Physicists popped champagne corks after a $125 million atom smasher made its first Z particle, a milestone in the quest to "understand what's in the mind of God" by studying the birth of the universe and the makeup of matter.

"I'm happy, with relief and joy," said Burton Richter, 1976 Nobel laureate in physics and director of the Stanford Linear Accelerator Center, southeast of San Francisco.Richter said scientists drank champagne Wednesday after confirming one Z particle was produced Tuesday by the 3-mile-long Stanford Linear Collider after an almost two-year delay.

While Z particles are incredibly tiny, they are the heaviest known fundamental particle of matter, weighing the same as 100 protons, the positively charged particles in the center of atoms. They can be thought of as "heavy light" because they are a heavier version of photons, particles that carry light.

Discovered by European scientists in 1983, Z particles are so heavy they could have existed naturally only for an instant after the "big bang" - the incomprehensible explosion scientists believe formed the universe up to 20 billion years ago.

Stanford's collider is designed to mass-produce Z particles by smashing together electrons - negatively charged particles that make a television picture - and their antimatter counterparts, called positrons. Mass production of thousands of Z particles will take months, Richter cautioned.

The collider will let physicists "peer deeper into matter and look back at what the universe was like nearer and nearer to its beginning," he added.

The particle, Stanford physicist Michael Riordan said, is "the key to understanding the birth of the universe. It helps us understand the forces that bind matter together and make it decay" radioactively.

Richter said last year that "what we're really after is trying to understand what's in the mind of God."