CERN / LHCb Collaboration

An exotic particle made up of five quarks has been discovered a decade after experiments seemed to rule out its existence.

The short-lived ‘pentaquark’ was spotted by researchers analysing 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. The finding, says LHCb spokesperson Guy Wilkinson, opens a new era in physicists’ understanding of the strong nuclear force that holds atomic nuclei together.

“The pentaquark is not just any new particle — it represents a way to aggregate quarks, namely the fundamental constituents of ordinary protons and neutrons, in a pattern that has never been observed before,” he says. “Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

Protons and neutrons are made up of three kinds of quarks bound together, but theorists calculate that, in principle, particles could be made of up to five quarks. Such particles would be rich testing grounds for quantum chromodynamics (QCD) — the theory that describes the forces that hold quarks together.

In 2002, researchers at the SPring-8 synchrotron in Harima, Japan, caused a stir when they announced that they had discovered a pentaquark, roughly 1.5 times heavier than a proton, inferring its existence from the debris of collisions between high-energy photons and neutrons. Within a year, more than ten other labs had reported finding evidence for the particle by reanalysing data.

But numerous others saw no evidence for such a state and, in 2005, the discovery was pronounced a mirage. The final straw came with an experiment at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, that repeated the SPring-8 measurement with more data and ruled out the pentaquark’s existence.

‘Curious episode’

The episode has since been held up as an example of how scientists can be tricked by data into seeing more than is there. In 2008, the annual Review of Particle Physics1 — the discipline’s official record — described the pentaquark as “a curious episode in the history of science”, and its most recent listings do not include a dedicated entry for the particle.

Those listings will have to be corrected, because the LHCb result leaves little doubt that pentaquarks are real, researchers say. Physicists saw a signal showing the unexpected appearance of two short-lived objects weighing 4.38 and 4.45 gigaelectronvolts (4.67 and 4.74 times heavier than a proton) during the decay of trillions of subatomic particles known as ‘Lambda B’ baryons, by analysing data that were recorded between 2009 and 2012.

After measuring the new objects’ properties and exhausting other known particles as candidates, the team concluded that they correspond to a pentaquark in two different configurations. The particle contains two ‘up’ quarks, a ‘down’ quark, and a ‘charm’ quark–antiquark pair, making it a ‘charmonium’ pentaquark. A preprint about the particle has been posted on the arXiv server2, and has been submitted for publication in Physics Review Letters.

Haunted by pentaquark

The result, which first caught the attention of physicists on LHCb in 2012 as a bump in their data, was a total surprise, says LHCb’s Sheldon Stone, at Syracuse University in New York. “In the old days we searched for new particles by bump–hunting, but in this case the bump found us!” he says. “We were studying something else at the time so at first we ignored it. For historical reasons we were quite haunted by the word pentaquark, so we did every conceivable check we could,” he says.

The researchers have high confidence in their finding: they say that there is a vanishingly small chance of the signal appearing if no new particles existed. The statistical bar — known as 9-sigma — is far higher than the 5-sigma usually required for a discovery in particle physics, such as the announcement of the Higgs boson in 2012. "If I have an immediate feeling of worry it is that they claim two states: is this because they have found a process that favours production of pentaquarks, or because they have not really found the best interpretation of the data?" says theorist Frank Close of the University of Oxford, UK.

“It’s about the most exciting discovery in QCD I could imagine,” says Frank Wilczek, of the Massachusetts Institute of Technology, and himself an architect of QCD. “To me the deep message is that ‘diquarks’ [hypothetical quark pairs] are a useful organizing principle within hadrons,” he says, adding that the scene is now set for rapid progress: “It’s like a phoenix rising from the ashes.”

The new pentaquark is not the one, known as the theta+, seen back in 2002, however: it is almost three times heavier, and contains different kinds of quarks.

“I think our result will energize the search for many different pentaquark states including the debunked theta+,” says Stone. “One important difference with the theta+ and other states is that our pentaquarks have charm–anticharm quarks present which can cause a much stronger binding. So pentaquarks with these constituents may exist while ones with lighter ones may not.”

Physicist Nick Manton, at Cambridge University, UK, cautions that the results are still preliminary. "We don’t really have a theoretical understanding of this beast, but if it really is there then there should be a whole pattern of similar objects that will likely take many years to find," he says.