A pentaquark and the first-ever pair of tetraquarks

The International Collaboration at the Large Hadron Collider (LHC) observed three particles that had never been seen before: a new type of pentaquark and the first-ever pair of tetraquarks, which includes a new type of tetraquark. The results, presented today at the CERN symposium, add three new alien members to the growing list of new hadrons found at the LHC. They will help physicists better understand how quarks are bound together in these composite particles.

quarks are elementary particles It comes in six flavors: up, down, charming, exotic, up and down. They usually combine together in binary and triple groups to form hadrons such as protons and neutrons that make up atomic nuclei. However, rarely, it can also combine into particles of four quarks and five quarks, or “quaternary quarks” and “pentaquarks”. Theorists predicted these strange hadrons at the same time as conventional hadrons, about six decades ago, but only recently, in the past 20 years, have they been observed by the LHCb and other experiments.

Most of the exotic hadrons discovered in the past two decades are tetraquarks or pentaquarks containing a charm quark and a charm antiquark, with two or three remaining subatomic particles Being an up, down, or odd quark. But in the past two years, the LHCb has discovered different types of exotic hadrons. Two years ago, the Collaboration discovered a quaternary quark made up of two charm quarks and two charm quarks, and two “charming open” quarks made up of a charm antiquark, an up quark, a down quark and a strange antiquark. And last year, I stumbled upon the first-ever example of a “double open magic” quadrant with two charm quarks and an up and down antiquark. Open magic means that the particle contains a charm quark without an equivalent antiquark.

The discoveries announced today by the LHCb collaboration include new types of exotic hadrons. The first type, observed in the analysis of the “decay” of negatively charged B mesons, is a pentagon made up of a charm quark, a charm antiquark, an up and down quark and a strange quark. It is the first pentagon that has been found to contain a strange quark. The result is of enormous statistical significance of 15 standard deviations, well in excess of the five standard deviations required to claim the particle’s observation in Particle physics.

The second type is a doubly electrically charged quadrupole. It is an open enchanted tetraquark consisting of a Charm Quarks, strange antiquarks, and up and down quarks Antiquark, which was observed with its neutral counterpart in a joint analysis of the decay of positively charged and neutral B mesons. The new tetraquarks, observed with statistical significance of 6.5 (double charged particle) and 8 (neutral particle) standard deviations, represent the first time a pair of tetraquarks have been observed.

“The more analyzes we do, the more exotic hadrons we find,” says LHCb physics coordinator Nils Toning. “We are experiencing a period of discovery similar to the 1950s, when the discovery of the “particle zoo” of hadrons began and eventually led to quark The classic hadron model of the 1960s. We are making “Particle Zoo 2.0”.

“Finding new types of tetraquarks and pentaquarks and measuring their properties will help theorists to develop a unified model of exotic hadrons, the exact nature of which is largely unknown,” says LHCb spokesman Chris Parks. “It will also help to better understand conventional hadrons.”

While some theoretical models Exotic hadrons are described as tightly bound single units of quarks, while other models envision them as pairs of modular hadrons loosely bound in a molecule-like structure. Only time and more studies of exotic hadrons will tell if these are grains one or the other or both.