How MiniBooNE works
No original scientific discovery can ever be considered as definitive. Other
scientists using different equipment must corroborate and expand the finding,
bringing their own skills and intelligence to the problem, probing and checking
the discovery using their own methods. Such motivations are behind the creation
of the MiniBooNE experiment.
Almost as soon as a second neutrino flavor had been imagined, scientists
began to wonder if neutrinos could “oscillate,” or spontaneously change
from one flavor to another. Other indicators also suggested the possibility
of neutrino oscillations: only about 50% of the electron neutrinos expected
to be coming from the sun are seen in experiments, and muon neutrinos interacting
in our atmosphere do not follow their expected distribution while electron
A 1995 experiment at the Los Alamos
in New Mexico stunned the physics community when it reported a few
instances where a neutrino of one flavor had indeed transformed into a
neutrino of another flavor. The
Liquid Scintillator Neutrino Detector
neutrino events from 1993 to 1998, and saw about seven dozen events
where neutrino oscillation had apparently occurred.
Such a finding would also imply that neutrinos have a tiny mass, and could
indicate the existence of an additional, fourth flavor of neutrino, a flavor
unaccounted for by the hallowed
of particle physics. Though statistically significant, the LSND finding
was based on only a few events and it begged for confirmation. So in 1997
the idea for MiniBooNE was born when Columbia University physicists
decided to chase after the controversial LSND result.
Hunting for oscillations
the first phase of the "Booster
located at the Fermi National Accelerator
in Illinois, is a large detector set up to hunt for neutrino oscillations--specifically,
the change of a muon neutrino into an electron neutrino. Ironically, picking
out oscillations produced by such infinitesimal particles requires a huge
apparatus weighing hundreds of tons.
Detecting oscillations requires a beam of neutrinos produced through
a series of steps:
1. First, bunches of very energetic
(moving at about 99.3 percent of the speed of light) will be produced
a small particle accelerator. Each bunch will consist of about 5 trillion
protons and will last only 1.5 millionths of a second, with five such bunches
produced each second.
Both muon and electron neutrinos will enter the MiniBooNE detector.
Most will fly through without stopping, but a small fraction will interact
and be observed.
2. These protons will strike a target made of beryllium.
3. When the protons strike the beryllium, particles known as pions
and kaons will be produced, and gathered by a magnetic focusing device into
a tight beam.
4. This second beam will travel inside a pipe about 50 meters (160 feet)
long. Inside the pipe about half the pions and kaons will decay into muons
and muon neutrinos.
5. The muons will be filtered out by a barrier made of about 5 meters of steel and
500 meters (1,600 feet) of earth. The neutrinos will zip unimpeded through
the barrier to the MiniBooNE detector. If LSND is correct, a tiny fraction of the muon neutrinos
will oscillate en route into electron neutrinos.
The MiniBooNE detector is a
sitting in a large underground vault at Fermilab. The tank, about 12
meters (40 feet) in diameter, is filled with 250,000 gallons of mineral
oil. (The oil was delivered to the site in
twelve railroad tanker cars.) A small fraction of the muon
and electron neutrinos will collide with atoms in the mineral oil, at a
rate of about one per 20 seconds. When they do, they will produce either
a highly energetic muon or a highly energetic electron, respectively.
These muons and electrons will rush through the mineral oil, moving in fact
faster than the speed of light in the oil. Any particle moving faster
than the speed of light in a material causes a particular kind of light to
be emitted, called Cerenkov light, a phenomenon similar to the sonic boom
that supersonic aircraft can produce.
Just inside MiniBooNE’s outer tank is an inner tank, dotted
with 1,280 light detectors. Called “photomultiplier tubes,” they can detect
the cones of Cerenkov light after they are produced.
Sorting the light cones
The Cerenkov light cones produced by muons and electrons are significantly
different--which allows MiniBooNE to tell the particles apart. Muons, being
almost 200 times heavier than electrons, produce sharply defined cones
of light. The much lighter electrons, however, do not travel nearly as far in the
mineral oil; they scatter about and eventually come to a stop. This means
that their Cerenkov light cones
fuzzy inner and outer edges. Since the light cones from electrons and
muons are different, the particles producing the light and therefore the
neutrinos engendering the particles can be distinguished.
The MiniBooNE experiment will receive about a billion trillion protons
each year, and should observe about a thousand neutrino transformations
in that time. It will collect much more data than LSND because of its more
intense neutrino beam; this will produce more events and a result of greater
As seen in an earlier section, confirming the LSND result would indicate
the existence of an additional kind of neutrino beyond the three known
flavors, requiring physicists to rewrite a large part of the theoretical
framework of the Standard Model.
-- David Appell