884 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 $\pi_0(H)$. In particular, we find that, unless
$2\sin^2\theta_W$ is a rational number, the thermal splitting of electroweak
Z-strings by magnetically neutral black holes is impossible, even though
$\pi_0(H)$ 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

### Topological Extensions of Noether Charge Algebras carried by D-p-branes

We derive the fully extended supersymmetry algebra carried by D-branes in a
massless type IIA superspace vacuum. We find that the extended algebra contains
not only topological charges that probe the presence of compact spacetime
dimensions but also pieces that measure non-trivial configurations of the gauge
field on the worldvolume of the brane. Furthermore there are terms that measure
the coupling of the non-triviality of the worldvolume regarded as a U(1)-bundle
of the gauge field to possible compact spacetime dimensions. In particular, the
extended algebra carried by the D-2-brane can contain the charge of a Dirac
monopole of the gauge field. In the course of this work we derive a set of
generalized Gamma-matrix identities that include the ones presently known for
the IIA case.
In the first part of the paper we give an introduction to the basic notions
of Noether current algebras and charge algebras; furthermore we find a Theorem
that describes in a general context how the presence of a gauge field on the
worldvolume of an embedded object transforming under the symmetry group on the
target space alters the algebra of the Noether charges, which otherwise would
be the same as the algebra of the symmetry group. This is a phenomenon recently
found by Sorokin and Townsend in the case of the M-5-brane, but here we show
that it holds quite generally, and in particular also in the case of D-branes.Comment: 45 pages, some minor misprints corrected, no modifications otherwis

### Unstable vortices do not confine

Recently, a geometric model for the confinement of magnetic charges in the
context of type II string compactifications was constructed by Greene, Morrison
and Vafa. This model assumes the existence of stable magnetic vortices with
quantized flux in the low energy theory. However, quantization of flux alone
does not imply that the vortex is stable, since the flux may not be confined to
a tube of definite size. We show that in the field theoretical model which
underlies the geometric model of confinement, static, cylindrically symmetric
magnetic vortices do not exist. While our results do not preclude the existence
of confinement in a different low-energy regime of string theory, they show
that confinement is not a universal outcome of the string picture, and its
origin in the low energy theory remains to be understood.Comment: Latex, 8 page

### Embedded Monopoles

Using the embedded defect method, we classify the possible embeddings of a 't
Hooft-Polyakov monopole in a general gauge theory. We then discuss some
similarities with embedded vortices and relate our results to fundamental
monopoles.Comment: 6 pages, LaTe

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