Analyzing data from the LHCb detector installed at the Large Hadron Collider, scientists have found reliable evidence for the existence of a unique particle - a doubly overtly charmed tetraquark. And no matter how outlandish its name sounds for a person who is not familiar with the Standard Model, it gives physicists even more strange information: it seems that modern scientific ideas about the structure of the world will again have to be pretty shaken up.
A physics course in a general education school gives a picture of the microworld as well as an incomplete picture. Every person interested in natural sciences sooner or later faced this shock - it turns out that everything is not limited to protons, neutrons, electrons, photons and neutrinos. There are also quarks, leptons, bosons, and the subatomic particles “familiar” to us are generally a “soup” of much smaller elements. Well, except for electrons and photons - everything is with them, as it was, they are indivisible.
Almost all matter around us consists of baryons, that is, heavy particles: stable neutrons and protons. They, in turn, have a relatively simple structure - three quarks in each. There are also unstable mesons composed of a pair of quark-antiquark, but in the framework of this story they are not so important. If we go completely deeper, then this picture is still not complete, since there is somewhat different information regarding the proton. In any case, physicists have long been determined about the role of quarks, having awarded them the title of fundamental "building blocks" of the universe, from which all large particles are "recruited". And this is definitely proven with a sufficient degree of reliability (but what they themselves are formed by is a question of long and so far not approaching the resolution of disputes).
The problem is as follows. There are no laws of nature that would forbid quarks to gather not in two or three, but in large quantities. Similar exotic particles were predicted by several theoretical physicists at once back in the 1960s. However, due to the fact that until recently scientists did not have advanced noise filtering tools on experimental installations, they could not be detected. The situation has changed dramatically in the 21st century against the background of the development of computer technology. And since 2003, discoveries have been pouring in like a horn of plenty: today, several dozen tetraquarks (four quarks) and a pair of pentaquarks (five quarks) are known.
All these exotic particles share several common properties at once. First, they are extremely unstable and their lifetime is calculated in zeptoseconds (sextillion fractions of a second). Secondly, although they do not contradict the Standard Model, they do not fit into it. In other words, their role is completely incomprehensible, and the existence of pentaquarks in nature is completely questionable (they were obtained only intentionally in the course of special experiments). But the recently discovered twice openly charmed tetraquark, even against this unusual background, managed to stand out.
The most exotic of all exotic particles
The discovery of an unusual tetraquark at the Conference on High Energy Physics of the European Physical Society (EPS-HEP) was reported by specialists working with the CERN LHCb experiment. It is the smallest of the main detectors of the Large Hadron Collider (LHC). The particle was found in archived data from 2011-18, when they were sifted through a routine search for "missed" discoveries. Unfortunately, if some phenomenon is not predicted in advance by theorists (as was the case with the Higgs boson and the planet Neptune), its detection requires processing a huge amount of information literally "for luck." Earlier, several dozen different particles were found by the same method.
Screening out the noise from the records of millions of particle collisions carried out at the LHC made it possible to identify with great confidence the Tcc + meson, the same twice openly charmed tetraquark. It differs from all other similar particles in composition. It combines two heavy, almost equal to the mass of a proton, c-quark (charmed), as well as light u-antiquark (upper) and d-antiquark (lower). The epithets in brackets denote the "flavors" of particles, that is, certain quantum numbers (parameters) that characterize their fundamental properties.
The strangeness here is this: never before have particles with an open charm been observed, it was believed that the c -quark should be balanced by the c -antiquark. But Tcc + broke not only this template, it also lives for an incredibly long time - a few attoseconds, which is two to three orders of magnitude longer than the decay time of other exotic hadrons. The great news is that with more than two hundred candidate events on hand for Tcc + detection, CERN staff have described clear criteria by which it can be detected. The particles into which this tetraquark decays are relatively easy to detect, so it will not be difficult to confirm the discovery with teams working on other accelerators. In addition, the mass of Tcc + is low enough that it is formed in installations with much lower operating energies than the LHC.
Breakthrough or Revolution?
The very existence of Tcc + poses another question for physicists - what if its structure is not unique and is a "template" for other similar particles? Then the existence of a meson is quite real not with two charmed quarks, but with one or a pair of even heavier b-quarks (adorable). Some of these particles completely break the picture of the world, since they must “live” at least an order or two longer. This means that the heavier "cousins" of Tcc + will be more likely to interact with other surrounding particles. Thus, they may have a role to play in the universe, not only as byproducts of subatomic interactions.
In addition to all of the above, Tcc + boasts a whole range of features that are dauntingly difficult to explain in popular science language. Among them, for example, there is a suspicious closeness of the mass of a doubly charmed tetraquark and a pair of D mesons. In addition to this, the internal structure of Tcc + has not been adequately clarified. It is unclear whether it is a “molecule” of two mesons, that is, a pair of structures of a heavy quark and a light antiquark, or it resembles an atom where heavy quarks are located compactly in the center and are surrounded by a cloud of antiquark superpositions.
One thing is clear for sure - the news about Tcc + will trigger a new round of research by physicists and the search for other similar particles. Its discovery cannot be called a completely fantastic sensation of the level of the Higgs boson discovery, but the situation is still intriguing and with far-reaching consequences. At the very least, this event is weighty and rather directly hints at a thought that scientists have been thinking about for a long time: the Standard Model is not much more complete than the quantum mechanical model of the atom. It works, but only at its own level, and the reality is much more complicated, so physicists still have to dig and dig.Well, for us - to follow the coming great revolutions in science, which herald such breakthroughs.