Last week, the world witnessed a fascinating new discovery. The Babar Collaboration announced the first compelling evidence that the charm quark, like its siblings strange and bottom, plays a role in the mixing of matter and antimatter. What follows is a slightly edited version of a description I sent to my father. I knew I would be disappointed with how the initial press releases would explain the meaning of this discovery, so I wrote this for my dad’s benefit when the paper was made public.
Like its siblings, charm mesons form heavier particles called “mesons” as they pair with another, less massive quark. The D0 (pronounced “dee zero”) meson is the lightest meson that can be formed from the charm quark, when it pairs up with its little sister, the up quark. For a long time, physicists have been searching for evidence that matter D0 mesons can change into antimatter D0 mesons, a process allowed by the weak force. The B0 and the K0 mesons, formed from bottom and strange quarks (respectively), have long been known to mix up their matter and antimatter states. This effect plays a critical role in the fact that our universe is dominated by matter.
BaBar has now definitive evidence that D0 mesons also mix between their matter and antimatter states. But why is this important?
Matter/antimatter mixing has been seen for bottom and strange mesons. This phenomenon is also predicted for the charm quark, first discovered at SLAC and Brookhaven in 1974. Unlike its strange and bottom cousins, discovered in the 1950s and 1979, respectively, evidence of matter/antimatter mixing has remained elusive. BaBar now has the first definitive evidence that charm-quark mesons experience the same phenomenon. This indicates yet another place in the early universe where matter and antimatter can be intermixed.
There is no evidence for the most important phenomenon – violation of matter/antimatter symmetry – which has been found for both the strange and bottom mesons. If it exists at all for charm, it will still have to be teased out of even more data. But this new discovery is a striking test of the Standard Model, which predicted that charm mixing could be as large as has been observed: given enough time, about 1% of all neutral charm mesons change to antimatter. It’s a teeny, tiny effect, and that’s why it took so long to find it.
But what could all of this mean? Time will tell, but BaBar’s spokesman, Dr. Hassan Jawahery, conveys the hope and excitement best: “[it] could be the result of the Standard Model, or it could be new physics.” (see http://sciencenow.sciencemag.org/cgi/content/full/2007/314/2)
