Undated (AP) _ 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, 30 miles 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 scientists at the European Laboratory for Particle Physics (CERN) found the particle by using a collider in which protons are smashed against the antimatter counterparts called antiprotons.

Since 1983, hundreds of Z particles have been produced at different kinds of particle smashers at CERN and at Fermi National Accelerator Laboratory near Chicago. Those machines produced some Z particles but mostly other products of atomic decay.

The Stanford collider and one being built at CERN's lab in Geneva are to produce mostly Z particles.

Temperatures approach 1 million-billion degrees at the tiny collision point inside Stanford's machine, coming close to re-creating conditions that existed in the first one-tenth of one-billionth of a second after the big bang, said California Institute of Technology physicist Alan Weinstein.

''We're doing this to try and understand more about the physical universe: the fundamental building blocks of matter and the forces that hold those building blocks together,'' Richter said.

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.''

The collider's successful creation of its first Z particle is a triumph for American physicists and for Richter. That's because the machine cost one- eighth as much as the $1 billion Large Electron-Positron collider, due to produce Z particles as soon as July at CERN.

There was no guarantee Z particles would be produced by the untested technology of Stanford's collider, which is shaped like a huge tennis racket instead of like CERN's conventional ring-shaped collider.

''This is a milestone,'' said Jeff Sherwood, spokesman for the U.S. Department of Energy, which paid for the collider. ''We're delighted to hear the news.''

Construction of Stanford's collider was completed in May 1987. Richter had hoped it would produce Z particles that summer. Scientists started it up a year ago, shut it down in September to correct technical glitches, then restarted it in February.

The modifications added $10 million to its original $115 million cost, Riordan said.

Weinstein said the collider should help physicists inch toward their ultimate goal of developing a ''theory of everything'' - a simple theory that would explain all the particles and four fundamental forces of nature. Many scientists believe the four forces are simply different forms of a single force that existed at the beginning of time.

The four fundamental forces are gravity; electromagnetism, which produces light, radio waves, X-rays and microwaves; the strong force, which binds protons and neutrons together in the center of atoms; and the weak force, which makes some atoms break down radioactively.

The discovery of Z and related W particles at another type of atom smasher at CERN in 1983 yielded the 1984 Nobel Prize in physics for Carlo Rubbia and Simon van der Meer because it helped verify ''the standard model.'' That theory says electromagnetism and the weak force really are two forms of a single ''electroweak'' force.

Stanford's immediate goal once it mass-produces Z particles is to confirm the standard model by testing predictions about the mass of the Z particle and existence of particles called Higgs bosons. Some scientists also believe the machine may help them discover the elusive ''top quark,'' one of six types of quarks in atoms.

The collider is run by Stanford University for the Department of Energy, with more than 100 scientists from many universities involved.