The Importance of MiniBooNE
After five years of intense construction involving over 50 people, MiniBooNE
began collecting data in the summer of 2002.
MiniBooNE’s primary goal is to definitively detect neutrino oscillations,
a few occurrences of which were seen in 1995 in the Liquid Scintillator Neutrino
Detector (LSND) experiment at Los
, New Mexico. Confirming the LSND result would have an important consequence:
it would require physicists to rewrite a large part of their current understanding
of how our universe is constructed, a picture known as the “
.” Because Fermilab's
accelerator will feed protons to MiniBooNE in large quantity--about a billion,
trillion protons a year--MiniBooNE will be able to detect neutrino oscillations
in large numbers. In doing so, it will be able to measure the exact details
of neutrino oscillations with large precision.
The Standard Model is meant to explain the interactions between elementary
particles in our universe. It includes a listing of the most basic particles
known--which now numbers a few dozen--their properties, such as mass--and
the four forces by which they interact with one another. Solidified over the
period from about 1960 to 1990, the Standard Model is capable of explaining
almost all experiments that physicists have carried out, and to an astonishing
Rewriting the Standard Model?
But if MiniBooNE
confirms neutrino oscillations, part of the Standard Model will need to
be rewritten. That’s because oscillations of muon neutrinos into electron
neutrinos will imply that a fourth, additional kind of neutrino must exist,
one beyond the three known “flavors” (electron neutrino, muon neutrino, and
tau neutrino). In turn, that will imply a fourth type of electron-like particle,
one beyond the electron, muon, and tau particles (a particle family collectively
How many more flavors might exist? Probably only one, at most. Scientists
who study the large-scale structure and evolution of the universe--its cosmology--have
calculated that, based on the details of how our universe is evolving, there
is at most one additional lepton besides the three already known. Still, finding
a fourth lepton and its associated neutrino would be quite surprising.
For decades physicists believed that neutrinos of all flavors were massless.
But the presence of neutrino oscillations also implies that at least some
of the different types of neutrinos must, in fact, have mass. This mass, if
it exists, is expected to be quite small, perhaps a million times smaller
than the electron. (The electron itself is already quite light: it accounts
for only about 0.05 percent of a hydrogen atom’s mass, and it would take about
a trillion trillion electrons to weigh as much as an amoeba.)
So if neutrinos have a mass, it will be very small indeed. But even that
small mass would be different from the Standard Model. But because there are
so many neutrinos in the universe--about a trillion just passed through your
body in the last second--they may still be a significant portion of the mass
of the universe. And, in fact, scientists have been puzzled because they
have been unable to account for almost all of the mass that the universe is
expected to have, based on the way it expands and the way its galaxies rotate.
Neutrinos with mass might be a sizeable fraction of such “missing mass,”
sometimes also called “dark matter.”
Hunting more than oscillations
MiniBooNE will also investigate other areas of science. One is as a supernova
are exploding stars that occur at the end of a typical star’s lifetime.
After a collapse inward, the star suddenly radiates out immense amounts of
energy in the form of photons (particles of light) and in neutrinos. Supernovae
have been seen throughout history, as in 1054 A.D., and astronomers expect
about three such exploding stars occur in our galaxy every 100 years.
One of the more recent notable supernovae occurred in 1987 and was named
.” It occurred about 160,000 light-years from earth (a billion billion miles),
and in fact several neutrino detectors then in operation, such as
in Japan, detected about two dozen neutrinos from SN1987A.
MiniBooNE can efficiently operate as a supernova neutrino detector without
interfering with its main task of testing the LSND signal. About 230 electron-antineutrinos
would be expected to be seen by MiniBooNE if a future supernova exploded in
our galaxy, about ten times more than were observed by Kamiokande II in 1987.
It could thus serve as an important cross check with other supernova neutrino
detectors that exist around the world (especially if, as is sometimes the
case, other detectors have been shut down for repairs). And because supernova
neutrinos arrive first, such combined information might tell astronomers where
to look to see the light from a supernova and all them to build upon their
theories of how stars evolve and die.
MiniBooNE could also help explain a puzzling result found by particle physics
in 1995. Physicists in England found a small anomaly in the distribution of
neutrinos from the decays of muons and pions. Over the years physicists have
learned not to ignore such relatively small discrepancies, because they often
provide hints and clues of new phenomenon. This particular discrepancy, called
the “KARMEN timing anomaly” after the experimental group that detected it,
has never been explained, although more recent experiments have not seen evidence
of the anomaly.
One explanation of the anomaly involves a new, apparently rare and as yet
unknown particle physicists call a Q0 (read “Q-zero”). The Q
0 would be have no electric charge, would have to be very massive,
and would have to interact with other particles only weakly. MiniBooNE will
be capable of broadly exploring for the KARMEN timing anomaly, and perhaps
either prove the existence of the Q0 or ruling it out.
MiniBooNE will also be able to more precisely measure the extent to which
muon neutrinos act as a tiny magnetic, its so-called "magnetic moment." As
well, it may be able to look for high-energy protons that come from high-energy
solar flares, explosions on the sun's surface that cast energy into space
and towards the sun.
Finally, as with any experiment done on the frontiers of science, MiniBooNE
has the possibility of uncovering new and exotic phenomena never before seen,
or even suspected. Curiosity is a motivation for all scientists, and it has
time and time again paid off in exciting new discoveries that have opened
windows into unexpected corners of nature. As exciting as the boom in neutrino
physics has been over the last several years, no one yet believes that all
the secrets of these ghostly particles have yet been revealed. MiniBooNE aims
to keep the secrets coming.
-- David Appell