Twenty years ago today, the CDF collaboration observed the first collisions between protons and antiprotons produced at the Tevatron. It was the dawn of the collider physics era at Fermilab; but it didn't come easy.

"I remember we were scrambling to get the vertex detector together," said Fermilab physicist Bob Kephart, who worked at CDF during that time. "Even as the detector rolled into the ring [at the speed of a few feet per hour], some of us were still working on it."

Long before this point, there were myriad challenges for the Accelerator Division. First there was the time-consuming process of making and storing enough antiprotons to run. Once that was done, batches of protons had to be injected, coalesced and accelerated to 150 GeV in the Main Ring before blasting into the Tevatron where they could be accelerated to an even greater energy of 800 GeV.

Then, after those protons were orbiting correctly, the stored antiprotons had to undergo the same process before being transferred to the Tevatron to circle in the opposite direction. The timing had to be perfect: If the antiprotons were moved from the Main Ring to the Tevatron at the wrong moment, their entry might interfere with the protons already inside. The fact that the particles were nearly impossible to see made the whole thing even more difficult.

"All along the way, things could go wrong," said Alvin Tollestrup, who was the CDF spokesman at the time. "And if they did, the scientists had to guess what the problem was and start all over again."

Even after all of these obstacles were overcome, there was the last-minute frustration of making the protons and antiprotons line up, or "cog," at the right place in the ring.

"What they did at the time is inject the antiprotons into the Tevatron, and accelerate them to full energy," said Tollestrup, who along with Helen Edwards, Dick Lundy and Rich Orr, won the National Medal of Technology for his work on the design and construction of the Tevatron. "But then the antiprotons were located in the wrong spot for collisions."

If the shot was lost, and the beams collided in the wrong place, they would use up another batch of valuable antiprotons and go through all of these steps again. Cogging was tricky because the scientists had to distinguish the proton-antiproton collisions from the proton-air molecule collisions that were inevitable inside the ring.

"We weren't sure what to expect because we had never seen it before," said Tollestrup, "but we knew it when we found it." When a proton, moving close to the speed of light, hit an air molecule, the vertex detector showed tracks of particles moving mostly in one direction. But when a proton made contact with an antiproton moving toward it at the same speed, tracks of particles flying in all directions dotted the picture.

Knowing that they were close to observing proton-antiproton collisions for the first time, CDF scientists worked all night tuning the particle beams and checking the detector readouts. Far past midnight, in the early hours of October 13, 1985, they finally succeeded (see graphic).

"It was the first time everything came together," Tollestrup recalled of the early-morning event. "Afterwards, I invited everyone over to my place for bacon and eggs."

—Siri Steiner

Click here to see logbook entries and more pictures of the first collision events.