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