Exotic particles containing five quarks discovered at the Large Hadron Collider



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Everything you see around you consists of elementary particles called quarks and leptons, which can combine to form larger particles such as protons or atoms. But that does not make them boring – these subatomic particles can also combine in an exotic way that we have never spotted. Now, CERN's collaboration with LHCb has announced the discovery of a group of new particles called pentaquarks. The results can help reveal many of the mysteries of quark theory, a key element of the standard model of particle physics.

Quarks were first proposed to explain the disordered flow of new particles discovered in cosmic ray and collider experiments in the middle of the 20th century. This growing "zoo" of seemingly fundamental particles has caused consternation among physicists, who have a natural inclination for simplicity and order – and hate to remember more than a few basic principles. The famous Italian physicist Enrico Fermi captured the mood of his colleagues stating, "Young man, if I could remember the names of all these particles, I would have been a botanist".

Fortunately, in the 1960s, the American physicist Murray Gell-Mann observed in the particle zoo patterns similar to those noted by Dimitri Mendeleev when he drew up the periodic table of chemical elements. Just as the periodic table implied the existence of things smaller than atoms, so Gell-Mann's theory suggested the existence of a new class of fundamental particles. Particle physicists have finally been able to explain that the hundreds of particles contained in the zoo were composed of a much smaller number of truly fundamental particles called quarks.

Mysterious Hadrons

There are six types of quarks in the standard model: low, high, strange, charm, low and high. These also have companions "antimatter" – it is thought that each particle has an antimatter version that is virtually identical to itself, but with the opposite charge. Quarks and antiquarks are linked to form particles called hadrons.

According to the Gell-Mann model, there are two major classes of hadrons. One consists of particles composed of three quarks called baryons (which include the protons and neutrons constituting the nucleus of the atom) and other particles composed of a quark and a antiquark called mesons.

Until recently, baryons and mesons were the only types of hadrons observed in experiments. However, in the 1960s, Gell-Mann also discussed the possibility of more exotic combinations of quarks, such as tetraquarks (two quarks and two antiquarks) and pentaquarks (four quarks and an antiquark).

In 2014, LHCb, which runs one of the four giant experiments at CERN's Large Hadron Collider, published a result showing that the name of Z (4430)+ the particle was a tetraquark. This triggered a wave of interest for new exotic hadrons. Then, in 2015, LHCb announced the discovery of the very first pentaquark, adding a brand new class of particles to the hadron family.

The results presented today by LHCb extend this first discovery of pentaquark by discovering such additional particles. This was possible thanks to a large number of new data recorded during the second execution of the Large Hadron Collider. Liming Zhang, an associate professor at Tsinghua University in Beijing and one of the physicists who carried out the measurement, said, "We now have ten times more data than in 2015, which we allows to see structures more interesting and finer than before. "When Liming and his colleagues looked at the pentaquark originally discovered in 2015, they were surprised to find that it had split in half. The original pentaquark was actually two separate pentaquark particles with masses so similar that they originally resembled a single particle.

LHCb.
Maximilien Brice et al./CERN

As if two pentaquarks for the price of one were not exciting enough, LHCb also found a third pentaquark with a slightly lower mass than the other two. The three pentaquarks are composed of a quark down, two quarks up, a charming quark and an antiquark of charm.

The big question now is: what is the precise internal structure of these pentaquarks? One option is that they actually consist of five quarks, all homogeneously mixed in a single hadron. Another possibility is that pentaquarks are actually a baryon and a meson glued together to form a weakly bound molecule, similar to the way protons and neutrons bind together in the atomic nucleus.

Tomasz Skwarnicki, a professor of physics at Syracuse University in New York, who also worked on measurement, said the new companion state "lies at a mass that provides insights into the internal structure pentaquarks ". The most likely option is that these pentaquarks are baryon-meson molecules, he added. To be quite sure, physicists will need more experimental data and new studies from theorists, which means that the history of these pentaquarks is far from over.

These results complete a week of exciting announcements from LHCb, which included the discovery of a new type of matter-antimatter asymmetry. The LHC has not yet discovered any particles other than the standard model that could help explain mysteries like dark matter, an invisible but unknown substance that constitutes the majority of matter in the universe.

But these exciting measurements show that there is still much to learn about the particles and forces of the standard model. Perhaps our best chance of finding answers to the great questions of fundamental physics in the 21st century is in more detailed studies of the particles we already know than in the discovery of new ones. Whatever it is, we still have a lot to discover.

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