Tevatron set to resume search for top quark

May 15, 1992

By Judith Goldhaber

This is the moment that LBL high-energy physics teams have been waiting for.

Earlier this week, Fermi National Laboratory's Tevatron collider went on line again after a three-year hiatus, with two giant state-of-the-art particle detectors -- the Collider Detector Facility (CDF) and the D Zero -- in the beam line.

Scientists in LBL's Physics and Engineering divisions are collaborators in both detector experiments. After supervising the design and construction of key components of the CDF and the D-Zero at LBL over the past decade, many of them will spend the next few years commuting to Fermilab, in Batavia, Ill., for a running period slated to continue (in several phases) through 1995.

The CDF is the original detector for the Tevatron collider. It began operating in 1987 and is back for a second run with a new, state-of-the-art, "SVX" vertex detector, much of which was designed and built at LBL (see Currents, Jan. 22, 1988; Feb. 3, 1989). And just around the ring now sits the CDF's friendly rival, the D-Zero detector, ready to operate for the first time after a research and development effort that began almost 10 years ago.

Though every experiment with new detectors and an improved accelerator is a step into the unknown, there is no question that this particular Tevatron run has a very specific goal -- the production and discovery of the elusive particle known as the top quark. In fact, both the CDF and the D-Zero detectors, while remaining alert for any unexpected physics that might come their way, have automatic "express lines" -- special tapes that will record likely top candidates the instant they are detected and move them to the head of the line for data analysis.

If the top quark exists in the discovery range of the Tevatron, both detectors should begin seeing the first events within a matter of months. Accumulating enough data to support an official discovery announcement might take more than a year, however.

The top quark is the missing member of the third (and possibly final) "generation" of elementary particles. The other members of this group are the b, or "bottom," quark (discovered in 1977 at Fermilab), the tau lepton (a kind of heavy electron discovered in 1975 by the SLAC-LBL Mark I collaboration at SLAC), and the tau lepton's neutrino.

The first generation of elementary particles, which makes up ordinary matter, contains the "up" and the "down" quarks and the electron with its neutrino. The second generation contains the "strange" and "charm" quarks and the muon with its neutrino. Recent experiments suggest that there probably are no additional generations of elementary particles to be discovered -- which makes the top quark a specially prized quarry.

The top quark has proved to be extraordinarily hard to pin down. Immediately after the discovery of its counterpart -- the b quark -- in 1977, there was widespread optimism that the mass of the top might be low enough to allow it to be seen at accelerators such as SLAC'S PEP or Europe's PETRA. When that didn't happen, the optimists pinned their hopes on the first run of the CDF at the Tevatron in 1987. That successful debut achieved some important physics results (including a new lower limit of 90 billion electron volts on the top quark's mass) but didn't find the top. Now, with five times as many particles in the beam at the Tevatron, a significantly improved CDF, and the addition of the D-Zero, hopes are high again.

The hunt for the top quark will take place in the explosion of particles that follows the collision of a proton and an antiproton in the Tevatron ring. Out of the energy released in this impact, large numbers of particles are created -- everything from the familiar protons and neutrons of everyday matter to the most exotic denizens of the subnuclear zoo. But a particle cannot be created if its mass (expressed in terms of energy, according to Einstein's familiar formula, E = MCsup2) is higher than the energy available. That's why the top quark has not been found in previous searches.

If the top quark is created at the Tevatron, it will decay within a fraction of an instant into b quarks and other particles. These particles will fly out from the center of the beam pipe, where collisions take place, and pass through a concentric series of specialized detecting systems in the CDF and D-Zero, beginning with vertex detectors very close to the origin of the event. In these instruments, the particles will have their charge, momentum, and energy measured. From these measurements, the particles will be identified. Certain particles or combinations of particles are signatures that point backwards to the fleeting presence of the top quark.

Though there's no question about what holds "top" priority in this Tevatron run, a lot of other interesting physics is expected to occur in this energy range, and both detector groups will be implementing strategies to study it.

Both the CDF and D-Zero are giant collaborations involving many institutions and hundreds of physicists. CDF lists 224 scientists from 18 institutions (including 15 from LBL) as co-authors on a recent publication; D-Zero numbers 323 members from 34 institutions (16 from LBL).

In the initial phase of the CDF, LBL participants were responsible for designing and building one section of the endcap calorimeters. Following the first run, a collaborative effort between Physics Division's CDF team and Engineering Division's advanced detectors group designed and constructed the electronic data acquisition system for the new SVX silicon-strip vertex detector -- the most advanced of its kind in the world.

In the D-Zero, LBL teams contributed two major components: the electromagnetic part of the endcap calorimeters, and the vertex chamber that tracks the particles as they emerge from the initial collision.