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NY Times, Oct. 20 2015
Struggling to Get a Handle on the Flavorful Neutrino
by George Johnson
It was 20 years ago that Art McDonald and I stopped at a Tim Hortons
near Sudbury, Ontario, for coffee and doughnuts on our way to his job at
the neutrino mine. Donning hard hats, we crowded into a rattling
elevator cage and descended 6,800 feet to an underground laboratory that
reminded me of the one in “The Andromeda Strain.”
The Sudbury Neutrino Observatory sat at the end of a long corridor at
the bottom of a working nickel mine. In the tunnels above us miners dug
and blasted through granite. Dr. McDonald was after a far less
substantial ore.
Neutrinos are said to pass through Earth almost as easily as through
empty space. During the flight, theorists believed, neutrinos were
capable of changing their identity. Dr. McDonald’s job was to catch them
in the act. It was six more years before he succeeded. Earlier this
month he won a share of a Nobel Prize, a climax to a long and perplexing
tale.
I had come to Sudbury on a philosophical quest. I wasn’t entirely sure I
believed in neutrinos, which were invented in 1930 to fill a hole in
physics. In a type of radioactive decay, experimenters had measured more
energy going in than coming out.
That is supposed to be impossible, so the fix was to conjure forth an
unseen particle with precisely the right characteristics to take up the
slack. The particles had gone unnoticed, the argument went, because they
had no handles for experimenters to grab onto — no charge and (it was
believed at the time) no mass.
Sometimes this explanation seemed a little too pat. I knew neutrinos
were eventually detected, but in ways so oblique that I wondered how
physicists could be sure they weren’t just seeing what they needed to see.
By now neutrinos are woven so tightly into the mesh of physics that you
would have to be a crank to doubt their existence. But when you start
peeling away the layers of theory, reality can start feeling pretty
abstract.
Of all the particles in the universe, the only ones our senses can
directly register are photons — particles of light that strike the
retina and send electrical signals to the brain. Our cruder senses
respond to whole globs of matter — larger globs for touch, invisibly
tiny globs for smell. Sound, for its part, is the vibration of matter, a
rumbling of the ground or a reverberation of the air.
Revealing the existence of theoretical stuff like neutrinos means
coaxing them into producing photons to be registered by our eyes or our
instruments. It took a quarter century to figure out how to do that, and
by a somewhat circuitous route.
Like other particles, neutrinos, the theory goes, have antimatter
counterparts. When an antineutrino collides with a proton, it should
transform it into a neutron while an antimatter electron is kicked out.
It quickly strikes a regular electron, exploding into two photons flying
in opposite directions — tiny flashes of light.
An instant later the neutron is sucked into the core of an atom,
resulting in another flash. If the timing and energy of these
scintillations are precisely in sync, you can say you have glimpsed a
neutrino.
In the mid-1950s two experimenters, Frederick Reines and Clyde Cowan,
put it all together. They measured neutrinos spewing from a nuclear
reactor. The reward was a Nobel Prize.
Then things started getting messy. Now that it was possible to detect
neutrinos, physicists had a way of testing their theory of sunlight —
that it was generated by nuclear fusion. That meant neutrinos should be
pouring from the sun in droves.
In the inverse of the reaction used by Reines and Cowan, a neutrino
striking a neutron should transform it into a proton and an electron. If
the neutron is contained within the core of a chlorine atom, it morphs
into an argon atom and emits gamma rays. Gamma rays are very high
frequency light.
With a vat of chlorine atoms (in the form of dry cleaning fluid),
Raymond Davis Jr. was first to find this circumstantial evidence
(resulting in yet another Nobel Prize). But his experiment is more
famous for seeing only a fraction of the neutrinos required by solar
theory — another hole in physics.
Maybe the sun wasn’t really powered by fusion. Or maybe neutrinos were
eaten by a black hole lurking inside. By the time I met Dr. McDonald,
theorists had rallied around a less radical thought.
By then there seemed to be three different “flavors” of neutrinos. Maybe
as neutrinos streamed from the sun, they “oscillated” between the
different types. Our instruments had been blind to all but one. That is
where the Sudbury detector came in. It succeeded in registering all
three flavors, as signatures of photons.
This solution to the solar neutrino problem required a troubling
trade-off. For years physicists had celebrated the neutrino’s massless
purity, which allowed it to pass at lightspeed through anything in its
path. But for neutrinos to oscillate they had to be saddled with a tiny
dab of mass. “That can’t be — it’s too ugly,” the great physicist Hans
Bethe remarked when he heard the proposal. But in the end the
alternatives seemed worse.
As I raise a doughnut and a cup of takeout coffee to Dr. McDonald and
his crew, I still feel a little uneasy. Quarks, gluons, Higgs bosons —
the story is the same. Behind the scenes, particles decay into other
particles, until at the end of the tunnel you see the light. Whether
we’re reading a meter or a computer screen, our knowledge ultimately
comes down to photons.
On that trip to Sudbury, Dr. McDonald gave me a nugget of nickel ore.
Solid as it seems it is made of atoms that consist mostly of empty
space. It shimmers a silvery gold because photons strike hollow shells
of electrons and ricochet into my eyes. All of our knowledge of the
world is so indirect.
I find some satisfaction in Boswell’s famous description of Samuel
Johnson disputing Bishop Berkeley’s contention that the world is all in
our minds. Kicking a rock and maybe stubbing his toe, he declared of the
theory, “I refute it thus.”
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