But on Monday, representatives of the Sudbury Neutrino Observatory in Canada announced that neutrinos made by nuclear reactions in the sun's core change from one type to another during their 93-million-mile journey to Earth. And only particles with mass can change form.

The neutrino's mass cannot be much, around a mere billionth of a proton's. But its mere existence has profound implications:

• The standard model, the reigning theory in particle physics, does not allow particles that change their flavor to have mass. So that theory will have to be patched up -- though not discarded -- to accommodate the new observations.
• Because they originate deep inside the Sun, neutrinos may provide an unprecedented view of what goes on there.
• They may not weigh much individually, but adding up all the neutrinos in existence changes the total estimated mass of the universe -- a figure of great interest to physicists. Neutrinos seems to account for a small but significant fraction -- possibly up to 18 percent -- of the mysterious "dark matter'' in the universe that cannot be observed by telescopes or other ordinary means.

About 100 physicists from the United States, Canada and the United Kingdom collaborated on the Sudbury experiment. They presented their results at a meeting of the Canadian Association of Physicists and in a paper submitted to the journal Physical Review Letters.

Physicists have wrestled with the "solar neutrino problem'' since the early 1970s, when experiments detected a shortfall of the particles coming from the sun. The neutrino shortage meant either that theories describing the nuclear furnace at the Sun's core were wrong, or that something was happening to the particles on their way to Earth.

Monday's announcement demonstrates with 99 percent confidence that it is the latter.

The Sun produces only one type of neutrino. But there are two other kinds that the earliest neutrino detectors could not see, and some of the ones made by the Sun turn into those other types on their way to Earth.

Three years ago, a Japanese experiment called Super-Kamiokande came up with indirect evidence that some of the neutrinos produced by the Sun were changing into those different types. But that experiment could not distinguish among those types.

Now the Sudbury Neutrino Observatory has directly observed those changed neutrinos.

Measurements taken between November 1999 and January 2001 indicate that about 60 percent of the Sun's neutrinos change.

The Sudbury observatory is a 10-story-tall cavity a mile underground in a Canadian nickel mine. Neutrino experiments have to be performed deep underground because at the Earth's surface a heavy rain of cosmic rays and other high-energy particles drowns out the meek particles.

Inside the rock-hewn cavity is an acrylic tank filled with heavy water. Most neutrinos pass through the heavy water, just as they do the rock surrounding it. But every hour or two a neutrino collides with a heavy water molecule, giving off a spark of light. By measuring that light, the detector can tell that a collision occurred and determine what kind of neutrino made it.