By Anna Gosline

For 13 years, the Mark II detector sat in retirement at the east end of the Collider Experimental Hall (CEH), patiently waiting to be dismantled. Life was not always so quiet for this 1800-ton feat of engineering glory. Now that it is being dismantled, we reflect again on the detector’s glorious past.

Sean Dyer (retired EFD) outfitting Mark II for SLC run. (Courtesy of Tech Pubs)

Constructed in part with steel salvaged from a sunken Pearl Harbor battleship, Mark II was installed in three different locations, survived a massive earthquake and saw reams of important physics whose legacy can still be felt in the halls of SLAC today.

Mark II had big shoes to fill upon installation at SPEAR in 1977. Its predecessor on the beam line, Mark I, was the revolutionary cylindrical detector that saw not one but two Nobel Prize winning discoveries: the charm quark and the tau lepton. The discovery of these two particles is responsible for our current understanding of the different generations, or families, of matter that exist at successively higher energy levels.

"Mark I blew the lid off particle physics. There was a lot of important complementary work that followed the discovery of the tau and the charm, which was done with Mark II," said SLAC Director Jonathan Dorfan, who was a spokesperson for the Mark II experiments from 1980 to 1989.

Mark II was similar to its older brother in basic design, but was much more sensitive. "Mark II was a large improvement from all concepts of Mark I," said Vera Luth (EC), who worked with both detectors. "It could measure neutral particles, like photons, really well and was much better at detecting leptons." These improvements allowed physicists to make a detailed study of the charm meson as well as to describe several different decay modes of the tau lepton.

"Those decay modes were interesting at the time because people weren’t quite convinced that there was a tau," said John Jaros (EA).

The Lifetime of a Particle

After its brief stint at SPEAR working with energies from 3-7 GeV, in 1979 Mark II found a new home in the 28 GeV PEP storage ring. Its job there was, among many other things, to measure the lifetimes of the tau, the charm and the B-meson. In 1981, Jaros and his group added a new piece of hardware called a vertex detector, which made these measurements possible. This precise central device gave physicists the ability to trace particle tracks back to the original point of decay

"The tau particle lifetime was well predicted by theory. But we didn’t know what the B-meson lifetime would be. The prejudice was that it would be very short. It turned out to be incredibly long. That was really surprising," said Jaros.

The lifetime of the B-meson determined a crucial missing parameter of the Standard Model. Its unexpected length is also the key to the B Factory physics program at SLAC today.

After Mark II’s successes at PEP, it was on the move again in 1987. This time to the 50 GeV Stanford Linear Collider (SLC)—the world’s first electron-positron linear collider, built to produce and study Z-bosons.

The hulking detector was removed from PEP and lowered by crane into the SLC pit. Dorfan remembers looking down at the dangling detector from an office high up in the CEH. "It was nerve-wracking to watch the crane inch across the 65 foot deep collider hall pit," he said. "The whole experiment was hanging by a chain. If it was dropped, I was responsible. And they decided to leave it hanging there overnight."

In November 1989, the hard work of the Mark II and SLC teams paid off when they published research that limited the families of matter to three, answering the nagging question first posed after the revolutionary discoveries made with Mark I in the 1970s. "It was certainly very competitive, but we did have the first significant measurement limiting the number of neutrino particles to fewer than four," said Dorfan.

After its final moments of glory, Mark II was rolled off the SLC beamline in 1990, soon to be replaced by the much larger and more complex Stanford Large Detector (SLD) in 1991.