Analytic Construction of Multi-brane Solutions in Cubic String Field Theory for Any Brane Number

We present an analytic construction of multi-brane solutions with any integer brane number in cubic open string field theory (CSFT) on the basis of the $KBc$ algebra. Our solution is given in the pure-gauge form $\Psi=UQ_\textrm{B}U^{-1}$ by a unitary string field $U$, which we choose to satisfy two requirements. First, the energy density of the solution should reproduce that of the $(N+1)$-branes. Second, the EOM of the solution should hold against the solution itself. In spite of the pure-gauge form of $\Psi$, these two conditions are non-trivial ones due to the singularity at $K=0$. For the $(N+1)$-brane solution, our $U$ is specified by $[N/2]$ independent real parameters $\alpha_k$. For the 2-brane ($N=1$), the solution is unique and reproduces the known one. We find that $\alpha_k$ satisfying the two conditions indeed exist as far as we have tested for various integer values of $N$ $(=2, 3, 4, 5, \cdots)$. Our multi-brane solutions consisting only of the elements of the $KBc$ algebra have the problem that the EOM is not satisfied against the Fock states and therefore are not complete ones. However, our construction should be an important step toward understanding the topological nature of CSFT which has similarities to the Chern-Simons theory in three dimensions.