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Exotic particles with five quarks discovered at the Large Hadron Collider



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've never seen before. Now, CERN's LHCb collaboration has announced the discovery of a new particle group called Pentaquarks. The results can help reveal many of the secrets of quark theory, a key element of the Standard Model of particle physics.

First, quarks were proposed to explain the messy amount of new particles that were discovered in experiments with cosmic rays and colliders in the mid-20th century. This growing "zoo" of seemingly fundamental particles has caused dismay among physicists who have a natural inclination for simplicity and order ̵

1; and hate having to remember more than a few basic principles. The famous Italian physicist Enrico Fermi captured the mood of his colleagues when he said: "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 noticed patterns in the particle zoo, similar to those discovered by Dimitri Mendeleev when he set up the periodic table of chemical elements. Just as the periodic table included the existence of things smaller than atoms, Gell-Mann's theory suggested the existence of a new class of fundamental particles. Finally, particle physicists were able to explain the hundreds of particles in the zoo, which consist of a much smaller number of truly fundamental particles called quarks.

Mystery hadrons

There are six types of quarks in the standard model – down weird, charm, down and up. These also have "antimatter counterparts" – it is assumed that each particle has an antimatter version that is virtually identical to itself, but with the opposite charge. Quarks and antiquarks combine to form particles known as hadrons.

According to Gell-Mann's model, there are two broad classes of hadrons. One is particles consisting of three quarks called baryons (including the protons and neutrons that make up the atomic nucleus) and the other particles made from a quark and an antiquark known as mesons.

Until recently, baryons and mesons were the only types of hadrons that had been seen in experiments. In the 1960s, however, Gell-Mann also addressed the possibility of more exotic combinations of quarks such as tetraquarks (two quarks and two antiquarks) and pentaquarks (four quarks and one antiquark).

In 2014, LHCb, which conducts one of CERN's four large-scale experiments on the Large Hadron Collider, published a finding showing that the particle named Z (4430) + was a tetraquark. This started a keen interest in new exotic hadrons. In 2015, LHCb announced the discovery of the first pentaquark and added a brand new class of particles to the Hadron family.

The findings presented by LHCb today expand this first pentaquark discovery by finding additional such particles. This was possible thanks to much new data taken during the second run of the Large Hadron Collider. Liming Zhang, an adjunct professor at Tsinghua University in Beijing and one of the physicists who made the measurement, said, "We now have ten times more data than 2015, which allows us to see more exciting and finer structures than we do previously could. When Liming and his colleagues examined the original Pentaquark discovered in 2015, they were surprised that he had split in half. The original pentaquark actually consisted of two separate pentaquark particles that had similar masses that initially looked like a single particle.

LHCb.
Maximilien Brice et al./CERN[19659012?Iftwopentaquarkswouldn'thavearousedanyexcitementfortheprizeLHCbauchathirdpentaquarkwithaslightlylowermassthantheothertwopentaquarksconsistsofadownquarktwoupquarkaharmharmiquarkandaCharmAntiquark

The big question is: What is the exact internal structure of these pentaquarks? One option is that they really consist of five quarks, all mixed evenly in a single hadron. Another possibility is that the pentaquarks are actually a baryon and a meson bound together to form a loosely bound molecule, much like protons and neutrons combine in the atomic nucleus.

Tomasz Skwarnicki, physics professor at Syracuse University In New York, who also worked on the measurement, he told me that the new escort state "has a mass that gives clues to the inner structure of pentaquarks". The most likely option is that these pentaquarks are baryon meson molecules, he added. To be absolutely sure, physicists need more experimental data and more research from theoreticians, meaning that the history of these pentaquarks is far from over.

These results complete a week of exciting new announcements from LHCb that included the discovery of a new kind of matter-antimatter asymmetry. The LHC has not discovered any particles that go beyond the standard model to explain mysteries like dark matter, an invisible but unknown substance that makes up most of the matter in the universe.

However, these exciting measurements show that there is still much to learn about the particles and forces of the Standard Model. Perhaps our best chance at finding answers to the big questions facing 21st century fundamental physics is to study the particles we already know in more detail than to discover new ones. Either way, we still have much to discover.


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