CERN's LHCb observes matter-antimatter asymmetry in baryons for the first time

In the Big Bang, the explosion that formed the early universe, matter and antimatter were created in equal proportions. However, all observations show that our current universe contains only matter (the traces of antimatter observed are completely negligible). After the Big Bang, matter and antimatter annihilated each other, giving rise to radiation, until the antimatter was exhausted. Fortunately, a tiny excess of matter remained, giving rise to the universe we live in, where we observe barely one particle of matter (protons, neutrons, electrons) for every billion particles of radiation (photons). Our very existence is due to this insignificant excess of matter, but how did it come about?

The physical laws of electromagnetism, strong (nuclear) interaction and gravitation are identical for particles of matter and antimatter. They are distinguished only by weak interaction. However, in order to explain the asymmetry observed between matter and antimatter, the physical laws must also change when a ‘CP transformation’ (charge plus parity inversion) is performed: a universe of antimatter observed through a mirror must be distinguishable from the universe of matter. Although the current theory of fundamental interactions (the Standard Model) satisfies this condition, we know that it does so insufficiently. Therefore, matter-antimatter asymmetry originates from some additional unknown interaction that is not symmetrical under CP transformation. Hence the enormous interest in experimentally searching for violations of CP symmetry.

CP symmetry violations measured in the laboratory are consistent with the predictions of the Standard Model, but until now they had only been observed in the decays of mesons, particles made up of a quark and an antiquark. However, ordinary matter is made up of particles composed of three quarks, called baryons (protons and neutrons), and electrons. The importance of the results announced by the LHCb experiment lies in the fact that this is the first time a CP violation has been observed in baryons. Specifically, a significant difference has been observed between the decays of the Lambda_b baryon and its corresponding antibaryon. The Lambda_b particle (a ‘b d u’ state) is like a neutron (‘d d u’) in which a light quark “d” has been replaced by a heavy quark ‘b’. The fact that the first observation was made in a heavy baryon is in line with expectations in the Standard Model, but it is still too early to draw conclusions.

We hope that this interesting measurement will be followed by other observations of CP-violating phenomena, providing us with valuable information about unknown physics beyond the Standard Model.

The first observation of an asymmetry between matter and antimatter in baryons by the LHCb collaboration at CERN is a fundamental discovery. This asymmetry had already been found in other types of particles, called mesons, which are composed of two quarks. The current theoretical framework describing particles and their interactions, called the Standard Model, predicts that this asymmetry should also be observed in baryons, which are composed of three quarks. However, it had not been discovered until now. This is therefore a pioneering result, which confirms our theory about the fundamental laws of nature and also represents a first step towards new, even more precise experimental measurements in the future, which may help us to uncover new physics.

This discovery also has large-scale implications. Our universe initially contained equal amounts of matter and antimatter, but in its evolution only galaxies, stars, planets, etc., made up of matter, have remained. Thus, it is possible that the matter-antimatter asymmetry in the interactions of subatomic particles such as baryons is responsible for the matter-antimatter asymmetry of the entire universe.

Is the study of good quality?

“Yes, it is a good quality study. It was carried out in the LHCb experiment, which is one of the four experiments at CERN's large-scale collider, and which is optimized for the study of flavor physics, that is, the processes in which one type of quark transforms into another. The greatest interest of these processes is that they may hold the key to why the universe is made of matter and not antimatter.”

Do these observations represent a milestone in particle physics?

“This is the first time that an asymmetry has been observed in the behavior of a type of baryon (made up, like the neutron, of three quarks, but replacing one d quark with a heavy b quark) and its corresponding antiparticle. The Standard Model predicts this phenomenon, which has previously been measured in many mesonic particles (made up of a quark and an antiquark), but, until now, not in baryons.”

What implications might this finding have (in this or other areas)?

“It is very interesting to study to what extent the Standard Model can quantitatively predict the observed asymmetry. This calculation is very complex because it involves strong interactions, and we currently do not know how to perform these calculations with sufficient precision. Contrasting the experimental result with theory is essential to establish whether or not there are effects beyond the Standard Model that could be contributing to this process. We know that the asymmetry between matter and antimatter in the Standard Model is not sufficient to explain the universe, so these effects are precisely what we are looking for.”

Is the study of good quality?

“Yes, of course, it is supported by the LHCb collaboration and in a highly statistically significant and extensively studied data sample (data from Run 1 and Run 2, spanning the years 2011, 2012, and 2015-2018). Not surprisingly, the data, as well as the code, will be made available to the general public as part of CERN's Open Data policy.”

Do these observations represent a milestone in particle physics?

“Yes, well, until now, these asymmetries predicted in the Standard Model had only been observed in samples of mesons (composed of two quarks) and not in baryons (composed of three quarks), the latter being those present in the conventional matter that makes up the universe, the human body... Every object on Earth is made of baryons (protons and neutrons).”

What implications might this discovery have (in this or other areas)?

“I think it represents another small step forward in the long road of science, in this particular case, as it was a long-sought and expected result, with very high precision. Understanding why we are made of matter and not antimatter (positive protons instead of negative antiprotons, for example) is one of the key elements in understanding our universe.”

What limitations does it have?

“Apart from the measurement uncertainty, which, although low because it is very precise (the combined systematic uncertainty is as low as 0.10%), can always be improved with more statistical or other analytical techniques, the result is reduced to a single type of baryon (type b) and its antiparticle, and in a given decay channel (proton, kaon, and two pions).”