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 in 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 origin of time, the universe was very dense and very hot. It was full of particles and antiparticles, annihilating each other in energy in the form of photons, the particles of light. But, certainly, a portion of matter survived, remaining from these annihilations. Since we are here, we are made of matter and we are not photons. Why was the annihilation between particles and antiparticles not perfect? What happened in the early universe that generated this imbalance, breaking the symmetry between matter and antimatter? The Large Hadron Collider beauty (LHCb) experiment at the CERN laboratory is searching for 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 them up (u), down (d), charm (c), strange (s), top (t), and bottom (b) (the latter is sometimes also called the beauty quark, 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 (two up quarks and one down quark), and it is the lightest baryon. But there are other types of baryons that are more massive, such as the so-called Lambda beauty baryons (made of one up quark, one down quark, and one bottom 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 a spatial inversion: just like mirrors that flip things around. So, if I have a particle, I swap it for its antiparticle, and then invert it in space (i.e., I imagine looking at it in a mirror), things don’t look the same. This phenomenon is known as “CP violation.” And it means that the number of decays is measured to be different for hadrons and antihadrons. And the fact that CP violation occurs in the world of particles is a necessary condition for explaining the imbalance between matter and antimatter that we observe in the universe.
Recently, CERN's LHCb collaboration observed, for the first time, that baryons also undergo this CP-breaking phenomenon. Until before this new discovery published in NatureCP-breaking had been measured only in meson decays or disintegrations. Today we show that this also happens to baryons. LHCb reports that this difference is very clear, and, statistically speaking, they are confident by 5.2 standard deviations that the difference in the number of baryon and anti-baryon beauty Lambda decays is not pure chance. In particle physics, reaching a statistical confidence of 5 standard deviations is considered a discovery, which is equivalent to a certainty of 99.9999%.
But measuring a necessary condition to explain the imbalance between matter and antimatter in mesons and baryons is not enough to explain the evident asymmetry between matter and antimatter in the cosmos. And this is the most interesting thing in my opinion, since it opens doors to the possibility of the existence of new elementary particles, which contribute even more to the imbalance. This is because the amount of CP breaking predicted by the Standard Model is not enough to explain the asymmetry. But to understand this discrepancy, we must measure it very well in the Standard Model, so that, if/when the new particles appear, we are sure how much they contribute to the imbalance. And this is where the importance of this new beautiful and symmetry-breaking measurement lies: we need to know how to quantify with high precision what is known, so that we can recognize when we are in the presence of what we do not know.
Source: Cooperativa.cl
Giovanna Cottin , Research Associate SAPHIR
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