Scientists have long muttered the word “pentaquark,” though its existence was debatable until Tuesday. Physicists now claim to have discovered the particle once and for all, laying to rest years of back-and-forth about whether or not it was just a mirage.
The pentaquark was discovered by scientists analyzing data on the decay of unstable particles in the LHCb experiment at the Large Hadron Collider (LHC) at CERN – Europe's particle-physics laboratory near Geneva.
Physicists believe the discovery could shed light on how everyday matter is constituted, and deepen the understanding of the strong nuclear force – one of the four basic forces in nature, along with gravity, electromagnetic force, and the weak nuclear force.
"The pentaquark is not just any new particle," Guy Wilkinson, the spokesperson for the LHCb experiment that carried out the research, said in a press release. "Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we're all made, is constituted."
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Although the word “pentaquark” isn't new, the confirmation of its existence marks an extremely important moment for the scientific community.
To understand the history of the pentaquark, you must rewind to 1964. This is when Nobel Prize winner Murray Gell-Man proposed that “quarks” exist in our everyday world.
He showed that every proton and neutron is made from combinations of three elementary particles known as quarks, and suggested that it could also be possible to make matter from five quarks, resulting in a pentaquark.
But until now, there was no evidence of the pentaquark's existence – though it wasn't down to lack of trying.
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In 2002, researchers at the Spring-8 synchrotron in Harima, Japan, announced they had discovered the pentaquark, stating that it was roughly 1.5 times heavier than a proton and inferred its existence from the debris of collisions between high-energy protons and neutrons.
Others followed suit, and within a year more than 10 labs had reported finding evidence for the particle.
But many in the science community weren't convinced, stating that they found no evidence that the pentaquark existed. An experiment at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, repeated the Spring-8 measurement with more data and suggested the 2002 discovery was a mistake.
However, the LHC team is now certain of the pentaquark's existence.
Their discovery actually began a few years ago, while studying the results of particle collisions at the LHC done between 2011 and 2012. This is when the team accidentally encountered a “bump” in the data. That is, a large spike in one of the readings.
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“Since the pentaquark has such a bad reputation, we didn’t take it seriously,” said Sheldon Stone, professor of physics at Syracuse University and one of the four members of the LHC team, as quoted by The Wall Street Journal.
But after six months of further work, the team decided that it had actually found the elusive pentaquark.
"Benefitting from the large data set provided by the LHC, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states," said LHCb scientist Tomasz Skwarnicki, a professor of physics at Syracuse University.
"More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark,” Skwarnicki added.
The team is still unsure how the quarks are bound together, and plans to investigate those details in further research.
“The quarks could be tightly bound,” said LHCb physicist Liming Zhang of Tsinghua University. “Or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei.”