1,282 research outputs found
The Weak Gravity Conjecture and Scalar Fields
We propose a generalisation of the Weak Gravity Conjecture in the presence of
scalar fields. The proposal is guided by properties of extremal black holes in
supergravity, but can be understood more generally in terms of
forbidding towers of stable gravitationally bound states. It amounts to the
statement that there must exist a particle on which the gauge force acts more
strongly than gravity and the scalar forces combined. We also propose that the
scalar force itself should act on this particle stronger than gravity. This
implies that generically the mass of this particle decreases exponentially as a
function of the scalar field expectation value for super-Planckian variations,
which is behaviour predicted by the Refined Swampland Conjecture. In the
context of supergravity the Weak Gravity Conjecture bound can be
tied to bounds on scalar field distances in field space. Guided by this, we
present a general proof that for any linear combination of moduli in any
Calabi-Yau compactification of string theory the proper field distance grows at
best logarithmically with the moduli values for super-Planckian distances.Comment: 25 pages. v2: Modified and extended section 4.1. v3: Clarifications
added, published versio
On Natural Inflation and Moduli Stabilisation in String Theory
Natural inflation relies on the existence of an axion decay constant which is
super-Planckian. In string theory only sub-Planckian axion decay constants have
been found in any controlled regime. However in field theory it is possible to
generate an enhanced super-Planckian decay constant by an appropriate aligned
mixing between axions with individual sub-Planckian decay constants. We study
the possibility of such a mechanism in string theory. In particular we
construct a new realisation of an alignment scenario in type IIA string theory
compactifications on a Calabi-Yau where the alignment is induced through
fluxes. Within field theory the original decay constants are taken to be
independent of the parameters which induce the alignment. In string theory
however they are moduli dependent quantities and so interact gravitationally
with the physics responsible for the mixing. We show that this gravitational
effect of the fluxes on the moduli can precisely cancel any enhancement of the
effective decay constant. This censorship of an effective super-Planckian decay
constant depends on detailed properties of Calabi-Yau moduli spaces and occurs
for all the examples and classes that we study. We expand these results to a
general superpotential assuming only that the axion superpartners are fixed
supersymmetrically and are able to show for a large class of Calabi-Yau
manifolds, but not all, that the cancellation effect occurs and is independent
of the superpotential. We also study simple models where the moduli are fixed
non-supersymmetrically and find that similar cancellation behaviour can emerge.
Finally we make some comments on a possible generalisation to axion monodromy
inflation models.Comment: 24 pages. V2: Appendix on backreaction in axion monodromy added.
References adde
Models of Particle Physics from Type IIB String Theory and F-theory: A Review
We review particle physics model building in type IIB string theory and
F-theory. This is a region in the landscape where in principle many of the key
ingredients required for a realistic model of particle physics can be combined
successfully. We begin by reviewing moduli stabilisation within this framework
and its implications for supersymmetry breaking. We then review model building
tools and developments in the weakly coupled type IIB limit, for both local
D3-branes at singularities and global models of intersecting D7-branes. Much of
recent model building work has been in the strongly coupled regime of F-theory
due to the presence of exceptional symmetries which allow for the construction
of phenomenologically appealing Grand Unified Theories. We review both local
and global F-theory model building starting from the fundamental concepts and
tools regarding how the gauge group, matter sector and operators arise, and
ranging to detailed phenomenological properties explored in the literature.Comment: 79 pages, Invited review article for the International Journal of
Modern Physics
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
On Gauge Threshold Corrections for Local IIB/F-theory GUTs
We study gauge threshold corrections for local GUT models in IIB/F-theory.
Consistency with holomorphy requirements of supergravity and the
Kaplunovsky-Louis formula implies that the unification scale is enhanced by the
bulk radius R from the string scale to M_X =RM_S. We argue that the stringy
interpretation of this is via a locally uncancelled tadpole sourced by the
hypercharge flux. This sources closed string modes propagating into the bulk;
equivalently open string gauge coupling running up to the winding scale M_X.
