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LHC spots mesons flipping between matter and antimatter

Charming change a small step to explaining why matter exists

The LHC might be inactive at the moment, but the science goes on: a group of researchers trawling the instrument's vats of data has spotted a matter-antimatter oscillation that completes their “zoo” of meson “flips”.

In a paper published at Arxiv and accepted by Physical Review Letters, the researchers say they've spotted D mesons oscillating between matter and antimatter states, a kind of interaction previously observed in K mesons and two types of B mesons.

The oscillation might sound odd, but it's an important observation in charge parity (CP) violation studies. For example, as this explanatory note at SLAC puts it: “Such 'oscillations' are allowed in the Standard Model and have been observed for neutral mesons containing strange (the K0 meson containing a strange antiquark and a d quark) or beauty quarks (both the B0 meson containing a beauty antiquark and a d quark).”

Mesons are particles with two quarks – one matter, the other antimatter. For example, D mesons combine a charm quark with an up antiquark. They also have a finite lifetime.

In their publication, “the first observation of (D meson / D anti-meson) oscillations from a single measurement”, the LHCb collaboration authors suggest “Together with the recent evidence of CP violation in charm decays, this can be interpreted as a hint for the presence of non-SM physics.” (It's D meson decays that also provided other hints at CP violation, the paper notes).

The D meson oscillation had been inferred from other experiments, but this is the first time a single measurement has exceeded five-sigma confidence. CP violation is part of the investigation into why matter exists, since if the proto-universe had produced all particle types in symmetry, matter and antimatter would have wiped each other out. ®

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