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

TL;DR

This paper constructs non-supersymmetric, tachyon-free Type I string models with standard model-like features, exploring low string scale scenarios and potential mechanisms for extra dimension stabilization.

Contribution

It presents the first semi-realistic string vacua with broken supersymmetry and a low string scale, incorporating standard model-like gauge groups and anomaly cancellation mechanisms.

Findings

Constructed three-generation models with standard model gauge groups.

Achieved models with a low string scale down to 1-100 TeV.

Proposed a new radius stabilization mechanism via brane-antibrane forces.

Abstract

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.