neutrino is one of nature's most peculiar particles. It has 1/2 unit of spin but
no electric charge, a near-zero rest-mass, and it interacts with other particles
only through gravity and the weak interaction. It can pass through light years
of lead without an interaction. There is good experimental evidence that the
Earth receives only about 1/3 of the neutrinos that the Sun should be producing
and sending in our direction.
written several previous columns about neutrinos. One discussed the then-current
evidence for a neutrino with a huge 17 keV rest-mass (see Analog, December,
1991). Several other columns considered the possibility that the electron
neutrino might have an imaginary rest-mass characteristic of faster-than-light
tachyon particles (see Analog, September, 1992 and October, 1993). I will start
this column by saying that from new experimental results, it is now clear that
neutrinos do not have a 17 keV mass (that was a detector artifact)
and are not tachyons (that was a subtle artifact of the physics
and chemistry of the tritium sources used). We now understand much more about
neutrinos, and this column will present some of that understanding.
me start by reviewing the Standard Model of particle physics as it applies to
neutrinos. There are two classes of fundamental spin 1/2 particles, the
strongly-interacting quarks and the weakly-interacting leptons. Three of the
leptons (e, m,
have significant masses, and all three have the same electric charge. The other
three leptons (ne,
have zero charge and are called neutrinos. The simplest form of
the Standard Model assumes that neutrinos, like photons and gluons, have zero
rest-mass. However, we have had to change that assumption, based on new
experimental evidence. Neutrinos have very small masses (probably a few
hundredths of an electron-volt), they usually travel at nearly (but not quite)
the speed of light, and they rarely interact with anything.
sun is a giant thermonuclear reactor that burns hydrogen into helium, making
lots of neutrinos in the process. You can think of the Sun's thermonuclear
reaction as applying heat and pressure that forces hydrogen nuclei to
"eat" their orbiting electron and change their charges to become
neutrons, spitting out the neutrino "lepton seeds" that are left over.
About 61 billion neutrinos per second made in the Sun pass through each square
centimeter of area on the surface of the Earth. If your body presents an area to
the sum of a square meter, this means that 610 trillion neutrinos are passing
right through your body in the second it takes to read this line. But you don't
notice this because there are no interactions.
pass through your body and through the Earth as if neither was there. As you
might imagine, this makes neutrinos very difficult to detect... but not
impossible. Over the last quarter of the twentieth century, a succession of
large underground neutrino detectors has demonstrated that (a) the neutrinos
from the Sun can be detected and that (b) there is a discrepancy of a factor of
three between the number of neutrinos predicted by astrophysical theories and
the number of neutrinos actually detected. This is known as the Solar Neutrino
Problem. At the end of the twentieth century, it was number five on my list of
the top ten things that we do not understand about physics (see Analog,
July/August, 1999). However, in the past few years, important new information
about neutrinos has come from observations by the SNO, KamLAND and WMAP
detectors, and the Solar Neutrino Problem has essentially been solved.
Some years ago, the Canadian government began a
program for developing and selling nuclear power reactors, called CANDU
reactors. These were fueled with natural uranium and moderated with "heavy
water," that is, with water made with deuterium instead of hydrogen. This
made economic sense, because
had lots of natural uranium and a great deal of hydroelectric power, and during
off-peak times this surplus electricity could be used to separate deuterium from
hydrogen by electrolysis to make heavy water. However, after the market for
nuclear power reactors diminished because of misguided environmentalism and
used a CANDU reactor in developing its nuclear bomb, sales of the CANDU units
dried up and
was left with a sizable quantity of surplus heavy water.
are opportunists. The large reservoir of Canadian heavy water represented an
experimental opportunity that could not be overlooked. After years of proposal
writing and some delicate negotiations between the
and Canadian governments, a consortium of US and Canadian physicists built the
SNO detector (Sudbury Neutrino Observatory), a large acrylic vessel filled with
$300,000,000 worth of Canadian heavy water and housed miles deep in a mine in
. The acrylic vessel is suspended in a tank of normal water and surrounded by
photomultiplier tubes that record light flashes from the vessel.
neutrinos pass through heavy water, they can interact in several ways. In the
first process, which is called a "charged-current interaction," the
incoming neutrino can convert the deuterium nucleus into two protons and an
electron. Essentially, a neutron and neutrino change charges through a W
boson to become a proton and electron. The electron carries off most of the
original neutrino's energy and will make a flash of Cerenkov light in the heavy
the second process, which is called a "neutral-current interaction,"
the in-coming neutrino breaks up the deuterium nucleus into a proton and a
neutron. Essentially, a neutron and neutrino interact through a Z boson. The
neutron subsequently is captured by another nucleus, producing a gamma ray that
can be detected from the flash of Cerenkov light by a photo-electron.
the third process, which is called a "neutrino elastic scattering,"
the incoming neutrino bounces off the electron of a deuterium atom. Here, the
electron and neutrino interact through a either a Z
or W boson. The electron carries off part of the neutrino's energy and
will make a flash of Cerenkov light in the heavy water. These three types of
interactions generate somewhat different signals and are distinguishable in the
SNO detector is selectively sensitive to electron neutrinos (ne) through the charged-current interactions, and is
unselectively sensitive to all three neutrino species (ne,
through the neutral and elastic interactions. Therefore, if the missing 2/3 of
the neutrinos from the Sun are absent because they have "oscillated"
from ne to nm
they should contribute to the neutral and elastic interactions, but not the
sure enough, when the SNO data was analyzed, the solar neutrinos missing in the
charged-current interactions turned up in the neutral and elastic interactions.
