293 research outputs found
Cosmological string models from Milne spaces and SL(2,Z) orbifold
The -dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with , . We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.Comment: 14 pages, harvma
Immunogenicity and Immunosensitivity of Urethane-induced Murine Lung Adenomata, in Relation to the Immunological Impairment of the Primary Tumour Host
The depression of the immunological status of BALB/c mice treated during infancy with two different doses of urethane, alone or combined with cortisone, was evaluated by counting the number of plaque forming cells at 30 or 50 days of age. The incidence of lung adenomatous nodules was directly related to the degree of immunological impairment at 50 days of age. Twenty-seven lung adenomata were tested in an in vitro system involving spleen cells immune against the same single tumour used as target cell. Eighty-six per cent of tumours in the most immunodepressed group of mice were positive compared with 20-40% in the less immunodepressed groups. Syngeneic cross-reaction tests showed that non-immunogenic tumours were immunosensitive since 66% positive tests were obtained when target cells belonging to the less immunodepressed groups were tested with spleen cells of mice immunized with immunogenic adenomata
Deep Inelastic Scattering in Conformal QCD
We consider the Regge limit of a CFT correlation function of two vector and
two scalar operators, as appropriate to study small-x deep inelastic scattering
in N=4 SYM or in QCD assuming approximate conformal symmetry. After clarifying
the nature of the Regge limit for a CFT correlator, we use its conformal
partial wave expansion to obtain an impact parameter representation encoding
the exchange of a spin j Reggeon for any value of the coupling constant. The
CFT impact parameter space is the three-dimensional hyperbolic space H3, which
is the impact parameter space for high energy scattering in the dual AdS space.
We determine the small-x structure functions associated to the exchange of a
Reggeon. We discuss unitarization from the point of view of scattering in AdS
and comment on the validity of the eikonal approximation.
We then focus on the weak coupling limit of the theory where the amplitude is
dominated by the exchange of the BFKL pomeron. Conformal invariance fixes the
form of the vector impact factor and its decomposition in transverse spin 0 and
spin 2 components. Our formalism reproduces exactly the general results predict
by the Regge theory, both for a scalar target and for gamma*-gamma* scattering.
We compute current impact factors for the specific examples of N=4 SYM and QCD,
obtaining very simple results. In the case of the R-current of N=4 SYM, we show
that the transverse spin 2 component vanishes. We conjecture that the impact
factors of all chiral primary operators of N=4 SYM only have components with 0
transverse spin.Comment: 44+16 pages, 7 figures. Some correction
Coleman meets Schwinger
It is well known that spherical D-branes are nucleated in the presence of an
external RR electric field. Using the description of D-branes as solitons of
the tachyon field on non-BPS D-branes, we show that the brane nucleation
process can be seen as the decay of the tachyon false vacuum. This process can
describe the decay of flux-branes in string theory or the decay of quintessence
potentials arising in flux compactifications.Comment: 5 pages, 2 figure
More on Membranes in Matrix Theory
We study noncompact and static membrane solutions in Matrix theory. Demanding
axial symmetry on a membrane embedded in three spatial dimensions, we obtain a
wormhole solution whose shape is the same with the catenoidal solution of
Born-Infeld theory. We also discuss another interesting class of solutions,
membranes embedded holomorphically in four spatial dimensions, which are 1/4
BPS.Comment: 7 pages, LaTeX; expanded to treat matrix membrane solutions with
electric flux, equivalently fundamental strings; to appear in Phys. Rev.
Four-point correlators with higher weight superconformal primaries in the AdS/CFT Correspondence
The four-point correlation function of two 1/2 BPS primaries of conformal
weight and two 1/2-BPS primaries of conformal weight is
calculated in the large 't Hooft, large limit. These operators are dual to
Kaluza-Klein supergravity fields with masses and .
Given that the existing formalism for evaluating sums of products of SO(6)
tensors that determine the effective couplings is only suitable for primaries
with small conformal dimensions, we make us of an alternative formalism based
on harmonic polynomials introduced by Dolan and Osborn.
We then show that the supergravity lagrangian relevant to the computation is
of sigma-model type (i.e., the four-derivative couplings vanish) and that the
final result for the connected amplitude splits into a free and an interacting
part, as expected on general grounds.Comment: 31 pages, 4 figure
Riemann Surfaces of genus g with an automorphism of order p prime and p>g
The present work completes the classification of the compact Riemann surfaces
of genus g with an analytic automorphism of order p (prime number) and p > g.
More precisely, we construct a parameteriza- tion space for them, we compute
their groups of uniformization and we compute their full automorphism groups.
Also, we give affine equations for special cases and some implications on the
components of the singular locus of the moduli space of smooth curves of genus
g.Comment: 28 pages, 5 figure
The Regge Limit for Green Functions in Conformal Field Theory
We define a Regge limit for off-shell Green functions in quantum field
theory, and study it in the particular case of conformal field theories (CFT).
Our limit differs from that defined in arXiv:0801.3002, the latter being only a
particular corner of the Regge regime. By studying the limit for free CFTs, we
are able to reproduce the Low-Nussinov, BFKL approach to the pomeron at weak
coupling. The dominance of Feynman graphs where only two high momentum lines
are exchanged in the t-channel, follows simply from the free field analysis. We
can then define the BFKL kernel in terms of the two point function of a simple
light-like bilocal operator. We also include a brief discussion of the gravity
dual predictions for the Regge limit at strong coupling.Comment: 23 pages 2 figures, v2: Clarification of relation of the Regge limit
defined here and previous work in CFT. Clarification of causal orderings in
the limit. References adde
Plane waves with weak singularities
We study a class of time dependent solutions of the vacuum Einstein equations
which are plane waves with weak null singularities. This singularity is weak in
the sense that though the tidal forces diverge at the singularity, the rate of
divergence is such that the distortion suffered by a freely falling observer
remains finite. Among such weak singular plane waves there is a sub-class which
do not exhibit large back reaction in the presence of test scalar probes.
String propagation in these backgrounds is smooth and there is a natural way to
continue the metric beyond the singularity. This continued metric admits string
propagation without the string becoming infinitely excited. We construct a one
parameter family of smooth metrics which are at a finite distance in the space
of metrics from the extended metric and a well defined operator in the string
sigma model which resolves the singularity.Comment: 22 pages, Added references and clarifying comment
On Abelian Multi-Chern-Simons Field Theories
In this paper a class of multi-Chern-Simons field theories which is relevant
to the statistical mechanics of polymer systems is investigated. Motivated by
the problems which one encounters in the treatment of these theories, a general
procedure is presented to eliminate the Chern-Simons fields from their action.
In this way it has been possible to derive an expression of the partition
function of topologically linked polymers which depends explicitly on the
topological numbers and does not have intractable nonlocal terms as it happened
in previous approaches. The new formulation of multi-Chern-Simons field
theories is then used to remove and clarify some inconsistencies and
ambiguities which apparently affect field theoretical models of topologically
linked polymers. Finally, the limit of disentangled polymers is discussed.Comment: 18 pages, plain LaTe
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