21 research outputs found
Worldsheet instantons and coupling selection rules in heterotic orbifolds
We review recent results on string coupling selection rules for heterotic
orbifolds, derived using conformal field theory. Such rules are the first step
towards understanding the viability of the recently obtained compactifications
with potentially realistic particle spectra. They arise from the properties of
the worldsheet instantons that mediate the couplings, and include stringy
effects that would seem 'miraculous' to an effective field theory observer.Comment: 4 pages, talk presented at SQS'13, JINR, Dubna, Russia, 29 July - 03
August, 201
Runaway quintessence, out of the swampland
We propose a simple, well-motivated and robust alternative to a metastable de
Sitter vacuum in string theory, consistent with current observations of dark
energy and naturally satisfying conjectured swampland constraints. Inflation
ends in a supersymmetric Minkowski minimum, with a flat direction that is
protected by non-renormalisation theorems. At some scale non-perturbative
effects kick in, inducing a runaway scalar potential. The tail of this runaway
potential cannot sustain a late-time dominating, slow-roll quintessence.
However, the potential always contains a dS maximum. If the modulus starts
close to the hilltop, it remains frozen there by Hubble friction for much of
the cosmological history, at first sourcing a tiny classical vacuum energy and
then constituting a rolling quintessence, with observable consequences. So long
as the modulus is localised away from the Standard Model in the extra
dimensions, there are no observable fifth forces nor is there time-variation of
fundamental constants, and the modulus mass is protected from radiative
corrections. We revisit concrete string models based on heterotic orbifold
compactifications, and show that their de Sitter extrema satisfy the recently
refined dS Swampland Conjecture.Comment: 12 pages, 5 figures. v2: minor modifications, references and comments
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Subleading effects and the field range in axion inflation
An attractive candidate for the inflaton is an axion slowly rolling down a
flat potential protected by a perturbative shift symmetry. Realisations of this
idea within large field, natural and monomial inflation have been disfavoured
by observations and are difficult to embed in string theory. We show that
subleading, but significant non-perturbative corrections can superimpose sharp
cliffs and gentle plateaus into the potential, whose overall effect is to
enhance the number of e-folds of inflation. Sufficient e-folds are therefore
achieved for smaller field ranges compared to the potential without such
corrections. Thus, both single-field natural and monomial inflation in UV
complete theories like string theory, can be restored into the favour of
current observations, with distinctive signatures. Tensor modes result
un-observably small, but there is a large negative running of the spectral
index. Remarkably, natural inflation can be achieved with a single field whose
axion decay constant is sub-Planckian.Comment: 18 pages, 15 figures; v2 references improve
SUSY Breaking and Moduli Stabilization from Fluxes in Gauged 6D Supergravity
We construct the 4D N=1 supergravity which describes the low-energy limit of
6D supergravity compactified on a sphere with a monopole background a la Salam
and Sezgin. This provides a simple setting sharing the main properties of
realistic string compactifications such as flat 4D spacetime, chiral fermions
and N=1 supersymmetry as well as Fayet-Iliopoulos terms induced by the
Green-Schwarz mechanism. The matter content of the resulting theory is a
supersymmetric SO(3)xU(1) gauge model with two chiral multiplets, S and T. The
expectation value of T is fixed by the classical potential, and S describes a
flat direction to all orders in perturbation theory. We consider possible
perturbative corrections to the Kahler potential in inverse powers of
and , and find that under certain circumstances, and when taken together
with low-energy gaugino condensation, these can lift the degeneracy of the flat
direction for . The resulting vacuum breaks supersymmetry at moderately
low energies in comparison with the compactification scale, with positive
cosmological constant. It is argued that the 6D model might itself be obtained
from string compactifications, giving rise to realistic string
compactifications on non Ricci flat manifolds. Possible phenomenological and
cosmological applications are briefly discussed.Comment: 32 pages, 2 figures. Uses JHEP3.cls. References fixed and updated,
some minor typos fixed. Corrected minor error concerning Kaluza-Klein scales.
Results remain unchange
Discrete R-symmetries and Anomaly Universality in Heterotic Orbifolds
We study discrete R-symmetries, which appear in 4D low energy effective field
theory derived from hetetoric orbifold models. We derive the R-symmetries
directly from geometrical symmetries of orbifolds. In particular, we obtain the
corresponding R-charges by requiring that the couplings be invariant under
these symmetries. This allows for a more general treatment than the explicit
computations of correlation functions made previously by the authors, including
models with discrete Wilson lines, and orbifold symmetries beyond
plane-by-plane rotational invariance. Surprisingly, for the cases covered by
earlier explicit computations, the R-charges differ from the previous result.
