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 LHCb magnet. Image credit: CERN.
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,” said Syracuse University’s Professor Sheldon Stone, member of the LHCb Collaboration.
A CP-symmetry transformation swaps a particle with the mirror image of its antiparticle. The LHCb Collaboration has observed a breakdown of this symmetry in the decays of the D0 meson (illustrated by the big sphere on the right) and its antimatter counterpart, the anti-D0 (big sphere on the left), into other particles (smaller spheres). The extent of the breakdown was deduced from the difference in the number of decays in each case (vertical bars, for illustration only). Image credit: CERN.
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.
“Looking for these two decay products in our unprecedented sample of D0 particles gave us the required sensitivity to measure the tiny amount of CP violation expected for such decays,” said Dr. Giovanni Passaleva, spokesperson for the LHCb Collaboration.
“Measuring the extent of the violation then boiled down to counting the D0 and anti-D0 decays and taking the difference.”
“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,” said Syracuse University’s Dr. Ivan Polyakov, member of the LHCb Collaboration.
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.
“What makes this study unique is that it is the first time anyone has witnessed particles with charmed quarks being asymmetrical. It’s one for the history books,” Professor Stone said.
“The result is a milestone in the history of particle physics,” said Dr. Eckhard Elsen, CERN Director for Research and Computing.
“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.”
The results were presented this week at the 54th Rencontres de Moriond Conference, in the CERN seminar presentation, and in the LHCb paper that will be published in the journal Physical Review Letters.
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R. Aaij et al (LHCb Collaboration). 2019. Observation of CP violation in charm decays. Phys. Rev. Lett, in press; CERN-EP-2019-042
