|
The
apparatus at Los Alamos used to
set a limit on the mass of the
electron antineutrino. The superconducting
solenoid containing the gaseous
tritium source is being examined
by John Wilkerson (right) and
Tom Bowles (center). In the background
Hamish Robertson (left) and Mel
Anaya are inside the cage of conductors
used to cancel the Earth's magnetic
field. Behind them is the high-resolution
magnetic spectrometer. |
|
Dr.
R.G. Hamish Robertson |
Neutrinos are created with electrons
when the nucleus of an atom disintegrates
through beta decay. Scientists know
roughly how many neutrinos exist as
leftovers from the Big Bang creation
of the universe; however, their mass
is a mystery. The mass range was narrowed
in 1991, when scientists at Los Alamos
National Laboratory announced a new
top limit (9.3 electron volts, or
eV) for the mass of the electron anti-neutrino
(one type of neutrino), discovered
through measurements of the beta decay
of molecular tritium. From these data,
the scientists determined that less
than 30 percent (and consistent with
zero) of the mass of the universe
comes from electron anti-neutrinos,
and thus, that they cannot constitute
the bulk of the "dark matter" that
makes up some 90 percent of the mass
of the universe. Dark matter is nonluminous
material that cannot be detected by
observing electromagnetic radiation,
but whose existence is inferred from
current models of the universe. The
electron anti-neutrino would need
a mass exceeding 10 eV for it to constitute
the primary form of dark matter. R.
G. Hamish Robertson won the 1997 Tom
W. Bonner Prize from the American
Physical Society largely for this
work.
Scientific Impact:
Determining if dark matter exists,
and in what quantity, is among the
most challenging problems in modern
astrophysics. This work is generally
viewed as a textbook example of how
to probe the nature of mass in the
universe, and its success paved the
way for improved experiments that
have lowered the electron anti-neutrino
mass limit to about 3 eV.
Social Impact: This
experiment gives insight into the
question of what the universe is made
of thus, one possible explanation
of what the universe is made of, thus
contributing to human understanding
of nature and improvements in science
education. This finding also helped
stimulate a worldwide research effort
in neutrino physics that has brought
scientists from many countries together
to work on the same problem.
Reference: "Limit
on Electron Antineutrino Mass from
Observation of the b decay of Molecular
Tritium," R. G. H. Robertson et al.,
Phys. Rev. Lett., 67: 957
(1991).
URL:
http://cupp.oulu.fi/neutrino/nd-mass.html
Technical Contact:
Dr. R.G. Hamish Robertson, rghr@u.washington.edu
Press Contact: Jeff
Sherwood, DOE Office of Public Affairs,
202-586-5806
SC-Funding Office:
Office of High Energy and Nuclear
Physics |