104 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
Vector-like exotics in F-theory and 750 GeV diphotons
AbstractThe recent excess in diphoton events around 750 GeV seen by the ATLAS and CMS experiments could be hinting at the existence of new vector-like charged matter around the TeV scale which couples to a singlet. Such a spectrum of exotics arises inevitably in certain classes of F-theory GUTs with hypercharge flux when the GUT symmetry is extended by a U(1) symmetry under which the Higgs fields of the MSSM are not vector-like. The exotics are not vector-like under the U(1) symmetry and therefore their mass is naturally related to its breaking scale. Previously this scale was taken to be close to the GUT scale which led to tension with proton decay, the μ-term magnitude, and too large R-parity violation. The 750 GeV excess provides new motivation for considering breaking the U(1) around the TeV scale, which additionally alleviates the previous problems. We study the possible TeV-scale spectrum in such an SU(5) GUT scenario and show that it is constrained and predictive. Gauge coupling unification can be retained at the accuracy of the MSSM at one loop even though typically the spectrum does not form complete GUT representations. For example the exotics cannot form a complete 10 multiplet but nonetheless happen to behave as one in the beta functions. We present an initial analysis of the diphoton production rates for the exotics spectra and find them compatible with data
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
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