Sometimes, when I get too excited about subatomic particles at home, I am told “Matter, antimatter … doesn’t matter to me!” To which I never, ever reply, “Oh beloved family member, you wouldn’t exist if there weren’t any difference between the two!”
Measured in the lab, matter and antimatter are nearly, but not exactly, the same. Seen in the universe, though, antimatter is extremely rare; the universe is made pretty much entirely out of matter. The step in the universe’s development that produced this outcome is called baryogenesis, which happened very early in the universe’s history.
The details of how baryogenesis works are still not clear; from the physics we know about, the universe shouldn’t necessarily come out to be composed entirely of matter. And a universe with large amounts of antimatter would be very different indeed, with regions where matter-antimatter annihilations give huge energy flares. For this reason, physicists continue to look for ways in which matter and antimatter differ.
In 2010, DZero got an unusual result in events where two muons are created in the collision of protons with antiprotons. The “like-sign dimuon asymmetry” measures the difference between matter and antimatter; the result was not the same as what is expected from the Standard Model, so it might be a result of some new kind of difference between matter and antimatter. On the other hand, the result was not so wildly different from what was expected that one could be certain of a discovery. Later studies from DZero continued to get the same result, but there are no other experiments that can repeat this exact same study, so independent confirmation does not exist.
What we can do, though, is look at different possible causes for that unusual result. One possible cause is that, when a b quark in a meson decays into a combination of three particles (a negative muon, a neutrino and an anti-D meson, or μ–νD0), that this is somehow different from the antimatter version of the same process, B–→μ+νD0.
Recently, DZero measured the difference in the rates of the decays B–→μ–νD0 vs. B+→μ+νD0. In the Standard Model, this difference should be zero; we do not know of any way in which the matter and antimatter should be different in these two decays. And indeed it isn’t; we found that the difference between the two is only 0.03 percent, with an uncertainty of 0.27 percent. Another reason for the unusual 2010 result needs to be found!