For nearly two decades, the mathematical whiz Peter Muller and his secretive band of traders have helped power Morgan Stanley to bigger profits. But now Morgan Stanley and Mr. Muller are in advanced talks about splitting up. Under the plan being discussed, Morgan Stanley would spin off Mr. Muller’s unit, called Process Driven Trading, and keep a minority stake. Mr. Muller and his team would get access to Morgan Stanley’s infrastructure, including its legal and other resources. Mr. Muller, 47, is part of a new breed of investors called quants — short for quantitative — who use high-speed computers to turbocharge their mathematically powered investing skills. They program their computers to track different data and variables like the historic relationship between certain pairs of stocks. If the relationship goes out of kilter, a quant might make a bet, amplified with borrowed money, that the historical relationship between the stocks will return.