Scientists at the Large Hadron Collider have announced the discovery of the pentaquark, a class of subatomic particle consisting of four quarks and one antiquark bound together. Like the Higgs boson before it, the pentaquark's existence has been theorized for years, but experiments in the early-2000s claiming to have detected the exotic form of matter were later invalidated. Many scientists had since given up on the pentaquark for good, but this time, say CERN physicists, there's no doubt it's been found.
As its name suggests, the pentaquark is a particle consisting of five quarks bound together. Quarks are elementary particles that exist in six variations known as flavors (these are given the unusual names of up, down, strange, charm, top, and bottom). These bind together in different combinations to form a range of composite particles, of which the most commonly known are neutrons and protons, consisting of three quarks each.
"The pentaquark... has never been observed before in over 50 years of experimental searches."
"The pentaquark is not just any new particle," said Guy Wilkinson, a spokesperson for the LHCb, one of the LHC's seven particle detectors and the site of the pentaquark's discovery. "It represents a way to aggregate quarks... in a pattern that has never been observed before in over 50 years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted."
The discovery was made by watching the decay of a type of subatomic particle known as the Lambda B baryon. The data for the resulting paper (which has been submitted for publication in the journal Physics Review Letters) was recorded between 2009 and 2012, but scientists were wary of the results given previous invalidated experiments. "We were studying something else at the time so at first we ignored it," LHCb physicist Sheldon Stone told Nature. "For historical reasons we were quite haunted by the word pentaquark, so we did every conceivable check we could."
Some scientists have suggested that there may yet be a better interpretation for the data that does not involve pentaquarks, but CERN's physicists are certain. "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 physicist Tomasz Skwarnicki.