Contents

17 January 2014
Vol 343, Issue 6168

About The Cover

Cover image expansion

COVER Artist's conception of the charge distribution associated with virtual particles surrounding an electron. Spin precession measurements in molecular thorium oxide show that any deviation from perfect roundness (as depicted here, greatly exaggerated) must be smaller than one part in a quadrillion. This finding strongly constrains proposed extensions to the Standard Model of particle physics. See pages 255 and 269. Image: Gary Pikovsky