28 research outputs found
Selection rules for splitting strings
It has been pointed out that Nielsen-Olesen vortices may be able to decay by
pair production of black holes. We show that when the abelian Higgs model is
embedded in a larger theory, the additional fields may lead to selection rules
for this process - even in the absence of fermions - due to the failure of a
charge quantization condition. We show that, when there is topology change, the
criterion based on the charge quantization condition supplements the usual
criterion based on . In particular, we find that, unless
is a rational number, the thermal splitting of electroweak
Z-strings by magnetically neutral black holes is impossible, even though
is trivial.Comment: 12 pages revtex -- Published version (some points clarified
A note on the stability of axionic D-term strings
We investigate the stability of a new class of BPS cosmic strings in N=1
supergravity with D-terms recently proposed by Blanco-Pillado, Dvali and Redi.
These have been conjectured to be the low energy manifestation of D-strings
that might form from tachyon condensation after D- anti-D-brane annihilation in
type IIB superstring theory. There are three one-parameter families of
cylindrically symmetric one-vortex solutions to the BPS equations (tachyonic,
axionic and hybrid). We find evidence that the zero mode in the axionic case,
or s-strings, can be excited. Its evolution leads to the decompactification of
four-dimensional spacetime at late times, with a rate that decreases with
decreasing brane tension.Comment: 6 pages, 5 figure
F-term uplifting and moduli stabilization consistent with Kahler invariance
An important ingredient in the construction of phenomenologically viable
superstring models is the uplifting of Anti-de Sitter supersymmetric critical
points in the moduli sector to metastable Minkowski or de Sitter vacua with
broken supersymmetry. In all cases described so far, uplifting results in a
displacement of the potential minimum away from the critical point and, if the
uplifting is large, can lead to the disappearance of the minimum altogether. We
propose a variant of F-term uplifting which exactly preserves supersymmetric
critical points and shift symmetries at tree level. In spite of a direct
coupling, the moduli do not contribute to supersymmetry breaking. We analyse
the stability of the critical points in a toy one-modulus sector before and
after uplifting, and find a simple stability condition depending solely on the
amount of uplifting and not on the details of the uplifting sector. There is a
region of parameter space, corresponding to the uplifting of local AdS {\em
maxima} --or, more importantly, local minima of the Kahler function-- where the
critical points are stable for any amount of uplifting. On the other hand,
uplifting to (non- supersymmetric) Minkowski space is special in that all SUSY
critical points, that is, for all possible compactifications, become stable or
neutrally stable.Comment: 20 pages, 1 figur
Semilocal and Electroweak Strings
We review a class of non-topological defects in the standard electroweak
model, and their implications. Starting with the semilocal string, which
provides a counterexample to many well known properties of topological
vortices, we discuss electroweak strings and their stability with and without
external influences such as magnetic fields. Other known properties of
electroweak strings and monopoles are described in some detail and their
potential relevance to future particle accelerator experiments and to baryon
number violating processes is considered. We also review recent progress on the
cosmology of electroweak defects and the connection with superfluid helium,
where some of the effects discussed here could possibly be tested.Comment: 86 pages; submitted to Phys. Re
On the viability of m**2 phi**2 and natural inflation
In the context of single field inflation, models with a quadratic potential
and models with a natural potential with subplanckian decay constant are in
tension with the Planck data. We show that, when embedded in a two-field model
with an additional super massive field, they can become consistent with
observations. Our results follow if the inflaton is the phase of a complex
field (or an angular variable) protected by a mildly broken U(1) symmetry, and
the radial component, whose mass is much greater than the Hubble scale, is
stabilized at subplanckian values. The presence of the super massive field,
besides modifying the effective single field potential, causes a reduction in
the speed of sound of the inflaton fluctuations, which drives the prediction
for the primordial spectrum towards the allowed experimental values. We discuss
these effects also for the linear potential, and show that this model increases
its agreement with data as wellComment: 14 pages, 7 figures. v2 added missing abstract in the pdf, references
and very minor changes. v3. typos corrected, references adde
Constraints on holographic multi-field inflation and models based on the Hamilton-Jacobi formalism
In holographic inflation, the cosmological dynamics is postulated to be
dual to the renormalization group flow of a Euclidean conformal field
theory with marginally relevant operators. The scalar potential of the
theory ---in which inflation is realized--- is highly constrained, with use of
the Hamilton--Jacobi equations. In multi-field holographic realizations of
inflation, fields additional to the inflaton cannot display underdamped
oscillations (that is, their wavefunctions contain no oscillatory phases
independent of the momenta). We show that this result is exact, independent of
the number of fields, the field space geometry and the shape of the
inflationary trajectory followed in multi-field space. In the specific case
where the multi-field trajectory is a straight line or confined to a plane, it
can be understood as the existence of an upper bound on the dynamical masses
of extra fields of the form up to slow roll corrections.
This bound corresponds to the analytic continuation of the well known
Breitenlohner--Freedman bound found in AdS spacetimes in the case when the
masses are approximately constant. The absence of underdamped oscillations
implies that a detection of "cosmological collider" oscillatory patterns in the
non-Gaussian bispectrum would not only rule out single field inflation, but
also holographic inflation or any inflationary model based on the
Hamilton--Jacobi equations. Hence, future observations have the potential to
exclude, at once, an entire class of inflationary theories, regardless of the
details involved in their model building.Comment: References added. Discussion expanded to include arbitrary number of
fields. Matches published version in PR
Universality and scaling in multi-field -attractor preheating
We explore preheating in multi-field models of inflation in which the
field-space metric is a highly curved hyperbolic manifold. One broad family of
such models is called -attractors, whose single-field regimes have been
extensively studied in the context of inflation and supergravity. We focus on a
simple two-field generalization of the -model, which has received renewed
attention in the literature. Krajewski et al. concluded, using lattice
simulations, that multi-field effects can dramatically speed-up preheating. We
recover their results and further demonstrate that significant analytical
progress can be made for preheating in these models using the WKB approximation
and Floquet analysis. We find a simple scaling behavior of the Floquet
exponents for large values of the field-space curvature, that enables a quick
estimation of the -model reheating efficiency for any large value of the
field-space curvature. In this regime we further observe and explain universal
preheating features that arise for different values of the potential steepness.
In general preheating is faster for larger negative values of the field-space
curvature and steeper potentials. For very highly curved field-space manifolds
preheating is essentially instantaneous.Comment: 43 pages, 21 figures; v2: published version with analysis extende
New class of de Sitter vacua in string theory compactifications
We revisit the stability of the complex structure moduli in the large volume
regime of type-IIB flux compactifications. We argue that when the volume is not
exponentially large, such as in K\"ahler uplifted dS vacua, the quantum
corrections to the tree-level mass spectrum can induce tachyonic instabilities
in this sector. We discuss a Random Matrix Theory model for the classical
spectrum of the complex structure fields, and derive a new stability bound
involving the compactification volume and the (very large) number of moduli. We
also present a new class of vacua for this sector where the mass spectrum
presents a finite gap, without invoking large supersymmetric masses. At these
vacua the complex structure sector is protected from tachyonic instabilities
even at non-exponential volumes. A distinguishing feature is that all fermions
in this sector are lighter than the gravitino.Comment: 6 pages, 2 figure