Johns Hopkins Gazette: March 6, 1995

Barnett Leads Hopkins Physicists In Search To Confirm Top Quark

By Emil Venere

     Score another point for particle physics.
     Last April scientists announced they had found "strong
evidence" for the existence of a subatomic particle called the
top quark, believed to be the last fundamental building block of
matter to have its existence confirmed by research. 

     Now it's official. Last Wednesday two teams of scientists at
Fermi National Accelerator Laboratory made a historic statement:
physicists have indeed observed the top quark, one of 12
particles from which, according to current theory, all matter is

     "This is a scientific milestone of great importance," said
Bruce Barnett, a physicist who led a group of seven Hopkins
researchers playing an important role in the discovery. "This
particle fills in the last of the basic building blocks in the
standard model."

     The standard model says that all matter consists of
elementary particles called leptons and quarks. There are six
types of quarks, grouped into three sets of "twins" labeled the
up and down, the strange and charm, the top and bottom quarks.
They are bound together by particles called gluons to make more
mundane particles like protons and neutrons. 

     The other elementary particles are six varieties of leptons,
a family that includes electrons and their lightweight kin,
called neutrinos, as well as their heavier cousins, known as
muons and taus.

     Out of all those constituents, the top quark was the final
missing piece of the subatomic puzzle.

     Not only have scientists found it, they have determined its
mass, information that could prove crucial in the study of matter
and the forces of nature.

     But last week's news was accompanied by an element of
mystery. The findings support confusing data reported last year:
the top quark's mass is about 10 times more than physicists had
predicted 15 years ago.

     "It's certainly surprising that it's this heavy," said Dr.
Barnett, vice chairman of the Department of Physics and
Astronomy. "It makes life interesting."

     The findings have implications beyond particle physics.
Understanding more about the top quark has a direct relationship
to uncovering secrets about how the universe evolved. Many
theorists believe that quarks and leptons were all that existed
in the very early universe, shortly after the Big Bang of cosmic
creation. In time, as the cosmos cooled, protons and neutrons
formed, eventually leading to matter as we know it today.

     To uncover the top quark's role in cosmology, more must be
learned about its mass, which can only be done with further
experiments. Fermilab, located near Chicago, is the only facility
with an accelerator powerful enough to study top quarks, Dr.
Barnett said.

     Hopkins' role in the research is linked to a device called
the silicon vertex detector. Developed in collaboration with
other American scientists and physicists from Italy, the detector
was instrumental in last year's findings. Since then, however,
the Hopkins researchers and their colleagues have made major
improvements to the device, enabling scientists to collect data
critical to the discovery.

     The detector is situated in the center of an 85-foot-long,
35-foot-high experiment called the Collider Detector at Fermilab
(CDF). CDF, in turn, is located at one of the points along a
3.9-mile circular pipe where protons collide with antiprotons,  a
proton's antimatter equivalent. A system of powerful
superconducting magnets called a Tevatron accelerator is used to
whip beams of protons and antiprotons around the circular track.
When they collide, they create "jets" of matter's most elementary
particles. These particles quickly decay into a wide array of
other kinds of exotic constituents, from muons to pions,
neutrinos to kaons, which are monitored by a girdle of nearly
100,000 detectors contained in the CDF.

     Top quarks can't actually be seen, but as they rapidly decay
they produce other particles that can be detected. The Hopkins
physicists are among 400 scientists on the CDF team, which found
the evidence that was announced last April.

     But at the time those findings were released, a team of
scientists working on another Fermilab experiment, called DZero,
had not yet found such evidence. More data had to be collected by
both teams before researchers could say with certainty that the
top quark does exist. Last Thursday, the CDF and DZero teams
announced that they had amassed all the data necessary to declare
the top quark real.

     The Hopkins group includes postdoctoral fellows John Skarha
and Rick Snider, and graduate students Douglas Glenzinski, Alan
Spies, Jeff Tseng and Jeff Cammerata.

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