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 tetraquark pair, which includes 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 discovered at the LHC. They will help physicists better understand how quarks bind together to form these composite particles.
Quarks are elementary particles and come in six flavors: high, low, charm, strange, high, and low. They usually combine in groups of two or three to form hadrons such as 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 conventional hadrons about six decades ago, but only relatively recently, within the last 20 years, have they been observed by LHCb and d ‘other experiences.
Most of the exotic hadrons discovered in the last two decades are tetraquarks or pentaquarks containing a charmed quark and a charmed antiquark, with the remaining two or three quarks being an up, down or strange quark or their antiquarks. But over the past two years, LHCb has discovered different kinds of exotic hadrons. Two years ago, the collaboration discovered a tetraquark consisting of two charmed quarks and two charmed antiquarks, and two “open charm” tetraquarks consisting of a charmed antiquark, an up quark, a down quark and of a strange antiquark. And last year he found the first-ever example of a ‘double open charm’ tetraquark with two charmed quarks and an up and down antiquark. Open charm means that the particle contains a charmed quark with no equivalent antiquark.
Discoveries announced today by the LHCb collaboration include new types of exotic hadrons. The first type, observed during an analysis of the “decays” of negatively charged B mesons, is a pentaquark composed of a charmed quark and a charmed antiquark and an up, a down and a strange quark. It is the first pentaquark containing a strange quark. The finding has a huge statistical significance of 15 standard deviations, well beyond the 5 standard deviations needed 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 charmed quark, a strange antiquark, an up quark and a down antiquark, and it was spotted with its neutral counterpart in a joint analysis of decays 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 perform, the more exotic hadron types we find,” explains Niels Tuning, LHCb physics coordinator. “We are seeing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons began to be discovered and eventually led to the quark model of conventional hadrons in the 1960s. We are creating the ‘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, the exact nature of which 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 view them as pairs of standard hadrons loosely bound in a molecule-like structure. Only time and further studies of exotic hadrons will tell if these particles are one, the other, or both.
A new particle of exotic matter, a tetraquark, discovered
Find out more on the LHCb website: lhcb-outreach.web.cern.ch/2022 … its-neutral-partner/
Quote: LHCb discovers three new exotic particles: the pentaquark and the very first pair of tetraquarks (2022, July 5) retrieved July 5, 2022 from https://phys.org/news/2022-07-lhcb-exotic-particles-pentaquark -first-ever.html
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