Rick Ayers doesn’t look much like the geek he says he has always been. A former golfer and rock guitarist, six feet tall, with a weight lifter’s build, long hair and a soul patch, Mr. Ayers, 37, stands out in the gray corridors of the National Institute of Standards and Technology…The obsessive qualities that drove him at the tee and onstage have made him an expert in the use of handheld devices like cellphones, BlackBerries and P.D.A.’s in solving crimes. It is a booming specialty in the fast-growing field of digital forensics…