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08/04/2004
Multi-Flavored Little One
One of the most intriguing particles that make up our world is the
tiny neutrino. Although I’ve discussed the neutrino in previous
columns (e.g., 12/19/2002), an article by Dennis Normile in the
July 16 issue of Science whetted my appetite to find out more on
this strange particle. Visits to the Web sites of the University of
California at Irvine and the Super-Kamiokande provided me with
a better understanding of the schizophrenic nature of the
neutrino. Physicists have an inimitable way of describing
fundamental particles; for example, they talk about quarks that
have “charm”. When were you last charmed by a quark? Well,
neutrinos coming in “flavors”.
Let’s play along with the flavor theme. What if you ordered a
scoop of vanilla ice cream and saw it turn into chocolate ice
cream as it was scooped onto the cone and then turn into butter
pecan as you started to eat it? Those quirky neutrinos come in
three flavors and do change flavors in transit. They also sail
right through anything without a hitch. Zillions of them will pass
through you today. They swish through our planet as if it
weren’t there. Let’s delve into the properties and history of the
neutrino, and how we’re able to detect such an elusive particle.
Back around 1930, famed theoretician Wolfgang Pauli was
bothered by the fact that in certain radioactive decay reactions
there was some missing energy. He proposed that the missing
energy was carried off by some kind of tiny neutral particle. In
1934, Enrico Fermi developed a theory of radioactive decay in
which he used Pauli’s particle. Fermi called it a “neutrino”,
which in Italian is “a little neutral one”. It took a quarter of
century before Clyde Cowan and Fred Reines, in 1959, actually
found a particle that matched the predicted neutrino.
It wasn’t realized that their neutrino was one of three kinds
(flavors) of neutrino. Their neutrino is associated with the
electron and would become the electron neutrino, the plain
vanilla of neutrinos. Neutrinos were also postulated to account
for missing energy from the nuclear reactions in the Sun. Sure
enough, in 1968, electron neutrinos from the Sun were detected.
However, there was less than half the number of neutrinos
expected from the theoretical model of our nuclear solar furnace.
This “solar neutrino problem” cried out for attention. One
unlikely explanation was that some of the neutrinos had changed
flavors in transit from the Sun. (Another type of neutrino
associated with a particle known as a muon had been found in
1962.) It was generally thought that a more likely explanation
was that the theoretical model needed revision.
In 1978, workers at the Stanford Linear Accelerator came up
with a new particle they called the tau particle. This tau particle
is very unstable and decays within a ridiculously small fraction
of a second. What do you know? There was missing energy in
its decay and a new type of neutrino, the tau neutrino, was born.
Now we had three flavors – electron, muon and tau and the stage
was set another quarter of a century of startling findings.
Let’s consider why a neutrino can pass through the Earth and
through us without stopping. Remember, it is the “little neutral
one”. With no electrical charge, it’s like a photon of light
passing through glass, unaffected by all the electromagnetic
fields in and around atoms, with their charged electrons and
protons. The neutrino was also thought not to have any mass,
like the photon. Now we know different.
If the neutrino goes sailing through everything how can we
detect it? The answer is that an extremely tiny fraction of
neutrinos will bang into a nucleus and charged particles will be
ejected. These charged particles can be detected. One facility
capable of detecting neutrinos is the Super-Kamiokande in Japan.
A huge tank containing 50,000 tons of pure water is buried in a
mine deep in the earth. Lining the tank are 11,200 so-called
photomultiplier tubes, each roughly 20 inches in diameter. These
tubes pick up a pale blue light called Cerenkov radiation that is
emitted when charged particles move in water at more than 75
percent of the speed of light. From the intensity and direction of
the light, researchers can figure out the particle interactions and
pick up any neutrino strikes. Distinctive ring patterns allow them
to distinguish between electron- and muon-neutrino interactions.
The tank is actually two chambers, one to filter out spurious
reactions from particles originating in the ground around the
facility. Super-K was put into operation in 1996. Because of its
huge volume (the tank is over 120 feet in height and in diameter),
Super-K can pick up over a hundred solar neutrino collisions a
day. In 1997, they confirmed the solar neutrino problem, finding
only 37 percent of the number predicted by theoretical models.
Were the missing neutrinos changing into a flavor not detectable
by Super-K? By 1997, other groups had come up with various
results indicating that “oscillation”, changing of flavors back and
forth, was a distinct possibility.
Then the Super-Kamiokande Collaboration, a number of groups
banding together, did a conclusive experiment. They decided to
study muon neutrinos formed when cosmic rays strike our
atmosphere. Since muon neutrinos are generated all around our
planet, they travel to Japan from distances ranging from a few
miles from directly above the mine to thousands of miles from
distant spots on the globe as they travel through the Earth to the
Super-K.
In 1998, the first results were announced and, sure enough, there
was a deficit of muon neutrinos coming from certain longer
distances. This is evidence that the muon neutrinos are
transforming into the tau flavor. (The detectors can’t pick up the
taus.) What’s new? Six years have passed and it seems that
Super-K has picked up some 14,000 muon neutrino hits. I figure
that’s nearly one a day. They’ve chosen the most reliable data,
about 20 percent of the total, and, as I understand it, have plotted
the number of hits against distance traveled and have found a dip
and a rise in the number of hits.
As I understand it, having seen the Science article and not the
actual data, this is what is expected. Muon neutrinos arriving
from overhead don’t have time to convert to taus. Those arriving
from longer distances may have converted. On the other hand,
those from even longer distances have had time to convert to taus
and then back to muon neutrinos. The dip and rise in the number
of muon neutrinos is strong evidence that oscillation from one
flavor to the other is taking place.
These results not only nail down the split personalities of the
neutrino but also confirm something else. According to the laws
of quantum mechanics, a particle can’t oscillate unless it has
mass. The neutrino has mass but I haven’t found any mention of
a definitive figure for the neutrino mass. Does the muon flavor
neutrino have a different mass from the tau? If so, I guess the
mass must also be changing as the neutrino moves along.
How is it possible for a particle to be one thing and change back
and forth to another? An article by Dave Casper on the UC
Irvine Web site makes me believe I could almost understand it.
Particles behave like waves. Normally, a wave rises and falls in
a typical sine wave. If we combine two waves, where a peak
meets a trough, the waves cancel and if a peak meets a peak they
add – combine the two and they give a wave of the same
frequency but bigger peaks and troughs.
That’s if the two waves have the same frequencies, that is, the
same distances between peaks and troughs. Suppose we
combine two waves that have different frequencies. Now things
get rough. At times the combined wave will have the same
frequency as the first wave, at other times the same frequency as
the second wave. I haven’t checked this out but I’ll take
Casper’s word for it. If true, as the wave moves along, it will
oscillate between being like the first and being like the second
waves.
Casper suggests to those who question this conclusion that they
should look at the math. I did. I was lost immediately when I
saw E = m. They just drop the c squared from Einstein’s
equation. Hey, I needed that! (If you think I would have
understood the other equations leaving it in, you’re out of your
mind.)
Allen F. Bortrum
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