150 research outputs found
Towards large r from [p,q]-inflation
The recent discovery of B-mode polarizations in the CMB by the BICEP2
collaboration motivates the study of large-field inflation models which can
naturally lead to significant tensor-to-scalar ratios. A class of such models
in string theory are axion monodromy models, where the shift symmetry of an
axion is broken by some branes. In type IIB string theory such models so far
utilized NS5 branes which lead to a linear potential with an induced
tensor-to-scalar ratio of . In this short note we study a
modification of the scenario to include [p,q] 7-branes and show that this leads
to an enhanced tensor-to-scalar ratio . Unlike 5-branes, 7-branes
are in-principle compatible with supersymmetry, however we find that an
implementation of the inflationary scenario requires an explicit breaking of
supersymmetry by the 7-branes during inflation. This leads to similar
challenges as in 5-brane models. We discuss the relation to high-scale
supersymmetry breaking after inflation.Comment: 8 pp; v2: references added, typos correcte
Hidden Selection Rules, M5-instantons and Fluxes in F-theory
We introduce a new approach to investigate the selection rules governing the
contributions of fluxed M5-instantons to the F-theory four-dimensional
effective action, with emphasis on the generation of charged matter F-terms.
The structure of such couplings is unraveled by exploiting the perturbative and
non-perturbative homological relations, introduced in our companion paper
arXiv:1506.06764, which encode the interplay between the self-dual 3-form flux
on the M5-brane, the background 4-form flux and certain fibral curves. The
latter are wrapped by time-like M2-branes representing matter insertions in the
instanton path integral. In particular, we clarify how fluxed M5-instantons
detect the presence of geometrically massive s which are responsible for
`hidden' selection rules. We discuss how for non-generic embeddings the
M5-instanton can probe `locally massless' symmetries if the rank of its
Mordell-Weil group is enhanced compared to that of the bulk. As a
phenomenological off-spring we propose a new type of non-perturbative
corrections to Yukawa couplings which may change the rank of the Yukawa matrix.
Along the way, we also gain new insights into the structure of massive
gauge fluxes in the stable degeneration limit.Comment: 42 pages; v2: references adde
Non-perturbative selection rules in F-theory
We discuss the structure of charged matter couplings in 4-dimensional
F-theory compactifications. Charged matter is known to arise from M2-branes
wrapping fibral curves on an elliptic or genus-one fibration Y. If a set of
fibral curves satisfies a homological relation in the fibre homology, a
coupling involving the states can arise without exponential volume suppression
due to a splitting and joining of the M2-branes. If the fibral curves only sum
to zero in the integral homology of the full fibration, no such coupling is
possible. In this case an M2-instanton wrapping a 3-chain bounded by the fibral
matter curves can induce a D-term which is volume suppressed. We elucidate the
consequences of this pattern for the appearance of massive U(1) symmetries in
F-theory and analyse the structure of discrete selection rules in the coupling
sector. The weakly coupled analogue of said M2-instantons is worked out to be
given by D1-F1 instantons. The generation of an exponentially suppressed F-term
requires the formation of half-BPS bound states of M2 and M5-instantons. This
effect and its description in terms of fluxed M5-instantons is discussed in a
companion paper.Comment: 49 pages, 9 figures; v2: references adde
Swampland Bounds on the Abelian Gauge Sector
We derive bounds on the number of abelian gauge group factors in
six-dimensional gravitational theories with minimal supersymmetry and in their
F-theoretic realisations. These bounds follow by requiring consistency of
certain BPS strings in the spectrum of the theory, as recently proposed in the
literature. Under certain assumptions this approach constrains the number of
abelian gauge group factors in six-dimensional supergravity theories with at
least one tensor multiplet to be (or in absence of
charged matter). For any geometric F-theory realisation with at least one
tensor multiplet we establish the bound by demanding unitarity of a
heterotic solitonic string which exists even in absence of a perturbative
heterotic dual. This result extends to four-dimensional F-theory vacua on any
blowup of a rational fibration. Our findings lead to universal bounds on the
rank of the Mordell-Weil group of elliptically fibered Calabi-Yau threefolds.Comment: 10 pages, 2-column forma
F-theory and 2d (0,2) Theories
F-theory compactified on singular, elliptically fibered Calabi-Yau five-folds
gives rise to two-dimensional gauge theories preserving N=(0,2) supersymmetry.
