The observed excess of matter over antimatter in the Universe is an enduring puzzle in physics. The imbalance implies a difference in the behaviour of matter and antimatter particles. This difference, or "asymmetry", is known as CP violation and is a fundamental part of the Standard Model of particle physics. But the amount of CP violation predicted by the model and observed so far in experiments is too small to explain the cosmic imbalance, suggesting the existence of as-yet-unknown sources and manifestations of CP violation beyond the Standard Model.
At the nineteenth beauty conference last month and at a seminar today at CERN, the LHCb collaboration reported the first observation of so-called time-dependent matter-antimatter asymmetry in particles known as Bs0 mesons, which contain a beauty antiquark and a strange quark.
CP violation was first observed more than five decades ago in particles called K0 mesons, and has since been observed in other types of particle - including in B0 mesons in 2001 by experiments at the SLAC laboratory in the US and the KEK laboratory in Japan, and recently by the LHCb collaboration in D0 mesons. The effect can manifest itself in two forms: time-integrated and time-dependent. In the time-integrated form, the number of transformations, or "decays", of a matter particle into certain particles differs from that of the corresponding antimatter particle. In the time-dependent form, the violation varies with the particle's lifetime due to the spontaneous oscillation of the particle into its antiparticle and back.
The new LHCb study provides the first observation of time-dependent CP violation in Bs0 mesons, in their decays into charged K mesons. The result, obtained by combining data collected during the first and second runs of the Large Hadron Collider, has a statistical significance of 6.7 standard deviations, which is beyond the threshold of 5 standard deviations used by particle physicists to claim an observation.
"The Bs0 mesons oscillate between particle and antiparticle three thousand billion times per second, but the excellent resolution of our detector made it possible to observe the effect of these oscillations. Our observation of time-dependent CP violation in Bs0 mesons represents a further milestone in the study of the differences between matter and antimatter," says LHCb spokesperson Chris Parkes, "adding to our previous observation of time-integrated CP violation in these mesons."
The next steps will be to compare the measurement with other measurements of CP violation and with predictions from the Standard Model and beyond. It's only after researchers make these comparisons that they will be able to tell whether or not the new measurement hides any surprises that might help to explain the matter-antimatter imbalance in the universe.
