The surprises of the tetraquark, “bonding” of elementary particles

The particle cauldron that is the LHC (Large Hadron Collider), the giant accelerator of the European Organization for Nuclear Research (CERN, near Geneva), may have been arrested for work for two years, his “old” data still reveals treasures. Thus, thanks to the smallest of its detectors, LHCb, researchers have just unearthed a new particle, predicted just forty years ago and whose atypical properties raise the hope of even more important discoveries.

“I was really excited when I saw it the first time, but it took a few weeks before we checked everything and convinced ourselves that we really had a new particle”, explains Ivan Polyakov, from Syracuse University (United States), member of LHCb, who has presented the result on July 29 at a conference. “When in 1981, with my two colleagues Pierre Taxil and Jean-Pierre Ader, we made this prediction, people were skeptical, notes Jean-Marc Richard, now professor emeritus at the University of Lyon. But I knew it would happen. Except that I thought it would be my grandchildren who would see it. So I was happy when colleagues from LHCb warned me. “

First peculiarity of the newcomer, unlike electrons, photons or the famous Higgs boson, it is not an elementary particle. It is a tetraquark, made up of four quarks, just as protons or neutrons are made of three quarks, and the lesser known, mesons, of only two quarks. Second distinctive sign, unlike other tetraquarks created in very intense collisions of particles over the past twenty years and which disappear almost instantly, this one survives 10 to 1,000 times longer. So much so that physicists think they have got their hands on a first assembly of well-linked quarks, rather than on a distended aggregate. A beautiful pearl necklace rather than a pierced bag containing beads. Or the equivalent of a stable molecule in chemistry.

Very massive charms

Making such a necklace is not easy. Two “glues” are available to assemble the elements. One, the electroweak interaction, is known to hold electrons around atoms together, or to act on protons and neutrons. The other, called “strong interaction”, maintains the quarks between them. The tetraquarks discovered so far mixed the two glues and “broke” easily, but not this last born, which seems to hold only by the strong glue. Its secret is that it contains two very heavy quarks, called “charms”, much more massive than the quarks found inside protons and neutrons. This would stabilize the building and give it both its barbaric name of Tcc + (for tetraquark with two charms, positively charged) and its mass of more than four times that of a proton.

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The surprises of the tetraquark, “bonding” of elementary particles

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