Rolling Tachyon Solution in Vacuum String Field Theory
  We construct a time-dependent solution in vacuum string field theory and investigate whether the solution can be regarded as a rolling tachyon solution. First, compactifying one space direction on a circle of radius R, we construct a space-dependent solution given as an infinite number of *-products of a string field with center-of-mass momentum dependence of the form e^{-b p^2/4}. Our time-dependent solution is obtained by an inverse Wick rotation of the compactified space direction. We focus on one particular component field of the solution, which takes the form of the partition function of a Coulomb system on a circle with temperature R^2. Analyzing this component field both analytically and numerically using Monte Carlo simulation, we find that the parameter b in the solution must be set equal to zero for the solution to approach a finite value in the large time limit x^0\to\infty. We also explore the possibility that the self-dual radius R=\sqrt{\alpha'} is a phase transition point of our Coulomb system. 
Masako Fujita and Hiroyuki Hata