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The Large Hadron Collider beauty (LHCb) experiment — which is investigating the slight differences between matter and antimatter by studying a type of particle called the “beauty quark”, or “b quark” — has observed three never-before-seen particles, CERN announced on July 5.
The three “exotic” additions — a new kind of “pentaquark” and the first-ever pair of “tetraquarks” — to the growing list of new hadrons found at the LHC will help physicists better understand how quarks bind together into these composite particles, CERN said in a release.
CERN — Conseil Européen pour la Recherche Nucléaire — is the original name of the European Organisation for Nuclear Research, which runs the particle accelerator complex that houses the LHC, the world’s largest and most complex collider.
The LHC, re-ignited after three years in April, was cranked up to unprecedented levels of energy on July 5, and has begun smashing together protons at almost the speed of light, which could throw up “new” physics beyond the Standard Model.
What are quarks?
Quarks are elementary particles that come in six “flavours”: up, down, charm, strange, top, and bottom. They usually combine together in groups of twos and threes to form hadrons such as the protons and neutrons that make up atomic nuclei.
But they can also combine into four-quark and five-quark particles, called tetraquarks and pentaquarks. These exotic hadrons were predicted by theorists about six decades ago — around the same time as conventional hadrons — but they have been observed by LHCb and other experiments only in the past 20 years.
What about tetraquarks and pentaquarks?
According to the CERN release, most exotic hadrons discovered in the past 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.
Two years ago, however, the LHCb experiment discovered an exotic tetraquark made up 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 instance of a “double open-charm” tetraquark with two charm quarks and an up and a down antiquark, CERN said.
Open charm means that the particle contains a charm quark without an equivalent antiquark.
And what are the new exotic hadrons announced by the LHCb collaboration?
The first kind was observed in an analysis of “decays” of negatively charged B mesons. It is a pentaquark made up of a charm quark and a charm antiquark, and an up, a down, and a strange quark, CERN said. It is the first pentaquark found to contain a strange quark.
The finding has a statistical significance of 15 standard deviations, far beyond the 5 standard deviations that are required to claim the observation of a particle in particle physics, the release said.
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.
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It was spotted together with its neutral counterpart in a joint analysis of decays of positively charged and neutral B mesons, the CERN release said.
“The more analyses we perform, the more kinds of exotic hadrons we find,” LHCb physics coordinator Niels Tuning said. “We’re witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons started being discovered and ultimately led to the quark model of conventional hadrons in the 1960s. We’re creating ‘particle zoo 2.0’.”
