D3/D7 Brane Inflation and Semilocal Strings

TL;DR

This paper proposes a generalized D3/D7 brane inflation model that produces semilocal strings, which are cosmologically benign and do not require fine-tuned gauge couplings, simplifying realistic model building.

Contribution

It introduces a new D3/D7 inflation variant leading to semilocal strings that are supersymmetric and cosmologically harmless, unlike traditional topological strings.

Findings

Semilocal strings are unbroken supersymmetric in supergravity with FI terms.

These strings are energetically independent of their thickness.

They disappear quickly after formation, avoiding cosmological issues.

Abstract

Among the inflationary models based on string theory, the D3/D7 model has the advantage that the flatness of the inflaton potential can be protected even with moduli stabilization. However, the Abrikosov-Nielsen-Olesen BPS cosmic strings produced at the end of original D3/D7 inflation lead to an additional contribution to the CMB anisotropy. To make this contribution consistent with the WMAP results one needs an extremely small gauge coupling in the effective D-term inflation model. Such couplings may be difficult to justify in string theory. Here we develop a generalized version of the D3/D7 brane model, which leads to semilocal strings, instead of the topologically stable ANO cosmic strings. We show that the semilocal strings have unbroken supersymmetry when embedded into supergravity with FI terms. The energy of these strings is independent of their thickness. We confirm the existing arguments that strings of such type disappear soon after their formation and do not pose any cosmological problems, for any value of the gauge coupling. This should simplify the task of constructing fully realistic models of D3/D7 inflation.