CERN Accelerating science

LHCb observes CP violation in charm

We live in a Universe of matter, which is a good thing but poses a problem: We know that in the Big Bang initially as much matter as antimatter was produced, and that the antimatter somehow disappeared. How this happened is one of the questions that the LHCb experiment is trying to answer.

Let us rewind. Every day’s physics is left-right symmetric. You cannot for instance tell if a film of a billiards game is shown left-right reversed unless social conventions, as numbers or words, give you a clue. This is because gravity and the electromagnetic interaction are symmetric under the exchange of left and right, an operation called parity or just P. However, in 1956 Lee and Yang noted that this symmetry had never been tested in weak interactions. “Madame” Wu (as she liked to be called) then performed the experiment and indeed found that parity is violated in radioactive beta decays. Physicists had to accommodate parity violation, but would assume that the laws of physics are symmetric under the combined operation of replacing left by right and swapping the charges of all particles. This is the CP operation, where C stands for charge replacement, which is the operation of turning particles into their antiparticles. But are the physics of matter and antimatter exactly the same?

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: CERN)

In 1964 in Brookhaven, Cronin and Fitch found that the K-long meson decays to two pi mesons. That was a huge surprise! The neutral kaon system comes in two species: a short-lived K-short and a longer lived K-long, respectively decaying to two and three pi mesons. In quantum mechanical terms, these states are the two eigenstates of the CP operator, so K-long decaying to two pi mesons instead of three is a violation of CP conservation. This is actually good news, as the existence of CP violation is one of three necessary conditions listed by Sakharov to obtain a matter-dominated universe. So, problem solved? Not really.

First one needs to explain how CP violation is possible at all. In 1973 Kobayashi and Maskawa implemented it as a complex phase in the quark-mixing matrix that parametrises the transitions of quarks by the weak interaction. For this to work requires the existence of at least six quarks. At the time only the up, down and strange quarks were known, so postulating three new quarks was quite audacious. The charm quark (observed in 1974)  had already been conjectured, as a solution to the the GIM mechanism. The next last two would only be discovered much later: beauty in 1979  and top in 1995.

Then, the model had to be tested: two so-called B-factory experiments were built at SLAC and KEK. They observed CP violation in beauty mesons in 2001, thus confirming that the theoretical description of CP violation was correct. This however posed another problem: there is far too little CP violation to explain the Universe. There must therefore be other processes that involve CP violation. CP violation in neutrinos may contribute, and the absence of CP violation in the strong interaction is also a mystery. New physics with new CP-violating interactions mediated by yet undiscovered particles is needed.

Finding these particles and interactions is one of the goals of the LHC. The LHCb experiment exploits the enormous yields of beauty and charm hadrons to perform precision measurements in rare processes. In 2013 LHCb reported the observation of CP violation in Bs mesons and in 2016 evidence for CP violation in decays of beauty baryons. But so far nothing was found in charm.

Due to a conspiracy in the quark-mixing matrix, CP violation in charm decays is extremely suppressed. The result reported last week by LHCb required 70 million decays of D mesons to pairs of pi or K mesons, a dataset collected between 2015 and 2018. It exploits the new trigger scheme introduced for Run 2 of the LHC: after a first filtering in real time, the data is stored on disk while the full offline-quality calibration is performed. This calibration is then used in the last stage of the trigger that reconstructs the D meson candidates. These are then ready for the use by analysts without any further processing required. 

The images show the so-called invariant-mass distributions used to count the number of decays that are present in the data sample. The area of each blue, bell-shaped (Gaussian) peak is proportional to the number of decays of that type recorded by the experiment. The final result, which uses essentially the full data sample collected by LHCb so far, is given by the quantity ΔACP=(-0.154±0.029)%, whose difference from zero quantifies the amount of CP violation observed. The result has a statistical significance of 5.3σ (Credits: LHCb collaboration, arXiv:1903.08726 [hep-ex]).

The CP asymmetry different is very stable in different data-taking periods, split by magnet polarity. The polarity of the LHCb magnet is swapped regularly to cancel detection asymmetries.​

However, to achieve a precision of 10-4, a trick has to be used to control systematic uncertainties. To cancel detection asymmetries, the difference in the amount of CP violation between the decays of D mesons to kaon and pion pairs is measured. Combining the new result with previous publications using 2011 and 2012 data, this difference is found to be (−15.4 ± 2.9) × 10-4, which is more than five standard deviations from zero. CP violation is thus observed in charm for the first time.

Is this compatible with the Standard Model or New Physics? We do not really know yet. Calculations involving charm quarks are extremely difficult. We hope that this result will trigger more efforts into making precise predictions.


For more information, see the LHCb website and the paper describing the results.