July 21, 2000
Scientists Detect Elusive Building Block of Matter
By JAMES GLANZ
hat many physicists consider to
be one of the last pieces of the theoretical puzzle that explains the structure of matter has been detected at
the Fermi National Accelerator Laboratory near Chicago.
An international team of scientists
will announce today that they have
detected the tau neutrino, considered
to be the most elusive member of
nature's most ghostly family of particles, the neutrinos.
The team of 54 physicists from
institutions in the United States, Japan, South Korea and Greece used
the world's most powerful particle
accelerator, Fermilab's Tevatron, to
fire an estimated 100 trillion tau neutrinos into an advanced emulsion
similar to photographic film.
Four of those neutrinos produced
minute but clearly recognizable
streaks in the emulsion.
Although their existence had been
suspected for 25 years, tau neutrinos
had escaped detection because it
takes a large amount of energy to
create them and because neutrinos
pass through most matter almost
without a trace.
"It's just been accepted that this
guy exists," said Dr. Regina Rameika, a physicist at Fermilab and a
member of the team. But, Dr. Rameika added, "the neutrinos are just
too strange to take for granted."
Neutrinos are particles with no
electrical charge and probably little
mass that rarely interact with other
matter. They are created by some of
the most basic physical processes of
the universe, such as decay of radioactive elements and fusion reactions
that power the sun.
Until now, the tau neutrino remained one of two major undetected
particles in the vast theoretical
structure called the Standard Model
of particle physics. The theory describes the properties of all the
known building blocks of matter,
from quarks, protons, neutrons and
electrons to the neutrinos.
The remaining unseen particle,
called the Higgs boson, is considered
a linchpin of the entire structure and,
according the theory, the source of
all mass in the universe. The new
finding clears the way for a race
between Fermilab and a European
laboratory, CERN, to detect the
Dr. Paul Langacker, a physicist at
the University of Pennsylvania who
was not part of the group, called the
neutrino experiment "very subtle
and difficult" and said that as recently as five years ago, many physicists believed that the detection of
the tau neutrino would be all but
"The remaining piece of the Standard Model itself is the Higgs particle," Dr. Langacker said. "There are
strong indirect indications that it is
right around the corner."
The experiment was also a step
toward clearing up some of the remaining mysteries concerning neutrinos themselves.
According to the Standard Model,
neutrinos have no mass. But two
years ago, a Japanese experiment
called Super-Kamiokande found evidence that neutrinos have at least a
small mass, without determining
what that mass is.
So the detection of the tau neutrino
was a crucial prerequisite for
planned experiments at Fermilab
and CERN to help determine the
mass of the neutrino.
Those experiments would involve
shooting beams of neutrinos hundreds of miles underground to distant detectors to see if one type of
neutrino changes into another en
route. According to advanced theories, any such transmutation would
be an indication of the mass.
Knowing the value of the mass
could help settle several mysteries,
including how much swarms of neutrinos in space might contribute to
the weight of the universe.
"To be able to make tau neutrinos
and detect them directly is going to
be very important in this whole new
world of neutrino physics," said Dr.
Martin L. Perl, a Nobel Prize-winning physicist at the Stanford Linear
Physicist Wolfgang Pauli first postulated the existence of neutrinos in
the 1930's to account for energy and
momentum that seemed to vanish
during the radioactive decay of various elements. So weakly do the particles interact with matter that physicists had to wait nearly 30 years for
the first detection of any neutrinos.
The first two types of neutrinos to
be seen are closely associated with
electrons and muons, much less bizarre particles that are grouped in a
classification called leptons within
the Standard Model.
Leptons are a class of particles
that do not interact strongly with
So when Dr. Perl and colleagues
discovered a new lepton, called the
tau particle, in 1975, they assumed
that the electron neutrino and muon
neutrino would soon have company.
But making a tau neutrino first
required making a tau particle --
and because it is much more massive than the other leptons, that required an accelerator with much
"Neutrino experiments are very
difficult to begin with," said Dr. Roger Rusack, a physicist at the University of Minnesota who is a member of
the team, "and we're looking at a
neutrino that's very hard to make."
The team did produce them in bulk
by smashing protons accelerated to
nearly the speed of light at the Tevatron into tungsten. Tau particles created in the maelstrom decayed into
tau neutrinos, which streamed
through thick layers of shielding that
blocked most other particles and
reached the emulsions.
There, very rarely, a tau neutrino
collided with an atomic nucleus and
produced other particles, including a
tau particle, that left a characteristic, kinked trail in the emulsion.
crucial part of that trail was only
about a millimeter long, and the
technique for finding such subtle features was developed at Nagoya University in Japan, one of the partners
in the experiment.
"It means that we've finally seen
the interactions from the tau neutrino, which was assumed to exist in
1975," said Dr. Byron Lundberg, the
Fermilab physicist who, with Dr. Vittorio Paolone of the University of
Pittsburgh, is a spokesman for the
Dr. David O. Caldwell, a physicist
at the University of California in
Santa Barbara, said that it would
have been "an incredible surprise" if
the tau particle did not have its own
neutrino, as the electron and the
Dr. Caldwell said that some speculative theories beyond the Standard
Model postulate yet another neutrino, a so-called sterile neutrino that
would be associated with no other
particle, but it had been important to
first confirm the existence of tau
Particle physicists are likely to
turn their immediate attention to the
search for the Higgs boson. The first
experiments with a chance of finding
it are scheduled to begin next spring
at Fermilab's Tevatron. If the particle is not seen there, it could turn up
at a much more powerful accelerator called the Large Hadron Collider,
which should begin operation at
CERN in 2005 or 2006.