University of Manchester: Large Hadron Collider project discovers three new exotic particles

The Large Hadron Collider beauty (LHCb) collaboration has announced the discovery of three new exotic particles.

Exotic particles, such as these, had only been theorised but not observed until recently. These exotic particles are built out of quarks.

“Like proton or neutrons, the particles that make up the nucleus of the atom, these new particles are made up of quarks”, explained Chris Parkes, Professor of Experimental Particle Physics at The University of Manchester. “However, protons and neutrons are made of three quarks, whereas exotic particles are made of four or five quarks”.

Exotic particles were predicted as possible by theorists about six decades ago, but only relatively recently, in the past 20 years, have they been observed by LHCb and other experiments.

“Finding exotic particles and measuring their properties will help theorists develop a model of how these particles are built, the exact nature of which is largely unknown,” according to Professor Parkes. “It will also help to better understand the theory for conventional particles such as the proton and neutron.”

The results presented today at a CERN seminar, add three new exotic members to the growing list of new particles found by experiments at the Large Hadron Collider (LHC). They will help physicists better understand how quarks bind together into these composite particles.

The LHCb collaboration is a collaboration of over 1000 scientists from twenty countries across the world. It has built and operates one of the four big detectors at the CERN LHC particle collider. The collaboration is led by Professor Parkes, while The University of Manchester has more than twenty members of staff and PhD students working on the project.

The new findings show that the international LHCb collaboration has observed three never-before-seen particles: a new kind of “pentaquark” and the first-ever pair of “tetraquarks”.

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Finding exotic particles and measuring their properties will help theorists develop a model of how these particles are built, the exact nature of which is largely unknown, it will also help to better understand the theory for conventional particles such as the proton and neutron.

Professor Chris Parkes
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Quarks are elementary particles and 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. More rarely, however, they can also combine into four-quark and five-quark particles, or “tetraquarks” and “pentaquarks”. Particles made of quarks are known as hadrons.

While some theoretical models describe exotic hadrons as single units of tightly bound quarks, other models envisage them as pairs of standard hadrons loosely bound in a molecule-like structure. Only time and more studies of exotic hadrons will tell if these particles are one, the other or both.

Most of the 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 an antiquark. But in the past two years, LHCb has discovered different kinds of exotic hadrons.

Two years ago, the collaboration discovered a 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. Open charm means that the particle contains a charm quark without an equivalent antiquark.

The discoveries announced today by the LHCb collaboration include new kinds of exotic hadrons. The first kind, observed in an analysis of “decays” of negatively charged B mesons, is a pentaquark made up of a charm quark and a charm antiquark and an up, a down and a strange quark. It is the first pentaquark found to contain a strange quark. The finding has a whopping 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 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 spotted together 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 LHCb experiment hopes to find further exotic particles in the future and start to understand the families in to which they form. The collaboration is starting collecting data with its new detector today for LHC Run 3. Critical elements of this new detector have been designed and assembled in Manchester over the past seven years.