STOCKHOLM, Sweden (AP) _ Americans David J. Gross, H. David Politzer and Frank Wilczeck won the 2004 Nobel Prize in physics on Tuesday for their exploration of the force that binds particles inside the atomic nucleus.

The trio, researchers at the University of California, Santa Barbara, the California Institute of Technology and the Massachusetts Institute of Technology, made important theoretical discoveries ``concerning the strong force, or the 'color force' as it is also called,'' the foundation said.

The ``strong force'' is the dominant force inside the nucleus that acts between the quarks inside the proton and the neutron, the foundation said in its citation.

Their discoveries, published in 1973, led to the theory of quantum chromodynamics, or QCD.

``This theory was an important contribution to the Standard Model,'' the citation said.

The Standard Model is the theory that describes all physics connected with the electromagnetic force, which acts between charged particles, the weak force, which is important for the sun's energy production, and the strong force, which acts between quarks.

Alfred Nobel, the wealthy Swedish industrialist and inventor of dynamite who endowed the prizes, left only vague guidelines for the selection committee.

In his will, he said the prize should be given to those who ``shall have conferred the greatest benefit on mankind'' and ``shall have made the most important discovery or invention within the field of physics.''

The academy, which also chooses the chemistry and economics winners, invited nominations from previous recipients and experts in the fields before cutting down its choices.

Last year physicists Vitaly L. Ginzburg of Russia and Americans A. Abrikosov and Anthony J. Leggett were honored for their work on superconductivity and superfluidity, the motion of a fluid without internal friction.

The two phenomena are linked, in that superconductivity arises from how pairs of electrons behave, while superfluidity comes about from pairings of atoms.

Researchers hope to harness superconductivity for such uses as power lines that conduct current without loss to resistance and high-speed trains that float above the tracks.