By Dr David Whitehouse BBC News Online science editor

Physicists have found a new subatomic particle, named Ds (2317). It will help them better understand the building blocks of matter.

Inside the BaBar detector

The particle consists of an unusual combination of more fundamental particles - quarks.

Two quarks form Ds (2317) and, curiously, its properties are not what theory predicted.

The announcement was made by physicist Antimo Palano to a packed auditorium at the Stanford Linear Accelerator Center (Slac) in the US.

The discovery was made by the BaBar international consortium, which operates a detector at Slac that analyses debris from subatomic particle collisions.

'Back to the drawing boards'

"Congratulations to BaBar," said Slac's director, Jonathan Dorfan.

"The existence of the particle is not a surprise, but its mass is lower than expected. This result will send theorists back to their drawing boards."

The peak in the data betrays the new particle's existence

Quarks are fundamental particles of which there are six types present in nature. The "up" and "down" quarks are the lightest, and are found within the nuclei of atoms of ordinary matter.

There are also the "charm", "strange", as well as the "top" and "bottom" quarks. These are heavier than the up and down quarks. Quarks can also have antiparticles such as anti-down, etc.

Heavier quarks were present in the early Universe and are created today in particle accelerators and in collisions of cosmic rays with atoms in the Earth's atmosphere.

The Ds (2317) combines a charm quark with another heavy quark - an anti-strange quark.

'From unexpected directions'

Physicists are hailing its discovery as important because it has unexpected properties that will provide insight into the force that binds the quarks together.

This force, unlike most others in nature, becomes stronger as the distance between the two quarks increases.

Marcello Giorgi, from the University of Pisa, Italy, who leads the BaBar collaboration, said: "Sometimes, the most exciting discoveries come from unexpected directions. There has been a buzz of excitement in the experiment in the past few weeks.

Slac fires particles along a track

"We have discovered a new charm particle in an experiment designed to probe the difference between matter and antimatter using bottom quarks."

Bob Cahn, a BaBar collaborator from Lawrence Berkeley National Laboratory, US, added: "The unexpected mass will make us look again at the forces between quarks and will stimulate new interest in charm-quark systems."

And Dr Raymond Orbach, director of the US Energy Department's Office of Science, said: "The BaBar experiment continues to produce important new knowledge adding to our fundamental understanding of the structure of matter."