the pentaquark and the very first pair of tetraquarks

The international LHCb collaboration at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new type of pentaquark and the first-ever pair of tetraquarks containing a new type of tetraquark. The results, presented today at a CERN seminar, add three new exotic members to the growing list of new hadrons found at the LHC. They will help physicists better understand how quarks combine to form these composite particles.

Quarks are elementary particles and come in six flavors: up, down, charm, strange, top, and bottom. They usually combine in groups of two and three to form hadrons like the protons and neutrons that make up atomic nuclei. More rarely, however, they can also combine to form four-quark and five-quark particles or “tetraquarks” and “pentaquarks”. These exotic hadrons were predicted by theorists at the same time as conventional hadrons some six decades ago, but only relatively recently, in the last 20 years, have they been observed by LHCb and other experiments.

Most of the exotic hadrons discovered in the last two decades are tetraquarks or pentaquarks, containing a charm quark and a charm antiquark, with the remaining two or three quarks being an up, down, or strange quark or their antiquarks. But in the last two years, LHCb has discovered different types of exotic hadrons. Two years ago, the collaboration discovered a tetraquark composed of two charm quarks and two charm antiquarks, and two “open charm” tetraquarks composed of a charm antiquark, an up quark, a down quark, and a strange antiquark. And last year it found the first-ever instance of a “double open charm” tetraquark with two charm quarks and an up and a down antiquark. Open charm 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 an analysis of “decays” of negatively charged B mesons is a pentaquark, which consists of a charm quark and a charm antiquark, as well as an up, a down, and a strange quark. It is the first pentaquark containing a strange quark. The finding has a whopping 15 standard deviations of statistical significance, well beyond the 5 standard deviations required to claim the observation of a particle in particle physics.

The second type is a doubly electrically charged tetraquark. It is an open charm tetraquark composed of a charm quark, a strange antiquark, an up quark and a down antiquark and together with its neutral counterpart in a joint analysis of positively charged and neutral B meson decays has been discovered. The new tetraquarks, observed with a statistical significance of 6.5 (doubly charged particle) and 8 (neutral particle) standard deviations, represent the first time a pair of tetraquarks has been observed.

“The more analyzes we do, the more types of exotic hadrons we find,” says LHCb physics coordinator Niels Tuning. “We are experiencing a discovery phase similar to that of the 1950s, when the discovery of a ‘particle zoo’ of hadrons began, and eventually led to the quark model of conventional hadrons in the 1960s. We create ‘Particle Zoo 2.0’.”

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

While some theoretical models describe exotic hadrons as single units of tightly bound quarks, other models see them as pairs of standard hadrons loosely connected in a molecule-like structure. Only time and further study of exotic hadrons will tell if these particles are one, the other, or both.

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