My experimental collaboration, Belle, has a paper presenting evidence for the mixing of charm mesons (D0 and anti-D0) in the current issue of Physical Review Letters.
This is an example of matter turning into antimatter, and vice versa. There are four known mesons for which this can occur without "breaking the rules", and the D0 system is the last of the four in which the effect has been observed. A related phenomenon may, or may not, occur among the neutrinos. (Mixing is known to occur between different neutrino types [called "flavours"], a process usually known as neutrino oscillation due to its characteristic signature. It's still unresolved whether neutrinos and anti-neutrinos mix.) It's a pretty big deal in any case, especially for those of us who've devoted time to studying the charm sector. Mixing is one of the principal concerns of the research group I founded at Belle (and ran for many years), and so finding it is a milestone for us. And personally, when a paper has taken a large fraction of your life for a year or more, it's very satisfying for it to be completed.
A semi-technical summary of the work is available in the news section of the CERN Courier. Our paper is PRL 98, 211803, also available as a preprint at arXiv:hep-ex/0703036. The competing experiment, BaBar, presents a different kind of evidence for the same phenomenon in a paper published back-to-back with ours. These results were the talk of the conference at the electroweak session of this year's Rencontres de Moriond in March, when they were first announced, and have provoked a lot of theoretical discussion since.