The Solar Neutrino Problem occurs because all three neutrino species have small
and slightly different rest-masses. As they propagate through space, the small
mass differences modify the quantum interference between species, and ne
are transformed into nm
(and back again). Before SNO
operated, there were several alternative theories for how neutrinos might
oscillate. The SNO results are
consistent with only one of these theories, the so-called LMA (Large Mixing
Angle) solution, and indicate that the electron neutrinos are oscillating into
another neutrino species (presumably nm)
that has a difference in mass-squared |m(ne)2
– m(nm)2| of 8 × 10-5 electron-volts2.
neutrinos detected by SNO travel 150 million kilometers, so there is a very long
path length over which the oscillations between neutrino species can occur.
Rather surprisingly, the SNO data suggests that the path distance over which
significant neutrino oscillations occur is fairly short, only a few hundred
rather short oscillation length has been tested by the KamLAND experiment
. In the old Kamioka cavity inside a Japanese mountain, where the predecessor of
the Super Kamiokande detector was housed, a new detector has been built, a
stainless-steel tank containing a spherical balloon holding one kiloton of
liquid scintillator. About 180 km from the Kamiokande site are several Japanese
nuclear power reactors. In the nuclear fission process that occurs in these
reactors, neutron-rich fission-fragment nuclei are produced, and these undergo
radioactive decays in which the excess neutrons are converted to protons,
emitting an electron and an anti-neutrino in the process. KamLAND is designed to
detect these anti-neutrinos as they convert protons in the detector into a
neutron and a positron. The signal of such an anti-neutrino event is the
detection of gamma rays from the positron annihilation, followed by the
detection of a 2.2-MeV gamma ray from the capture of the neutron by hydrogen.
KamLAND detector will continue to operate for some time with improving
statistics, but in the initial period of its operation the consortium has
reported evidence for neutrino oscillations consistent with the LMA solution,
with a difference in mass-squared |m(ne)2
of about 7 × 10-5 electron-volts2. Thus, neutrino oscillations have
been detected for both neutrinos from the Sun and anti-neutrinos from nuclear
power reactors and give consistent results.
WMAP(Wilkinson Microwave Anisotropy Probe)
problem with the new SNO and KamLAND results is that they measure |m(ne)2 – m(nm)2| rather than m(ne) and m(nm).
In a column published in the October
2003 issue of Analog, I described the results of the WMAP satellite probe, which
mapped the small angle fluctuations of the cosmic microwave background, the
reverberating "sound of the Big Bang" when our universe was about
380,000 years old.
turns out that the WMAP data also has something to say about the rest-mass of
neutrino species. This is because massive neutrinos fall into the general
cosmological category of "hot dark matter," which tends to smear out
the "clumpiness" of the early universe. The
WMAP data, because it views this initial clumpiness, places fairly stringent
limits on how much hot dark matter could have existed in the early universe. The
data indicate that the mass density of neutrinos in the universe cannot be
larger than about 2% of critical density. This
translates to 1.0 electron-volts as the sum of the rest-masses of all three
that the electron neutrino (ne)
is no more massive than the other two neutrino types, this result limits its
mass to no more than 0.33 electron-volts. This is to be compared with laboratory
measurements of tritium beta decay, which place a corresponding upper limit of
2.2 electron-volts. Thus, cosmology
has beaten laboratory physics in placing a limit on the rest-mass of the
electron neutrino by almost an order of magnitude.
results from SNO, KamLAND, and WMAP are not the final word on neutrino physics,
but they have answered some outstanding questions. We know that about 2/3 of the
neutrinos reaching the Earth from the Sun are in the form of nm
We know that neutrinos are not tachyons and have positive mass-squared values
with differences that suggest (but do not prove) that the masses are around 0.01
electron-volts. We know from WMAP
that in no case can the electron neutrino have more mass than around 0.33
electron-volts. We also know that
about 93% of the universe's mass-energy is in the form of some mysterious dark
energy and cold dark matter, with only 7% left for ordinary matter including
neutrinos. We now know that we live
in a very peculiar universe.
Physics: an Update," Wick C. Haxton and Barry R. Holstein, American Journal
of Physics 72, 18-24 (2004), preprint hep-ph/0306282.
Cramer's new book: a non-fiction work describing his Transactional
Interpretation of quantum mechanics, The Quantum Handshake - Entanglement,
Nonlocality, and Transactions, (Springer, January-2016)
is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406