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 very first pair of tetraquarks, including a new type of tetraquark. The findings, 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 bond together to form these composite particles.

Quarks are elementary particles and come in six flavors: up, down, charm, strange, above and below. They usually combine together in groups of twos and threes to form hadrons, such as the protons and neutrons that make up atomic nuclei. More rarely, though, they can also combine into four-quark and five-quark particles, or “tetraquarks” and “pentaquarks.” These exotic hadrons were predicted by theorists at the same time as conventional hadrons about six decades ago, but only relatively recently, in the past 20 years, have they been observed by 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, while the remaining two or three quarks an up, down or strange quark or their antiquarks. But over the past two years, LHCb has discovered several types of exotic hadrons. Two years ago, the collaboration discovered a tetraquark consisting of two charm quarks and two charm antiquarks, and two “open-charm” tetraquarks consisting of a charm antiquark, an up quark, a down quark and a strange antiquark. And last year it found the very first 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 species of exotic hadrons. The first species, observed in an analysis of “decay” of negatively charged B mesons, is a pentaquark consisting of a charm quark and a charm antiquark and an up, a down and a strange quark. It is the first pentaquark to contain a strange quark. The finding has a whopping 15 standard deviations, well above the 5 standard deviations needed to observe a particle in particle physics†

The second kind is a doubly electrically charged tetraquark. It is an open-charm tetraquark composed of a charm quark, a strange antiquark, and an up quark and a down antiquark, and it was seen along with its neutral counterpart in a joint analysis of the decay of positively charged and neutral B mesons. 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 species of exotic hadrons we find,” says LHCb physics coordinator Niels Tuning. “We are witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons was discovered and eventually led to the quark model of conventional hadrons in the 1960s. We are creating ‘particle zoo 2.0’.”

“By finding new species of tetraquarks and pentaquarks and measuring their properties, theorists can develop a unified model of exotic hadrons, the exact nature of which is largely unknown,” said 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 consider them as pairs of standard hadrons loosely bound in a molecular-like structure. Only time and more studies of exotic hadrons will tell if these particles be one, the other, or both.