In a most interesting way…
The particle cauldron that is the LHC (Large Hadron Collider), the giant particle accelerator of the European Organization for Nuclear Research (CERN, near Geneva), may have been shut down for work for the past two years, but its "old" data still reveal treasures. Thanks to the smallest of its detectors, LHCb, researchers have just discovered a new particle, predicted just 40 years ago, whose atypical properties raise hopes of even more important discoveries.
“ I was really excited when I first saw it, but it took a few weeks to check everything out and convince myself that we really had a new particle. ”
Explains Ivan Polyakov, from Syracuse University (USA), member of LHCb, who presented the result on July 29 during 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 a 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 informed me. ”
The first particularity of the newcomer is that, unlike electrons, photons or the famous Higgs boson, it is not an elementary particle. It is a tetraquark, composed of four quarks, just as protons or neutrons are made of three quarks, and the less known mesons, of only two quarks. The second distinctive feature is that, unlike the other tetraquarks created in very intense particle collisions over the last twenty years, which disappear almost instantly, this one survives 10 to 1,000 times longer. So much so that physicists think they have found a first assembly of quarks that are well linked together, rather than a distended aggregate. A beautiful pearl necklace rather than a pierced bag of 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 together the electrons around the atoms, or act on the protons and neutrons. The other, called the "strong interaction", holds quarks together. The tetraquarks discovered so far mixed the two glues and "broke" easily, but not this latest one, which seems to hold together by the strong glue alone. 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 edifice 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.
The researchers hope that replacing the "charm" quarks with two other even heavier quarks, called "beauties", will make it possible to obtain a truly stable tetraquark, the first of its kind. “ With Tcc +, we are at the limit, we are almost there. We will soon cross a ridge and discover a new landscape, ” Jean-Marc Richard enthuses. This would indeed open up new perspectives for physicists, who are currently quite embarrassed with the "strong" glue : they cannot calculate and predict its effects exactly. With stable tetraquarks - or even pentaquarks -, nature would provide them with references to better test their theories and calculations, or to find flaws in them. And who knows if a better understanding of the strong interaction and its quark assemblies will not lead to the discovery of new mechanisms or new effects?
The LHCb experiments will resume in a little over a year. In Japan, the new version of the Belle experiment, which started the interest in tetraquarks in 2003, is already on its way to confirm the presence of Tcc+, and especially to continue to string quarks.
