Level truncation and the quartic tachyon coupling

Abstract

We discuss the convergence of level truncation in bosonic open string field theory. As a test case we consider the calculation of the quartic tachyon coupling $γ_4$. We determine the exact contribution from states up to level L=28 and discuss the $L\to\infty$ extrapolation by means of the BST algorithm. We determine in a self-consistent way both the coupling and the exponent $ω$ of the leading correction to $γ_4$ at finite $L$ that we assume to be $\sim 1/L^ω$. The results are $γ_4 = -1.7422006(9)$ and $|ω-1|\lesssim 10^{-4}$.}