BCFT moduli space in level truncation

TL;DR

This paper introduces a new non-perturbative approach to explore the BCFT moduli space in open string field theory by analyzing a non-universal branch of the tachyon potential, extending the understanding of marginal deformations.

Contribution

It proposes a novel method focusing on a non-universal tachyon branch to better parametrize the BCFT moduli space in level truncated open string field theory.

Findings

Effective potential becomes increasingly flat in the non-universal sector.

The marginal field coefficient reaches a maximum compatible with existing solutions.

The full periodic moduli space of the cosine deformation is covered.

Abstract

We propose a new non-perturbative method to search for marginal deformations in level truncated open string field theory. Instead of studying the flatness of the effective potential for the marginal field (which is not expected to give a one-to-one parametrization of the BCFT moduli space), we identify a new non-universal branch of the tachyon potential which, from known analytic examples, is expected to parametrize the marginal flow in a much larger region of the BCFT moduli space. By a level 18 computation in Siegel gauge, we find an increasingly flat effective potential in the non-universal sector, connected to the perturbative vacuum and we confirm that the coefficient of the marginal field (lambda_SFT) has a maximum compatible with the value where the solutions stop existing in the standard Sen-Zwiebach approach. At the maximal reachable level, the effective potential still deviates from flatness for large values of the tachyon, but the Ellwood invariants stay close to the correct BCFT values on the whole branch and the full periodic moduli space of the cosine deformation is covered.