New additions to the ‘particle zoo 2.0’

July 5. () –

The LHCb experiment at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new ‘pentaquark’ and the first pair of ‘tetraquarks’, which includes a new type of tetraquark.

The findings, presented 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 the quarks in these composite particles are joined.

Quarks are elementary particles and come in six types, or ‘flavours’: up, down, charm, weird, 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 into four- and five-quark particles, or “tetraquarks” and “pentaquarks”.

These exotic hadrons were predicted by theorists at the same time as the conventional hadrons, about six decades ago, but only 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 kinds 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 consisting of a charm antiquark, an up quark, a down quark, and a strange antiquark. And last year he found the first instance of a “double open charm” tetraquark with two charm quarks and an up and down antiquark. Open charm means that the particle contains a charm quark without an equivalent antiquark..

The discoveries announced this July 5 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 composed of a charm quark and a charm antiquark and an up quark, down quark, and a strange quark. It is the first pentaquark that contains a strange quark.

The find has a huge statistical significance of 15 standard deviationswell beyond the 5 standard deviations that are required to claim 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 was detected together with its neutral counterpart in a joint analysis of neutral and positively charged B-meson decays. The new tetraquarks, observed with a statistical significance of 6.5 (doubly charged particles) and 8 (neutral particles) standard deviations, they represent the first time a pair of tetraquarks has been observed.

“The more analyzes we do, the more types of exotic hadrons we find,” he says. it’s a statement Niels Tuning, Physics Coordinator at the LHCb. “We are witnessing a period of discovery similar to the 1950s, when a hadron ‘particle zoo’ began to be discovered and ultimately led to the quark model of conventional hadrons in the 1960s. We are creating a ‘particle zoo 2.0’.”