Buckethead, who began a stint opening for Particle on March 31, has started to sit in with the quartet as well. On Saturday night at Chicago’s Park West he joined the group for a version of Kraftwerk’s ‘We are the Robots,’ initially demonstrating his signature robot dancing before engaging Particle’s Charlie Hitchcock in some guitar interplay. Buckethead will continue to open Particle gigs through April 16 including tonight at Mr. Smalls Theatre in Pittsburgh.