Behold the pentaquark

By Dr David Whitehouse
BBC News Online science editor

Physicists have discovered a new class of subatomic particle that will provide unexpected insights into the fundamental building blocks of matter.

Theory predicted where the particle should emerge

The discovery involves quarks - particles that make up the protons and neutrons usually found in the nuclei of atoms.

The new particle is the so-called pentaquark - five quarks in formation. Until now, physicists had only seen quarks packed into two- or three-quark combinations.

They say the discovery of this new particle should have far-reaching consequences for our understanding of how the Universe is put together.

Confirmed discovery

Until recently, no firm evidence of pentaquarks existed, even though physicists have searched for these objects for over 30 years.

In 2002, the first tentative evidence of the pentaquark was put forward at an international scientific conference in Japan. Earlier this year, a report of this work was submitted for publication to the journal Physical Review Letters.

The report says that pentaquarks were created by blasting carbon atoms with X-rays. The work was done by a Japanese team, led by Takashi Nakano of Osaka University.

Other evidence for the pentaquark has recently been reported by other experiments, with perhaps the strongest evidence coming from the Jefferson Lab in Virginia, US.

Physicist Ken Hicks, of Ohio University, who took part in both the experiment and the confirmatory work at the Jefferson Lab, says it took him two months to convince himself that the pentaquark was real.

We are quarks

More than 99.9% of the mass of everyday objects is contained within the nuclei of atoms. This means that most of your body mass comes from subatomic particles that are made up of quarks.

Carbon atoms were struck with X-rays

There are hundreds of subatomic particles known and most are composites of simpler particles. They all fit into two categories - baryons and mesons.

Baryons are made of three quarks and mesons comprise two quarks - a quark and an anti-quark.

For a long time, scientists have been puzzled as to why only these quark combinations existed. Some predicted other combinations such as the pentaquark, which consists of five quarks, including an anti-quark.

The discovery of the pentaquark, also known as a new exotic baryon state, should have far-reaching consequences for our theory of particle interactions that attempt to explain the structure of matter.