First glimpse of tetraquark pair at Large Hadron Collider

A Large Hadron Collider experiment at CERN’s particle accelerator in Switzerland has led to the first observation of a doubly charged tetraquark and its neutral partner.

Measurements of elusive and exotic particles may open up new avenues of research and help scientists develop fundamental theories such as the Standard Model of particle physics which aims to explain the relationships between the elementary particles and forces of nature.

Included in the Standard Model are particles known as quarks.

Quark model

The quark model was introduced in 1964. Its theory predicts the existence of elementary particles that can be found in different configurations to form other particles.

For example, neutrons and protons – the subatomic particles that make up the atomic nucleus – are actually composite particles made up of 3 quarks bound together by the strong nuclear force. Protons are made up of 2 up quarks and 1 down quark. Neutrons are composed of 1 up quark and 2 down quarks.

But the theory around quarks and anti-quarks (the anti-matter equivalent of quarks) also predicts more complex particles made up of four or five quarks – tetraquarks and pentaquarks, respectively.

Scientists have been looking for these composite particles for more than a decade in an experiment called LHCb – Large Hadron Collider beauty. Now, researchers involved in the experiment have reported in Physical Review Letters the first observation of a doubly charged tetraquark and its neutrally charged anti-matter partner.

Cern lhcb large physics experiment hadron collider particle accelerator detector
Opening of the LHCb detector in 2018. Credit: CERN, LHCb Collaboration.

“In the past decade, the LHCb experiment has done pioneering work in the discovery of so-called exotic particles,” team member Dr Yasmine Sara Amhis, physics coordinator of the LHCb experiment, says in an article on “The LHCb discovered the first pentaquark in 2015, and this opened the road to many other findings. The primary objective of LHCb’s research into exotic particles is to discover which tetraquarks and pentaquarks exist and to map their properties, especially uncovering the other particles they decay into and their quantum numbers.”

Investigations could help determine which of the different models used to describe tetraquarks and pentaquarks are correct and any discrepancies or inaccuracies between them.

Future of LHCb is bright

The analysis which found the tetraquark pair used data collected during the LHC’s first two experimental runs which spanned two years between 2011 and 2018.

“The analysis that led to this discovery is very sophisticated and is fair to say is an example of one of the most ‘difficult’ studies performed within our collaboration,” Amhis says.

“Papers like this one show that discoveries, including some unexpected ones, remain possible. More than 70 new hadronic particles have been discovered at the LHC, by far most of them at LHCb. As we learn more about which hadrons exist and their properties, we gain new understanding of the strong force, one of the four fundamental forces in nature, which binds quarks into hadrons.”

Amhis says that such advances in our understanding of the basic forces and particles of the universe could help physicists develop more complete theories than those we currently have.

“This understanding in turn opens new doors in searching for physics beyond the Standard Model, by reducing theoretical uncertainties associated with such searches which are caused by an imperfect understanding of the strong force.”

In its third run, the LHCb is expected to gather 5 times more data per year. Scientists believe that this and better detectors will lead to even more discoveries in coming years.

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