EXPLAINER-U. S. yield curve reaches deepest inversion since 1981: What . . . By David Randall and Davide Barbuscia NEW YORK, March 7 (Reuters) - Hawkish comments by Federal Reserve Chairman Jerome Powell helped push a closely watched part of the U S Treasury yield curve to its deepest inversion since 1981 on Tuesday, once again putting a spotlight on what many investors consider a time-honored recession signal The U S central bank has hiked interest rates