What Is a Pentaquark and Why Are Physicists so Excited About It?

For fifty years scientists have thought they existed, and now they finally have proof

An artist's rendering of what a pentaquark structure might look like. CERN

This week researchers at CERN’s Large Hadron Collider (LHC) in Switzerland announced evidence of a new type of particle called a pentaquark, Ian Sample reports for The Guardian.

Quarks are tiny particles that bind together to form different types of larger particles you might be more familiar with. Three quarks make a proton, for example. And, when five quarks combine, that’s called a pentaquark. This might not seem all that exciting, but for physicists it is. 

“The pentaquark is not just any new particle,” Guy Wilkinson, a physicist and spokesperson for the LHC experiment, said in a statement. “Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

Quarks come in six “flavors”: up, down, top, bottom, strange and charm. Anti-quarks also come the same six flavors. A pentaquark seems to consist of two up quarks, one down quark, a charm quark and an anti-charm quark. 

The discovery confirms 50 years scientists’ suspicions that more exotic particles might exist, Tia Ghose explains for Live Science:

In 1964, physicist Murray Gell-Mann proposed that a group of particles known as baryons, which include protons and neutrons, are actually made up of three even tinier charged subatomic particles known as quarks. Meanwhile, the theory went, another group of particles called mesons were composed of quarks and their antimatter partners, antiquarks.

Gell-Mann’s theory implied that even more complex quark structures could form larger particles, Ghose explains. If a trio of quarks make a proton, then what could five or six or seven quarks form? There were earlier hints at evidence of a pentaquark, but nothing conclusive.

The LHC researchers were studying how baryons (another particle made of three quarks) break down, but during the particle decay the quarks were forming intermediate structures. Based on the signal patterns they received, these intermediate structures had to be pentaquarks, the researchers report in the journal Physical Review Letters.

So far, physicists have only observed this one type of pentaquark in the LHC data, but there could be many other varieties.

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