1,188 research outputs found
The Landscape of M-theory Compactifications on Seven-Manifolds with Holonomy
We study the physics of globally consistent four-dimensional
supersymmetric M-theory compactifications on manifolds constructed via
twisted connected sum; there are now perhaps fifty million examples of these
manifolds. We study a rich example that exhibits gauge symmetry and a
spectrum of massive charged particles that includes a trifundamental. Applying
recent mathematical results to this example, we compute membrane instanton
corrections to the superpotential and spacetime topology change in a compact
model; the latter include both the (non-isolated) flop and conifold
transitions. The conifold transition spontaneously breaks the gauge symmetry to
, and associated field theoretic computations of particle charges make
correct predictions for the topology of the deformed manifold. We discuss
physical aspects of the abelian landscape broadly, including aspects of
Higgs and Coulomb branches, membrane instanton corrections, and some general
aspects of topology change.Comment: v1: 37 pages. v2: references added. v3: more discussion of instanton
zero modes. JHEP version v4: references correcte
Anergy in self-directed B lymphocytes from a statistical mechanics perspective
The ability of the adaptive immune system to discriminate between self and
non-self mainly stems from the ontogenic clonal-deletion of lymphocytes
expressing strong binding affinity with self-peptides. However, some
self-directed lymphocytes may evade selection and still be harmless due to a
mechanism called clonal anergy. As for B lymphocytes, two major explanations
for anergy developed over three decades: according to "Varela theory", it stems
from a proper orchestration of the whole B-repertoire, in such a way that
self-reactive clones, due to intensive interactions and feed-back from other
clones, display more inertia to mount a response. On the other hand, according
to the `two-signal model", which has prevailed nowadays, self-reacting cells
are not stimulated by helper lymphocytes and the absence of such signaling
yields anergy. The first result we present, achieved through disordered
statistical mechanics, shows that helper cells do not prompt the activation and
proliferation of a certain sub-group of B cells, which turn out to be just
those broadly interacting, hence it merges the two approaches as a whole (in
particular, Varela theory is then contained into the two-signal model). As a
second result, we outline a minimal topological architecture for the B-world,
where highly connected clones are self-directed as a natural consequence of an
ontogenetic learning; this provides a mathematical framework to Varela
perspective. As a consequence of these two achievements, clonal deletion and
clonal anergy can be seen as two inter-playing aspects of the same phenomenon
too
H-Spaces, Loop Spaces and the Space of Positive Scalar Curvature Metrics on the Sphere
For dimensions n greater than or equal to 3, we show that the space of
metrics of positive scalar curvature on the n-sphere is homotopy equivalent to
a subspace which takes the form of a H-space with a homotopy commutative,
homotopy associative product operation. This product operation is based on the
connected sum construction. We then exhibit an action of the little n-disks
operad on this subspace which, using results of Boardman, Vogt and May implies
that when n=3 or n is at least 5, the space of metrics of positive scalar
curvature on the n-sphere is weakly homotopy equivalent to an n-fold loop
space.Comment: 43 pages, 32 figures. In version 2 we added a line to the
introduction acknowledging a relevant new result in the field. In version 3,
we correct an error in the proof of the second main resul
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