The collaboration at the Large Hadron Collider (LHC) has observed three new particles, a new kind of “pentaquark” and the first-ever pair of “tetraquarks,” which includes a new tetraquark. The findings, presented today at a CERN seminar, add three new members to the expanding list of new hadrons found at the LHC. In addition, they will assist physicists in understanding how quarks bind together into these composite particles.
Most of the exotic hadrons found in the last 2 two decades and tetraquarks or pentaquarks contain a charm quark and a charm antiquark, with the remaining two or three quarks being an up, strange, or down quark and a strange antiquark. However, LHCb has discovered various exotic hadrons in the last two years. Two years back, the collaboration led to the discovery of tetraquarks made 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 first-ever “double open-charm” tetraquark with two charm quarks and an up and a down antiquark. Open charm means that the particle comprises charm quark without an equivalent antiquark.
Today’s discovery revealed by the LHCb collaboration offers a new type of exotic hadrons. The first pentaquark with a strange quark has provided a statistical significance of 15 standard deviations, far beyond the five standard deviations that are needed to claim the analysis of a particle in particle physics.
Some models describe exotic hadrons as single units of tightly bound quarks; others envisage them as pairs of standard hadrons loosely bound in a molecule-like structure. But only time and more research on exotic hadrons can tell if these particles are one, another, or both.