A balloon 60 stories tall carrying 5,000 pounds of instruments 120,000 feet over the American Southwest has found the first evidence in nature of antimatter, particles that are the exact opposite of all matter and whose existence has eluded science for the last 50 years.

Instuments sent aloft by scientists at New Mexico State University July 21 detected no fewer than 29 particles of antimatter during an eight-hour balloon flight over Texas and New Mexico. The 29 particles had precisely the same mass as the positively charged particle known as the proton, but not one of the 29 possessed a positive charge.

"They carried negative charges, the opposite of the proton," Dr. Robert Golden of New Mexico State said yesterday in announcing the finding. "We have found antimatter protons."

Antimatter is made up of atomic particles with the same mass and same-size electrical charge as protons (positive) and electrons (negative), with one big difference: they have the opposite charge. An antimatter proton is negatively charged, and an antimatter electron carries a positive charge.

Predicted by theory for most of the 20th century, antimatter is produced routinely in the laboratory of high-speed collisions of accelerated particles like protons with atoms of hydrogen gas, but have never before been found in nature. One reason is that watever a particle of antimatter collides with a particle of matter they annihilate each other.

The team of New Mexico State scientists who claim to have found the first natural antimatter has been working nine years to devise a way to detect antimatter.

"We started trying to make actual observations with balloons in 1976, but things didn't go our way at first," Golden said by telephone from Las Cruces, where New Mexico State is located. "We're only able to fly twice a year when the winds are right, and we had five balloon failures in a row before we finally succeeded."

The balloon flown by Golden and his team has a gasbag 600 feet high holding 28 million cubic feet of helium when fully inflated. Attached to the balloon is an instrument package that weighs 5,000 pounds, most of it cosmic ray sensors and counters, but including a 300-pound superconducting magnet. $ the magnet is so powerful that it points the entire 5,000 pounds of instruments toward the north," Golden said. "It is the world's largest compass."

The instuments were flown to an altitude of 120,000 feet, where there are almost no air particles to collide with antimatter striking the Earth from deep space. Scientists have long theorized that antimatter produced by an exploding star would reach the Earth but be annihilated as soon as it struck matter in the upper atmosphere.

The instuments in Golden's balloon were built to measure the flight path and speed of all particles caught by the powerful magnetic field produced by the superconducting magnet. Protons caught by the magnet were moved, or bent, one way, electrons another.

"The amount of bending told us its energy, and we already knew its velocity," Golden said, "and by knowing the two we could calculate the mass of each particle we caught."

The 29 antimatter protons trapped by the magnet were all bent the same way protons were bent. There was a sigle difference: they all had negative charges, meaning they were antiprotons.

Where did the antimatter come from? Golden said its high velocity and high energy means it traveled great distances and probably was produced by the collision of debris from stellar explosions with hydrogen clouds between the stars.

Theory has it that at the time of creation there were equal amounts of matter and antimatter produced. The question is, where did the antimatter go? One theory is that it flew to the edge of the universe, where there are stars and entire galaxies made of antimatter.