black hole

5 quark particles may exist in black holes.
© Hubble / NASA

New subatomic species found

Collision debris yields five-quark particle.

By David Cyranoski, Nature
July 2, 2003

A class of subatomic particle, consisting of five quarks rather than the normal two or three, has been discovered by physicists in Japan. Theorists had expected combinations of four or more to exist, but experiments over the past 30 years had failed to detect them.

A team from the SPring-8 synchrotron in Harima, near Kobe, created the particles by firing gamma-rays at a carbon target. The researchers, led by Takashi Nakano of Osaka University's Research Center for Nuclear Physics, say that the five-quark particle probably formed when two particles — a neutron, which is made up of three quarks, and a K+ meson, which contains two — fused during the collision. Evidence of the particle, which decayed rapidly, was found by analysing debris from the collision.

The discovery, which is due to be published on 4 July in Physical Review Letters, will give a huge boost to particle-physics research, and could have important implications for our understanding of the early Universe.

Nakano himself might never have thought to look for the five-quark particle had it not been for a series of lunchtime conversations with Dmitri Diakonov of the Nordic Institute for Theoretical Physics in Copenhagen, while the pair attended a conference in 2000. Diakonov thought that the experiments that Nakano was planning could produce a five-quark particle, and suggested a new data-analysis method that he argued would reveal the particle's presence. "Dmitri suggested that the particle could exist as a smaller particle at a lower energy level, so that's where we looked," says Nakano. "It was a novel way of interpreting the data."

Nakano presented his findings last October at the International Conference on Particles and Nuclei in Osaka. "We got a pretty negative response," he says. "Most people thought it was impossible." But in the months after the conference, a group at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, confirmed Nakano's results by revisiting data from previous experiments. The group presented its data, now submitted for publication, at the Conference on the Intersection of Nuclear and Particle Physics, held in New York in May.

Nakano says that the discovery could help to pin down some theories of the early Universe — such as how quarks came together to form matter after the Big Bang. "Studies on two- and three-quark matter had hit a wall," he says. "This might provide more clues."

But he adds that the five-quark particles are not likely to be found in many places in today's Universe. "It's very unlikely that these exist anywhere freely, except maybe in a black hole," says Nakano.