In this paper we initiate the study of such compactifications and determine the
dictionary between the geometric data of the elliptic fibration and the 2d
gauge theory such as the matter content in terms of (0,2) superfields and their
supersymmetric couplings. We study this setup both from a gauge-theoretic point
of view, in terms of the partially twisted 7-brane theory, and provide a global
geometric description based on the structure of the elliptic fibration and its
singularities. Global consistency conditions are determined and checked against
the dual M-theory compactification to one dimension. This includes a discussion
of gauge anomalies, the structure of the Green-Schwarz terms and the
Chern-Simons couplings in the dual M-theory supersymmetric quantum mechanics.
Furthermore, by interpreting the resulting 2d (0,2) theories as heterotic
worldsheet theories, we propose a correspondence between the geometric data of
elliptically fibered Calabi-Yau five-folds and the target space of a heterotic
gauged linear sigma-model (GLSM). In particular the correspondence between the
Landau-Ginsburg and sigma-model phase of a 2d (0,2) GLSM is realized via
different T-branes or gluing data in F-theory.Comment: 124 pages, 5 figures, v2: ref added, v3: typos corrected, discussion
of interactions extended, v4: updated section 12, v5: typos corrected and
refs adde
Heterotic GUT and Standard Model Vacua from simply connected Calabi-Yau Manifolds
We consider four-dimensional supersymmetric compactifications of the E8 x E8
heterotic string on Calabi-Yau manifolds endowed with vector bundles with
structure group SU(N) x U(1) and five-branes. After evaluating the
Green-Schwarz mechanism and deriving the generalized Donaldson-Uhlenbeck-Yau
condition including the five-brane moduli, we show that this construction can
give rise to GUT models containing U(1) factors like flipped SU(5) or directly
the Standard Model even on simply connected Calabi-Yau manifolds. Concrete
realizations of three-generation models on elliptically fibered Calabi-Yau
manifolds are presented. They exhibit the most attractive features of flipped
SU(5) models such as doublet-triplet splitting and proton stability. In
contrast to conventional GUT string models, the tree level relations among the
Standard Model gauge couplings at the GUT scale are changed.Comment: 46 pages, 2 figures, 6 tables; v2: references added, typos corrected;
v3: typos corrected, final version published in Nucl.Phys.
Algebraic Cycles and Local Anomalies in F-Theory
We introduce a set of identities in the cohomology ring of elliptic
fibrations which are equivalent to the cancellation of gauge and mixed
gauge-gravitational anomalies in F-theory compactifications to four and six
dimensions. The identities consist in (co)homological relations between complex
codimension-two cycles. The same set of relations, once evaluated on elliptic
Calabi-Yau three-folds and four-folds, is shown to universally govern the
structure of anomalies and their Green-Schwarz cancellation in six- and
four-dimensional F-theory vacua, respectively. We furthermore conjecture that
these relations hold not only within the cohomology ring, but even at the level
of the Chow ring, i.e. as relations among codimension-two cycles modulo
rational equivalence. We verify this conjecture in non-trivial examples with
Abelian and non-Abelian gauge groups factors. Apart from governing the
structure of local anomalies, the identities in the Chow ring relate different
types of gauge backgrounds on elliptically fibred Calabi-Yau four-folds.Comment: 45 page
(Non-)BPS bound states and D-brane instantons
We study non-perturbative effects in four-dimensional N=1 supersymmetric
orientifold compactifications due to D-brane instantons which are generically
not invariant under the orientifold projection. We show that they can yield
superpotential contributions via a multi-instanton process at threshold. Some
constituents of this configuration form bound states away from the wall of
marginal stability which can decay in other regions of moduli space. A
microscopic analysis reveals how contributions to the superpotential are
possible when new BPS states compensate for their decay. We study this
concretely for D2-brane instantons along decaying special Lagrangians in Type
IIA and for D5-branes instantons carrying holomorphic bundles in Type I theory.Comment: 39 pages, 4 tables, 2 figures; v2: Equ. (20) corrected, related
discussion adjusted, 1 reference added; v3: JHEP version with more refs adde
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