22-Mar-2019: Physicists observe Matter-Antimatter Asymmetry in Charm Particle decays

Physicists from the Large Hadron Collider beauty (LHCb) Collaboration at CERN have observed, for the first time, the matter-antimatter asymmetry known as charge-parity (CP) violation in the decays of a D⁰ meson, a subatomic particle made up of a charm quark and an up antiquark.

The term CP refers to the transformation that swaps a particle with the mirror image of its antiparticle. The weak interactions of the Standard Model of particle physics are known to induce a difference in the behavior of some particles and of their CP counterparts, an asymmetry known as CP violation. This asymmetry is one of the key ingredients required to explain why today’s Universe is only composed of matter particles, with essentially no residual presence of antimatter.

The phenomenon was first observed in 1964 in the decays of particles called neutral K mesons, which contain a strange quark, and the two physicists who made the discovery, James Cronin and Val Fitch, were awarded the Nobel Prize in physics in 1980. Such a discovery came as a great surprise at the time, as it was firmly believed by the community of particle physicists that the CP symmetry could not be violated.

In the early 1970s, Makoto Kobayashi and Toshihide Maskawa realized that CP violation could be included naturally in the Standard Model. Their fundamental idea was confirmed eventually three decades later by the discovery of CP violation in the decays of neutral B mesons, which contain a bottom quark, by the BaBar and Belle collaborations, leading to the award of the 2008 Nobel Prize in physics to Kobayashi and Maskawa.

There have been many attempts to measure matter-antimatter asymmetry, but, until now, no one has succeeded. It’ a milestone in antimatter research.

To observe the CP asymmetry in the D⁰ meson, the physicists used the full dataset delivered by the Large Hadron Collider to the LHCb experiment between 2011 and 2018 to look for decays of the D⁰ meson and its antiparticle, the anti-D⁰, into either kaons or pions.

We don’t see antimatter in our world, so we have to artificially produce it. The data from these collisions enables us to map the decay and transformation of unstable particles into more stable byproducts.

The result has a statistical significance of 5.3 standard deviations, exceeding the threshold of 5 standard deviations used by particle physicists to claim a discovery.

Ever since the discovery of the D meson more than 40 years ago, particle physicists have suspected that CP violation also occurs in this system, but it was only now, using essentially the full data sample collected by the experiment, that the LHCb Collaboration has finally been able to observe the effect.”