After a 15-year search, physicists working in Switzerland yesterday announced the discovery of a new, elementary subatomic particle, one of only four particles believed to be responsible for all the forces acting in the universe.

Called the "W particle," it is thought to be responsible for triggering a variety of events in nature, including radioactivity and the first step in the nuclear-fusion reaction that fuels the stars.

The discovery confirms theories made beginning in 1967 about the underlying unity of the forces in nature, such as electromagnetism, and about how these forces work. It is also expected to open a whole new field of physics experiments.

The event is "the discovery of a decade," said Herwig Schopper, director of the laboratory where the experiments were conducted.

The announcement was made by Harvard University's Carlo Rubbia at a meeting of the American Institute of Physics in New York. Rubbia, along with David B. Cline of the University of Wisconsin and Simon van der Meer of the University of Guelph, leads a team of physicists and technicians so large it has its own telephone book.

Rubbia first proposed searching for the new particle in the American particle accelerator, or atom smasher, in Batavia, Ill., but the money could not be found, according to Drasgo Jovanovich, chairman of the physics department at Fermilab in Batavia.

Congress has since appropriated the funds, but the Europeans now have a significant lead in this field.

"This is the first time that the Europeans have beaten the Americans to something big in high-energy physics," Jovanovich said.

The experiments were run chiefly last September through November at the giant particle-accelerating machine in Switzerland called CERN, the French initials for the European Organization for Nuclear Research.

The machine--along with its predecessor at Batavia--is one of the largest machines of any kind on earth. It consists of a doughnut-shaped steel ring four miles in circumference, in which atomic particles circle hundreds of thousands of times, picking up speed with each revolution.

The accelerated atomic particles in this experiment, protons and anti-protons, were smashed into one another to produce an array of debris, the constituents and energy of which can be both predicted and measured precisely by physicists.

Rubbia said that the physicists have found, among the billions of collisions in their experimental machinery, six "events" that produced the W particle. "It is remarkable," he said, "because theory predicted that by now we should have seen 5.2 events. Exactly right, within statistical limits." He also said the particle was exactly as heavy as predicted, about 90 times the mass of the proton.

"This is a very important experiment," said Steven Weinberg, physicist at the University of Texas at Austin. Weinberg shared the Nobel Prize in 1979 with Sheldon Glashow of Harvard University and Abdus Salam of the University of London for contributing to the theory of the existence and properties of the W particle.

"This theory made a tremendous extrapolation from known experimental facts into an unexplored area, and predictions like that just have to be checked," he said.

A negative result in the experiment would have brought down years of work by physicists, and would have also overturned predictions on which much recent research in elementary physics is based.

The theory behind the new discovery dominates the thinking of physicists today as Albert Einstein's theories of relativity did earlier in the century.

Physics students were once taught that atoms were the universe's basic building blocks and that they were made of protons, neutrons and electrons. In the new view, two groups of particles are thought to be the most basic units: those that make up matter and those that carry forces.

The particles that make up matter are mostly varieties of quarks, hypothetical particles that are thought to bunch together to make up the protons and neutrons of the atom.

It is now believed there are only four ways in which basic particles interact with one another to cause all action in the universe, from creation itself to chemical reactions in the cells of our bodies. These four reactions are triggered by "force-carrying" particles that pass between any two interacting particles.

Each of the four fundamental forces is believed to be carried by its own characteristic particle. Electromagnetism, the most obvious of the forces to the human senses, causes light, electricity, magnetism and all chemical reaction, and is carried by the particle called the photon.

Gravity is believed to be carried by a theorized but undiscovered particle called the graviton. The so-called "strong nuclear force," which holds the nuclei of atoms together and is tapped in nuclear bombs, is carried by a particle dubbed the gluon.

The fourth fundamental force in the universe, the "weak force," is responsible chiefly for natural radioactivity, the spontaneous bursting-apart of some atoms. The W particle is named after this force.

In 1967 and 1968, Weinberg, Glashow and Salam created a new mathematical theory that brought together, in one set of linked equations, both the electromagnetic and the weak forces. The theory began what is called the "grand unification," under which the theories of all particles and forces in physics may finally be linked in one set of equations describing the behavior of matter and energy.