Numerical universal solutions in $a$-gauge in open string field theory

In bosonic open string field theory, we construct numerical universal solutions in $a$-gauge corresponding to ``double brane'' and ``ghost brane'' solutions in Siegel gauge in addition to the tachyon vacuum solution, and evaluate their gauge invariants, which are energy and gauge-invariant observable. The $a$-gauge condition, which contains a real parameter $a$, was introduced by Asano and Kato. In earlier works it has been applied to find the tachyon vacuum solution with the level truncation method up to level $14$. The ``double brane'' and ``ghost brane'' solutions were constructed by Kudrna and Schnabl in Siegel gauge, which corresponds to ($a=0$)-gauge, up to level $28$. Starting from these solutions, by varying $a$ little by little, we have constructed numerical solutions in $a$-gauge for various values of $a$ including $a=\infty$ up to level $20$. Contrary to naive expectation, the gauge invariants of ``double brane'' and ``ghost brane'' solutions in $a$-gauge seem to be non-constant for $a$. In particular, although the normalized energy $E$ of the ``double brane'' solution in $a$-gauge is approximately two for $a\sim 0$, we find that $E$ becomes almost one for $0.5<a<1$. The gauge-invariant observable also behaves similarly. It might imply that the ``double brane'' solution varies to a single brane solution in such $a$-gauges.