Systematic traders who tie their fortunes to the ebbs and flows of stock markets are experiencing some of their worst returns in eight years, according to one Wall Street estimate. Factor investing -- which slices and dices equities based on traits like profitability and price volatility -- has buckled while the broader market has stayed afloat. For example, AQR Capital Management LLC’s $1.9 billion mutual fund, one of the largest in the sector, last month nursed its steepest loss since inception.