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 detectorís
Sean Dyer (retired EFD) outfitting Mark II for SLC
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 werenít
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 didnít
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 IIís successes at PEP, it was on the move again in 1987. This
time to the 50 GeV Stanford Linear Collider (SLC)óthe worldís 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."
Despite this precarious picture, Mark IIís installation at the SLC went
smoothly. It began taking data in 1989. A great consortium of physicists
had worked diligently to upgrade Mark II for the SLC. The silicon vertex
detector, a technology now used in B