The LHCb collaboration at CERN has seen, for the first time, the matter–antimatter asymmetry known as CP violation in a so-called D0 meson. LHCb is one of the four large experiments performed at the Large Hadron Collider (LHC) with Swiss participation of Ecole polytechnique fédérale de Lausanne (EPFL) and University of Zurich.
“The result is a milestone in the history of particle physics”, said CERN Director for Research and Computing Eckhard Elsen according to a todays CERN press release. The D meson itself had been discovered more than 40 years ago. Particle physicists have suspected CP violation to occur in this system, but it was only now, that the has finally been able to observe the effect.
Mesons are subatomic particles each composed of one quark and one antiquark. Mesons have the ability to make spontaneously a transition to their , called “oscillations”. Only four mesons allow to measure such effects: the neutral kaon (K0), the neutral B meson (B0), the neutral Bs meson (Bs0), and the neutral D meson (D0). The symbol zero means that these particles are electrically neutral, which is a necessary condition to allow for the oscillations to occur (because electric charge is conserved).
CP violation was observed in the neutral kaon (already in the 1960s in the US), in the neutral B0 mesons (in the 2000s in the US and Japan), and in the neutral Bs meson (in 2013 at CERN). But now the CP violation for the first time has been observed in a D0 meson, composed of a charm quark and an up antiquark. This means that the recent discovery represents the first observation of CP violation in a “charmed” particle.
Different behaviour of a particle and its antiparticle
Explained in the simplest way, the CP violation (short for: charge-parity violation) is the observed difference between the behaviour of a particle and its antiparticle. “We could say that the nature of the 'content' of a particle and an antiparticle is identical, but with opposite charges (electric charge, but also other properties that can change sign, such as what we call “flavour” for quarks)”, says Fred Blanc, senior researcher at and member of the LHCb collaboration.
“There are multiple forms of CP violation, but we can illustrate what one of them means in neutral mesons”, the particle physicist continues. “We have ways to measure the probability for a particle to oscillate into its antiparticle before decaying. The same can be measured for the antiparticle to oscillate to the particle and decay. If the two probabilities are different, we conclude that there is CP violation. CP violation is therefore a manifestation of the different behaviour between particles and antiparticles.”
The ball goes back to the theorists
The CP violation is a key element for a better understanding of the universe, since it may explain why there is an abundance of matter over antimatter in the universe. The size of CP violation observed so far in Standard Model interactions, however, is too small to account for the present-day matter–antimatter imbalance, suggesting the existence of additional as-yet-unknown sources of CP violation. “With the recent discovery we have confirmed a source of CP violation, but this most likely contributes only a tiny amount of the CP violation needed to explain the matter-antimatter asymmetry observed in the Universe”, Fred Blanc says.
What is the consequence of the discovery of CP violation in D0 mesons for today’s particle physics? The answer to this question is not clear yet, as Fred Blanc quotes: “The different observed CP asymmetries in the D0→K+K− and D0→π+π− final states was not expected, although there had been hint of this effect a few years ago. The ball is now in the camp of theorists, who will hopefully be able to offer explanations that can subsequently be validated experimentally with additional measurements. It will be interesting to see if the observed effect can be explained theoretically within the Standard Model, or if new physics must be invoked.”
Author: Benedikt Vogel