Puzzling new pieces inthe antimatter mystery

New measurements from DZero add pieces to the puzzle of matter-antimatter asymmetry.

One of the most puzzling properties of matter in our universe is that there’s so much of it in the first place. When new matter is created during a particle collision, an equal amount of antimatter accompanies it nearly every time. The Standard Model allows only a very slight deviation from this symmetry, and not nearly enough difference between matter and antimatter to explain the abundance of matter in our universe today.

The DZero Collaboration has previously shown evidence for a greater matter-antimatter asymmetry than predicted by the Standard Model in the decays of neutral B mesons, particles that combine a bottom quark with either a down quark (Bd0) or strange quark (Bs0). Two new measurements, which focus separately on the Bd0 and Bs0, now add complementary pieces to this puzzling matter.

Both analyses measured the difference in the number of times a neutral B meson decay included a muon versus an antimuon. If there were no preference for matter over antimatter, then the difference would be zero. Precisely measuring this asymmetry requires accounting for any differences between the ability to find and measure the muon and antimuon versions of B decays.

Muons and antimuons have opposite electric charges, so they will bend in opposite directions when moving in a magnetic field and take different paths through the DZero detector, which can affect their chances of being detected. Because reversing the magnetic field swaps these particle paths, periodically reversing the fields over the course of Run II allowed the analyzers to mitigate this effect on the asymmetry measurement. Complementing this with careful studies of detector performance enabled analyzers to significantly improve upon the previous world’s best measurements for the Bd0 and Bs0 asymmetries.

The new measurements are consistent with the previous evidence for matter-antimatter asymmetry reported by the DZero collaboration, which measured a particular mixture of the Bd0 and Bs0 system asymmetries in an independent channel. In combination, these results differ from the Standard Model by about 3 standard deviations. These new pieces hint at a very interesting picture of the universe, but more precise measurements will be required to finally solve this puzzle.

—Mike Cooke

These physicists made major contributions to this analysis.

The muon identification group’s efforts are critical to all analyses that look for muons, like those described above. They determine the criteria used to find muons with the DZero detector and ensure that muons are well modeled in event simulations.