and the effect of epothilone, gemcitabine, 5-fluorouracil and cyclophosphamide on tumour growth and body weight determined. Results: Epothilone alone had a minimal effect on tumour growth rate, but this was potentiated by DHA, while for 5-fluorouracil and cyclophosphamide tumour growth inhibition was enhanced by EPA. The antitumour effect of gemcitabine was not altered by either fatty acid. EPA arrested the development of cachexia, while DHA had no effect and the same was true for their effect on tumour growth rate. The anticachectic effect of EPA was only seen in combination with 5-fluorouracil. Conclusion: These results suggest that n-3 fatty acids do not interfere with the action of chemotherapy and may potentiate the effect of certain agents
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
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
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
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
D-instantons and Matrix Models
We discuss the Matrix Model aspect of configurations saturating a fixed
number of fermionic zero modes. This number is independent of the rank of the
gauge group and the instanton number. This will allow us to define a
large- limit of the embeddeding of D-instantons in the Matrix Model
and make contact with the leading term (the measure factor) of the supergravity
computations of D-instanton effects. We show that the connection between these
two approaches is done through the Abelian modes of the Matrix variables.Comment: harvmac (b), 26 pages. v5 : polished final version for publication.
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