The enhancement to R M_s is tied to GUT breaking by a globally trivial
hypercharge flux and will occur in all models realising this mechanism.Comment: 4 pages; v2. journal versio
Hypercharge Flux in IIB and F-theory: Anomalies and Gauge Coupling Unification
We analyse hypercharge flux GUT breaking in F-theory/Type IIB GUT models with
regards to its implications for anomaly cancellation and gauge coupling
unification. To this aim we exploit the Type IIB limit and consider 7-brane
configurations that for the first time are guaranteed to exhibit net
hypercharge flux restriction to matter curves. We show that local F-theory
models with anomalies of type U(1)_Y-U(1)^2 in the massless spectrum can be
consistent only if such additional U(1)s are globally geometrically massive (in
the sense that they arise from non-Kahler deformations of the Calabi-Yau
four-fold). Further, in such cases of geometrically massive U(1)s hypercharge
flux can induce new anomalies of type U(1)_Y^2-U(1) in the massless spectrum,
violating constraints in local models forbidding such anomalies. In particular
this implies that it is possible to construct models exhibiting a U(1)_{PQ}
global symmetry which have hypercharge flux doublet-triplet splitting and no
further exotics. We also show that the known hypercharge flux induced splitting
of the gauge couplings in IIB models at tree-level can be reduced by a factor
of 5 by employing a more F-theoretic twisting of U(1) flux by hypercharge flux
bringing it to well within MSSM 2-loop results. In the case of net restriction
of hypercharge flux to matter curves this tree-level splitting becomes more
involved, is tied to the vacuum expectation values of certain closed-string
fields, and therefore gauge coupling unification becomes tied to the question
of moduli stabilisation.Comment: 27 pages. v2: Expanded discussion on anomalies and showed that
geometrically massive U(1)s of Peccei-Quinn type are compatible with
hypercharge flux doublet-triplet splitting with no exotic
U(1) symmetries in F-theory GUTs with multiple sections
We present a systematic construction of F-theory compactifications with
Abelian gauge symmetries in addition to a non-Abelian gauge group G. The
formalism is generally applicable to models in global Tate form but we focus on
the phenomenologically interesting case of G=SU(5). The Abelian gauge factors
arise due to extra global sections resulting from a specific factorisation of
the Tate polynomial which describes the elliptic fibration. These
constructions, which accommodate up to four different U(1) factors, are worked
out in detail for the two possible embeddings of a single U(1) factor into E8,
usually denoted SU(5) x U(1)_X and SU(5) x U(1)_PQ. The resolved models can be
understood either patchwise via a small resolution or in terms of a
P_{1,1,2}[4] description of the elliptic fibration. We derive the U(1) charges
of the fields from the geometry, construct the U(1) gauge fluxes and exemplify
the structure of the Yukawa interaction points. A particularly interesting
result is that the global SU(5) x U(1)_PQ model exhibits extra SU(5)-singlet
states which are incompatible with a single global decomposition of the 248 of
E8. The states in turn lead to new Yukawa type couplings which have not been
considered in local model building.Comment: 46 pages, 3 figures; v2 typos corrected, citations adde
Effective action of (massive) IIA on manifolds with SU(3) structure
In this paper we consider compactifications of massive type IIA supergravity
on manifolds with SU(3) structure. We derive the gravitino mass matrix of the
effective four-dimensional N = 2 theory and show that vacuum expectation values
of the scalar fields naturally induce spontaneous partial supersymmetry
breaking. We go on to derive the superpotential and the Kaehler potential for
the resulting N = 1 theories. As an example we consider the SU(3) structure
manifold SU(3)/U(1)xU(1) and explicitly find N = 1 supersymmetric minima where
all the moduli are stabilised at non-trivial values without the use of
non-perturbative effects.Comment: 25 pages, 2 figures. References added and typos corrected to match
published versio
M-theory Compactifications to Three Dimensions with M2-brane Potentials
We study a class of compactifications of M-theory to three dimensions that
preserve N=2 supersymmetry and which have the defining feature that a probe
space-time filling M2 brane feels a non-trivial potential on the internal
manifold. Using M-theory/F-theory duality such compactifications include the
uplifts of 4-dimensional N=1 type IIB compactifications with D3 potentials to
strong coupling. We study the most general 8-dimensional manifolds supporting
these properties, derive the most general flux that induces an M2 potential,
and show that it is parameterised in terms of two real vectors. We study the
supersymmetry equations when only this flux is present and show that over the
locus where the M2 potential is non-vanishing the background takes the form of
a Calabi-Yau three-fold fibered over a 2-dimensional base spanned by the flux
vectors, while at the minima of the potential the flux vanishes. Allowing also
for non-vanishing four-form flux with one leg in the internal directions we
find that the Calabi-Yau three-fold in the fibration is replaced by an
SU(3)-structure manifold with torsion classes satisfying 2 W_4=-W_5.Comment: 40 page
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