"It shows us that we really don't know nothin'," said Nobel
physics laureate Leon Lederman in response to last week's announcement from Japan of
evidence that neutrinos have mass.
Results from two years of data collected on the Super-Kamiokande neutrino detector
indicate that the elusive subatomic particle has a mass at least one-ten billionth that of
an electron. Most interpretations of the Standard Model of particles and forces hold that
neutrinos have no more mass than a wraith.
 More
will be known about the neutrino when results begin coming in from the Sudbury Neutrino
Observatory in Canada (pictured here). SNO will be able to directly detect fluxes of
neutrinos emanating from the sun. | Photo by Roy Kaltschmidt
|
"These results really do shake up the Standard Model in a serious way," said
Lederman, former director of Fermilab who shared a Nobel prize for showing there was more
than one type of neutrino. The Standard Model says there are three types or
"flavors" of neutrinos--the electron, muon, and tau neutrinos. However, the
Super-Kamiokande results imply that there might be a fourth, fifth, or even a sixth
flavor--"sterile" neutrinos that could be detected only through their minuscule
gravitational effects.
Despite its tiny mass, the neutrino story packed a mighty wallop with the media. Major
stories appeared in the likes of The New York Times, Los Angeles Times, Washington Post,
San Francisco Chronicle, and Time magazine. There were reports on network television and
radio and on the Internet. Science as a whole can be gladdened that the public shows keen
interest when exciting research findings are announced. Even when the findings involve a
subject as esoteric as the neutrino.
Theorized in 1930 but not detected until 1956, neutrinos, which were created in the Big
Bang and in the nuclear fusion process that powers the stars, are the most plentiful
residents of the universe. There are thought to be some 50 billion of them for every
electron, and their collective mass could be comparable to that of all the visible stars
and galaxies. Yet neutrinos interact so faintly with ordinary matter that they are much
like ghosts, able to travel through millions of miles of lead without causing a stir.
The international team of about 100 U.S. and Japanese scientists at the
Super-Kamiokande did not actually measure mass in the neutrinos, but instead observed a
deficit and lower energies in muon neutrinos created in the atmosphere by cosmic radiation
after the neutrinos traveled hundreds of kilometers through the Earth. This indicates that
muon neutrinos oscillate--and to oscillate they must have mass.
"People will now have to take the idea of neutrino mass more and more
seriously," Berkeley Lab physicist and neutrino authority Eric Norman told Science
magazine.
Still, the Super-Kamiokande experiment did not provide an exact mass for the neutrino,
nor, in studying atmospheric neutrinos, did it answer the mystery of solar neutrinos. Only
a fraction of the neutrinos that the sun must be producing, according to what is known
about nuclear fusion, reaches Earth. Scientists would like to know why. These answers
won't be forthcoming until the Sudbury Neutrino Observatory comes on line next year.
Unlike the Super-Kamiokande detector which could only detect muon and electron neutrinos,
SNO can detect tau-neutrinos as well, and will directly detect fluxes of neutrinos coming
from the sun.
Says Kevin Lesko, a Lab physicist and project leader for the Berkeley part of the SNO
experiment, "Super-K has given us a tremendous statistical knowledge of neutrino
(oscillation) events. With respect to atmospheric neutrinos, their announcement is
exciting, but I'm still skeptical. There's an awful lot of theory that goes into the
production of muons and ions when cosmic rays strike the atmosphere, which is hard to
understand really well."
Any discussion involving the enigmatic neutrino seems bound to generate a difference of
opinions. As far back as 1960, author John Updike wrote of neutrinos:
At night, they enter at Nepal
And pierce the lover and his lass.
From underneath the bed -- you call
It wonderful; I call it crass."
