Our universe is made primarily of matter, not antimatter. We are made of atoms, not anti-atoms. However, antimatter does exist. Ever since Paul Dirac, in 1928, wrote a famous equation that treats space and time in the same way (combining Einstein’s Special Theory of Relativity with Quantum Mechanics), we have anticipated that for every elementary particle, there is another particle of the same mass but with the opposite electric charge. For example, Carl D. Anderson confirmed Dirac’s prediction that for every electron, there is a positron, or “positive electron,” which is the electron’s antiparticle. And so it is with all the particles of the Standard Model of Particle Physics, the theory that summarizes our current understanding of subatomic scales.Every particle has a corresponding antiparticle. If the two come into contact, they annihilate into energy.
At the dawn of time, the universe was extremely dense and extremely hot. It was teeming with particles and antiparticles, annihilating each other into energy in the form of photons, the particles of light. But certainly, a portion of matter survived, remaining after these annihilations. Since we are here, we are made of matter and are not photons.Why wasn’t the annihilation between particles and antiparticles complete? What happened in the early universe that created this imbalance, breaking the symmetry between matter and antimatter?The Large Hadron ColliderBeauty(LHCb) experiment at CERN is searching for the answers to these questions.
The Large Hadron Collider—or LHC—is an accelerator and collider for particles called hadrons. Hadrons are made up of fundamental particles of matter called quarks. There are six different types of quarks. We call themup (u),down (d),charm (c),strange (s),top (t), andbottom (b)(the latter is sometimes also calledthe beautyquark, which is the origin of the name of the LHCb experiment). Hadrons are classified into two main types: mesons and baryons. Mesons are made of two quarks. Baryons are made of three quarks. For example, the proton in our atoms is made of three quarks (twoupquarks and onedown quark), and it is the lightest baryon. But there are other types of baryons that are more massive, such as the so-called beauty baryons (made of oneup quark, onedownquark, and onebottom quark).
Our Standard Model of Particle Physics predicts quarks, and, among many other phenomena, predicts that states composed of quarks and antiquarks (i.e. matter and antimatter) behave differently if one exchanges them. This is because the breaking of two simultaneous symmetries of nature has been measured: the so-called charge conjugation (C) and parity (P). If a system or a theory looks the same when transformed in a certain way, we say that it possesses symmetries. If it does not look the same under transformation, we say that the symmetry is broken.
Charge conjugation C transforms particles into antiparticles. Parity is an inversion in space: just like mirrors that turn things around. So, if I have a particle, exchange it for its antiparticle, and invert it in space (i.e. imagine looking at it in a mirror), things don't look the same. This phenomenon is known as "CP breaking". And it translates into the fact that it is measured that the number of decays is not the same for hadrons and antihadrons. And the fact that CP breaking occurs in the particle world is a necessary condition to explain the imbalance between matter and antimatter that we observe in the universe.
Recently, the LHCb collaboration at CERN observed, for the first time, that baryons also exhibit this CP-violation phenomenon. Prior to this new discovery,published in*Nature*, CP violation had only been measured in meson decays. Today we have evidence that this also occurs in baryons. LHCb reports that this difference is very clear, and, statistically speaking, they are 5.2 standard deviations confident that the difference in the number of decays of baryons andbeautyLambda antibaryons is not mere chance. In particle physics, achieving a statistical confidence of 5 standard deviations is considered a discovery, which is equivalent to a certainty of 99.9999%.
Butmeasuring a necessary conditionto explain the imbalance between matter and antimatter in mesons and baryonsis not enoughto accountfor the evident asymmetry between matter and antimatter in the cosmos. And this is what I find most interesting, asit opens the door to the possibility that new elementary particles exist, which contribute even more to the imbalance. This is because the amount of CP violation predicted by the Standard Model is not sufficient to explain the asymmetry. But to understand this discrepancy, we must measure it very carefully within the Standard Model, so that, if/when new particles appear, we can be certain of how much they contribute to the imbalance. And this is where the importance of this beautiful, symmetry-breaking new measurement lies:we need to be able to quantify what we know with high precision, so that we can recognize when we are in the presence of the unknown.
