A yoctosecond (ys) is a septillionth of a second or 10–24 s*. Yocto comes from the Latin/Greek word octo/οκτώ, meaning “eight”, because it is equal to 1000−8. Yocto is the smallest official SI prefix.
A yoctosecond is the shortest lifetime measured, so far. The elementary particle Z-boson has a mean lifetime of less than a yoctosecond, 0.26 ys.
W and Z bosons mediate the weak interaction. This is responsible for nuclear radiation and particle decays. W bosons have a positive and negative electric charge of one elementary charge. The Z boson is electrically neutral and is its own antiparticle.
* in Europe septillionth is called a quadrillionth; this is due to use of long and short scales.
 M. Quack, Fundamental Symmetries and Symmetry Violations from High Resolution Spectroscopy, Chapt. 18, in Handbook of High Resolution Spectroscopy, Vol. 1 (Eds.: M. Quack, F. Merkt), Wiley, Chichester, New York, 2011, pp. 659-722.
 M. Quack, Molecular Femtosecond Quantum Dynamics Between Less than Yoctoseconds and More than Days: Experiment and Theory, chapter 27, in Femtosecond Chemistry (Eds.: J. Manz, L. Woeste), Verlag Chemie, Weinheim, 1994, p. 781.
 M. Quack, J. Stohner, M. Willeke, Annu. Rev. Phys. Chem. 2008, 59, 741–769.