Large bulk matter search for fractional charge particles
  We have carried out the largest search for stable particles with fractional electric charge, based on an oil drop method that incorporates a horizontal electric field and upward air flow. No evidence for such particles was found, giving a 95% C.L. upper limit of $1.17\times 10^{-22}$ particles per nucleon on the abundance of fractional charge particles in silicone oil for $0.18 e \le |Q_{residual}| \le 0.82 e$. Since this is the first use of this new method we describe the advantages and limitations of the method. 
Irwin T. Lee, Sewan Fan, Valerie Halyo, Eric R. Lee, Peter C. Kim,   Martin L. Perl and Howard Rogers, Dinesh Loomba, Klaus ...