  We explore the vacuum structure in bosonic open string field theory expanded
around an identity-based solution parameterized by $a(>=-1/2)$. Analyzing the
expanded theory using level truncation approximation up to level 20, we find
that the theory has the tachyon vacuum solution for $a>-1/2$. We also find
that, at $a=-1/2$, there exists an unstable vacuum solution in the expanded
theory and the solution is expected to be the perturbative open string vacuum.
These results reasonably support the expectation that the identity-based
solution is a trivial pure gauge configuration for $a>-1/2$, but it can be
regarded as the tachyon vacuum solution at $a=-1/2$.

High Energy Physics - Theory