Exotic tetraquark: CERN physicists discover novel particle | Science | In-depth reporting on science and technology | DW | 07.07.2020

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Exotic tetraquark: CERN physicists discover novel particle

Once again, particle physicists at CERN have discovered a previously unknown state of matter. It consists of four charm quarks and is probably the first find in a new class of particles.

At the particle detector LHCb, which forms part of the large accelerator ring Large Hadron Collider (LHC),  particle physicists from the European Organization for Nuclear Research (CERN) have succeeded in detecting a tetraquark consisting of four identical quarks. That is a particle consisting of two so-called charm quarks  and two charm antiquarks.

The researchers presented the discovery at a symposium and published it in an unreviewed preprint on arXiv on July 1.

They have described the findings in detail  on the LHCb project website. 

Read more: CERN: Large Hadron Collider gets stronger, better, brighter

Dark matter

What does it teach us?

Quarks are the smallest known elementary particles besides leptons and bosons. Knowing how quarks bind together to form larger particles is a step toward solving the huge jigsaw puzzle posed by the great mysteries of physics.

These include the question of what dark matter is and of how we can explain physical observations that do not seem to fit into the standard model of physics — and maybe there is even an answer to the big question: Is there a theory of everything?

 Tetra- and pentaquarks are not completely new

Until a few years ago, it had only been certain that quarks typically combine in groups of two or three to form hadrons. But for decades, physicists had been using models to calculate that there must also be particles with four or five quarks.

Then, the first tetraquarks were discovered by Japanese and Chinese researchers in 2013. Researchers at CERN were able to confirm their existence a year later. However, these tetraquarks consisted of different quarks than those making up the particle that has now been found.

"Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type," Giovanni Passaleva, the outgoing spokesman for the LHCb project, explained. "Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type."

In 2017, incidentally, LHCb particle physicists also succeeded in finding a pentaquark.  This is a particle consisting of four quarks and one antiquark. 

Read more: Boson predictor Peter Higgs: A fundamentally modest physicist

Theory of Everything

What force holds the atoms together?

Pentaquarks and tetraquarks can help physicists to better understand one of the four basic forces of physics: the strong nuclear force that holds the atoms together.

It binds protons, neutrons and atomic nuclei, which make up the matter we know.

The discovery of the particle could help to develop models to "to explain the nature of ordinary matter particles, like protons or neutrons," Passaleva's successor Chris Parkes said.

Huge crash test center for the tiniest particles

At CERN, researchers are discovering the new particles — including the Higgs boson, whose discovery in 2012 caused a sensation in the science world — by colliding particles at almost the speed of light  in a strong magnetic field in a huge ring-shaped accelerator. In the process, they disintegrate into their individual parts.

Using gigantic detectors — similar to the photosensors of a digital camera — the physicists then record in which direction and how far apart the fragments of the atoms fly. From this data, they can reconstruct the properties of the elementary particles. 

Read more: The KATRIN Tritium Neutrino experiment: A giant scale for the tiniest particles starts

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