21 March 2019
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)
(Credit: Credit: CERN)
The LHCb experiment has announced the first ever observation of CP violation in the decays of charm mesons. This represents a milestone in particle physics and UK physicists have been at the heart of this research.
The research finding from the LHCb collaboration at CERN shows, for the first time ever, the differences in matter–antimatter behaviour known as CP violation in a particle dubbed the D0 meson. The finding is sure to make it into the textbooks of particle physics.
Dr Marco Gersabeck, Lecturer in Physics, The University of Manchester said of the finding:
"The discovery of CP violation in charm is a major breakthrough in charm physics and for particle physics as a whole. Having pioneered the use of charm particles produced directly in the LHC collisions and having worked on the present analysis method before, it is great to have achieved this discovery. This marks the start of the era of precision physics in charm and I’m looking forward to many more exciting results."
Professor Tara Shears, who leads the University of Liverpool LHCb group made the point that
“It’s taken a full analysis of all the data we’ve collected so far to finally see this tiny but profound effect. Understanding the nature of CP violation is one of the biggest mysteries we have – and seeing it displayed here, in charm mesons, provides a whole new laboratory to study it in. This observation is just the first step in our exploration. Now we want to see what charm quarks can tell us about it.”
The term CP refers to the transformation that swaps a particle into its antiparticle. The weak interactions of the Standard Model of particle physics are known to induce a difference in the behaviour of some particles and of their CP counterparts, an asymmetry known as CP violation.
Currently the LHCb detector at CERN is undergoing a substantial upgrade ready to come back online in 2021. The upgrade will allow it LHCb to analyse ten times more data than it has taken so far.
UK Spokesperson for the LHCb is Professor Tim Gershon from the University of Warwick and he is keen to see just what the massive increase of data will be able to tell us.
“This CP violation is necessary to induce the processes that, following the Big Bang, established the abundance of matter over antimatter that we observe in the present-day universe. We have a lot more work to do in this research field to fully understand what is taking place and this means that when the LHC comes back online in 2021 the new capabilities of LHCb could bring us closer to answering key questions around the Standard model of Physics.”
The size of CP violation observed so far in Standard Model interactions is too small to account for the present-day matter–antimatter imbalance, suggesting the existence of additional as-yet-unknown sources of CP violation.
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, and proved that we finally have the capability to study the delicate effects of CP violation in charm quarks. But a discovery is just a beginning. In the larger datasets the upgraded LHCb will collect, charm quarks will allow a search for possible new sources of CP violation. The results will spur theoretical work on to see if they also reveal cracks in the Standard Model, of it stands firm again.
The finding was presented today at the annual Rencontres de Moriond conference in France and in a dedicated CERN seminar.
LHCb is one of the four main experiments at the Large Hadron Collider at CERN. LHCb was built in a cavern 100m below ground near Ferney-Voltaire in France.
It is investigating the subtle differences between matter and antimatter. One of the most fundamental questions is why is our Universe made of matter? It is widely thought that initially equal amounts of matter and antimatter were created, and currently there is no evidence opposing this. Experimental measurements and theoretical calculations hint that almost all matter and antimatter would have annihilated each other leaving behind a residual amount of matter to form our Universe. The mechanism that would have favoured matter against antimatter is called CP violation.
UK partners in LHCb include teams from:
Last updated: 21 March 2019