Nuclear Physics: The Search for *

The Search for

In the alphabet-soup world of subatomic physics, only one letter was missing. The equations of quantum theory had indicated the existence of 17 pairs of basic “building blocks”—particles and antiparticles, balanced by opposite electrical charges, and physicists had long since spotted and labeled the 17 normal particles. Once they got their huge, high-energy accelerators working in the late 1950s, they matched 16 of these bits of matter with their antiparticle mates. All that remained was the elusive (anti-Xi-zero). In September 1961, a group of Yale University and Brookhaven National Laboratory physicists set out to find it.

Working with Brookhaven’s powerful Alternating Gradient Synchrotron, they slammed a stream of antiprotons into a bubble chamber full of liquid hydrogen. As the antiprotons hit the stationary hydrogen nuclei—which were also protons—they annihilated each other, giving off energy and filling the 20-in. chamber with a sudden splash of new, extremely short-lived particles.

Somewhere among all those collisions, if theory was correct, the anti-Xi-zero particle had been born, had lived and died—all in one ten-billionth of a second. The physicists began searching through some 300,000 photographs of the reaction to find the elusive particle. Last week, nearly two years later, they reported success in Physical Review Letters.

What had made the search so difficult was the fact that the anti-Xi-zero particle carries no charge, therefore left no track on the photographic plate as it careened through the bubble chamber. To locate anti-Xi-zero, the physicists first had to find the byproducts of the collision that produced it. These visible byproducts could only have been balanced by the invisible anti-Xi-zero, which would decay an instant later, leaving visible debris. Of the 300,000 photographs they examined, only three showed the predicted evidence of collision and decay that could only have come from anti-Xi-zero.

By verifying the existence of anti-Xi-zero, the Yale-Brookhaven team-completed the alphabet of the first family of subatomic particles. But during their long search, other physicists also were busy. Several new arrangements of fundamental building blocks have already been postulated, and some of them leave gaps that may be filled by still more particles. The long search seems certain to continue.

*Including Yale Physics Professor Horace D. Taft, son of Ohio’s late Republican Senator Robert A. Taft.