STOCKHOLM, Oct 5: Three American scientists were awarded the Nobel prize for physics, the Royal Swedish Academy of Sciences announced on Tuesday.

David Gross, David Politzer and Frank Wilczek will equally share the prize, worth 10 million kronor (1.35 million dollars), for their work in the field of quark research.

A quark refers to any of a group of sub-atomic particles carrying a fractional electric charge. Quarks are nature's smallest building blocks. Gross and Politzer come from California institutes.

Gross, 63, is from the University of California, Santa Barbara. "It is a centre for physics," Gross told reporters in Stockholm by telephone after the announcement that his institute had been awarded its fourth Nobel physics prize in recent years.

Politzer is from the California Institute of Technology and Wilczek, 53, is from the Massachusetts Institute of Technology. Discovery of what is known in the field of quark research as the 'strong force' within the smallest building blocks in nature eventually led to the trio winning the Nobel award, according to the Swedish academy.

"This discovery was expressed in 1973 in an elegant mathematical framework that led to a completely new theory, Quantum Chromo Dynamics (QCD)," the academy announced. The discovery was a major contribution to the standard model - the theory that describes all physics connected with the electromagnetic force, the weak force and the strong force, the academy said.

The electromagnetic force acts between charged particles, the weak force is important for the sun's energy production while the strong force acts between quarks. -dpa