The stock market’s second half of 2015 is clearly off to a poor start. Wednesday’s decline was a perfect reminder that traders are nervous about the latest headlines. The Nasdaq composite paced the sell-off, falling nearly 1.8%. The S&P 500 was off 1.7% and slumped back below its 200-day moving average. (See Page B2) The IBD 50 dropped 1.9%. Volume fell below Tuesday’s levels. But as seen on Page B2, turnover was still heavy on the Nasdaq.