1,689 research outputs found

### S-Branes, Negative Tension Branes and Cosmology

A general class of solutions of string background equations is studied and
its physical interpretations are presented. These solutions correspond to
generalizations of the standard black p-brane solutions to surfaces with
curvature k=-1,0. The relation with the recently introduced S-branes is
provided. The mass, charge, entropy and Hawking temperature are computed,
illustrating the interpretation in terms of negative tension branes. Their
cosmological interpretation is discussed as well as their potential instability
under small perturbations.Comment: 10 pages. Talks given at: SUSY'02, ``the 10th International
Conference on Supersymmetry and Unification of Fundamental Interactions'',
DESY, Hamburg, Germany, 17-23 June 2002, and ``The 1st International
Conference on String Phenomenology'', Oxford, July 6 - 11, 2002. References
adde

### String Inflation After Planck 2013

We briefly summarize the impact of the recent Planck measurements for string
inflationary models, and outline what might be expected to be learned in the
near future from the expected improvement in sensitivity to the primordial
tensor-to-scalar ratio. We comment on whether these models provide sufficient
added value to compensate for their complexity, and ask how they fare in the
face of the new constraints on non-gaussianity and dark radiation. We argue
that as a group the predictions made before Planck agree well with what has
been seen, and draw conclusions from this about what is likely to mean as
sensitivity to primordial gravitational waves improves.Comment: LaTeX, 21 pages plus references; slight modification of the
discussion of inflection point inflation, references added and typos
correcte

### A D-Brane Alternative to the MSSM

The success of SU(5)-like gauge coupling unification boundary conditions
$g_3^2=g_2^2=5/3 g_1^2$ has biased most attempts to embed the SM interactions
into a unified structure. After discussing the limitations of the orthodox
approach, we propose an alternative that appears to be quite naturally implied
by recent developments based on D-brane physics. In this new alternative: 1)
The gauge group, above a scale of order 1 TeV, is the minimal left-right
symmetric extension $SU(3)\times SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ of the
SM; 2) Quarks, leptons and Higgs fields come in three generations; 3) Couplings
unify at an intermediate string scale $M_s= 9\times 10^{11}$ GeV with boundary
conditions $g_3^2=g_L^2=g_R^2=32/3 g_{B-L}^2$. This corresponds to the natural
embedding of gauge interactions into D-branes and is different from the
standard SO(10) embedding which corresponds to $k_{B-L}=8/3$. Unification only
works in the case of three generations;
4) Proton stability is automatic due to the presence of $Z_2$ discrete
R-parity and lepton parities. A specific Type IIB string orientifold model with
the above characteristics is constructed. The existence of three generations is
directly related to the existence of three complex extra dimensions.
In this model the string scale can be identified with the intermediate scale
and SUSY is broken also at that scale due to the presence of anti-branes in the
vacuum.
We discuss a number of phenomenological issues in this model including Yukawa
couplings and a built-in axion solution to the strong-CP problem. The present
framework could be tested by future accelerators by finding the left-right
symmetric extension of the SM at a scale of order 1 TeV.Comment: 50 pages, 7 figures. References adde

### Standard-like Models with Broken Supersymmetry from Type I String Vacua

We construct D=4 Type I vacua with massless content remarkably close to that
of the standard model of particle physics. They are tachyon-free
non-supersymmetric models which are obtained starting with a standard D=4, N=1
compact Type IIB orientifold and adding the same number of Dp-branes and
anti-Dp-branes distributed at different points of the underlying orbifold.
Supersymmetry-breaking is felt by the observable world either directly, by
gravity mediation or gauge mediation, depending on the brane configuration. We
construct several simple three generation examples with the gauge group of the
standard model or its left-right symmetric extensions. The models contain a
number of U(1) gauge groups whose anomalies are cancelled by a generalized
Green-Schwarz mechanism. These U(1)'s are broken but may survive as global
symmetries providing for a flavour structure to the models. The value of the
string scale may be lowered down to the intermediate scale (as required in the
gravity mediation case) or down to 1-100 TeV for the non-SUSY models. Thus the
present models are the first semirealistic string vacua realizing the
possibility of a low string scale. The unbalanced force between the pairs of
Dp- and anti-Dp-branes provides for an effect which tends to compactify some of
the extra dimensions but no others. This could provide a new mechanism for
radius stabilization.Comment: 58 pages, latex (typos corrected and references added

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