We study the anomalies associated with these R-symmetries, and comment on the
results.Comment: 21 pages, 2 figures. Minor changes, typos corrected. Matches JHEP
published versio
Revisiting Coupling Selection Rules in Heterotic Orbifold Models
We study L-point couplings between twisted sector fields in heterotic
orbifold compactifications, using conformal field theory. Selection rules
provide an easy way to identify which couplings are non-vanishing. Those used
in the current literature are gauge invariance, R-charge conservation and the
space group selection rule, but they are not the whole story. We revive and
refine a fourth selection rule, due to symmetries in the underlying torus
lattice, and introduce a fifth one, due to the existence or not of classical
worldsheet instanton solutions to mediate the couplings. We consider briefly
the phenomenological consequences of the additional rules, in particular for
recent orbifold constructions whose field content correspond to that of the
MSSM. The structure of the exotic mass matrices is unaltered and many
dimension-5 proton-decay operators vanish.Comment: 27 pages, v2: several clarifications, matches JHEP version. v3:
supercedes journal version, erratum to appear in JHEP; correction to "rule 5"
equations, main ideas unchange
Mechanisms for primordial black hole production in string theory
We consider mechanisms for producing a significant population of primordial black holes (PBHs) within string inspired single field models of inflation. The production of PBHs requires a large amplification in the power spectrum of curvature perturbations between scales associated with CMB and PBH formation. In principle, this can be achieved by temporarily breaking the slow-roll conditions during inflation. In this work, we identify two string setups that can realise this process. In string axion models of inflation, subleading non-perturbative effects can superimpose steep cliffs and gentle plateaus onto the leading axion potential. The cliffs can momentarily violate the slow-roll conditions, and the plateaus can lead to phases of ultra slow-roll inflation. We thus achieve a string motivated model which both matches the Planck observations at CMB scales and produces a population of light PBHs, which can account for an order one fraction of dark matter. In DBI models of inflation, a sharp increase in the speed of sound sourced by a steep downward step in the warp factor can drive the amplification. In this scenario, discovery of PBHs could indicate non-trivial dynamics in the bulk, such as flux-antibrane annihilation at the tip of a warped throat
Squeezed tensor non-Gaussianity in non-attractor inflation
We investigate primordial tensor non-Gaussianity in single field inflation,
during a phase of non-attractor evolution when the spectrum of primordial
tensor modes can be enhanced to a level detectable at interferometer scales.
Making use of a tensor duality we introduced in arXiv:1808.10475, we
analytically compute the full bispectrum of primordial tensor fluctuations
during the non-attractor era. During this epoch the shape of the tensor
bispectrum is enhanced in the squeezed limit, its amplitude can be amplified
with respect to slow-roll models, and tensor non-Gaussianity can exhibit a
scale dependence distinctive of our set-up. We prove that our results do not
depend on the frame used for the calculations. Squeezed tensor non-Gaussianity
induces a characteristic quadrupolar anisotropy on the power spectrum of the
stochastic background of primordial tensor perturbations. As a step to make
contact with gravitational wave experiments, we discuss the response function
of a ground based Michelson interferometer to a gravitational wave background
with such a feature.Comment: 34 pages, 4 figure
Anti-D3 branes and moduli in non-linear supergravity
Anti-D3 branes and non-perturbative effects in flux compactifications
spontaneously break supersymmetry and stabilise moduli in a metastable de
Sitter vacua. The low energy 4D effective field theory description for such
models would be a supergravity theory with non-linearly realised supersymmetry.
Guided by string theory modular symmetry, we compute this non-linear
supergravity theory, including dependence on all bulk moduli. Using either a
constrained chiral superfield or a constrained vector field, the uplifting
contribution to the scalar potential from the anti-D3 brane can be
parameterised either as an F-term or Fayet-Iliopoulos D-term. Using again the
modular symmetry, we show that 4D non-linear supergravities that descend from
string theory have an enhanced protection from quantum corrections by
non-renormalisation theorems. The superpotential giving rise to metastable de
Sitter vacua is robust against perturbative string-loop and
corrections.Comment: 33 page