Antineutrinos From Distant Reactors Simulate the Disappearance of Solar Neutrinos
First results from the Kamland detector in Japan home in on the parameters of solar-neutrino flavor oscillation.
After 36 years of solar neutrino experiments, the inescapable conclusion is that a large fraction of the electron neutrinos (ne)
produced by nuclear processes in the Sun's core are metamorphosing into
other neutrino varieties somewhere en route to the detectors on Earth
(see Physics Today, December 2002, page 16).
The cause is very likely neutrino oscillation, resulting from the
existence of three putative neutrino mass eigenstates with three
different masses. But until recently, the data have allowed
considerable latitude in the oscillation parameters, leaving open a
variety of possible mechanisms as the neutrinos traverse solar and
terrestrial matter as well as 150 million kilometers of vacuum.
Now, however, the first results from Kamland--a new kind of
reactor-neutrino experiment--have dramatically narrowed the range of
possible solar-neutrino parameters and thus made it clear that the
decisive metamorphosis takes place in the Sun itself.1
Kamland, a liquid-scintillator detector deep inside the Kamioka mine in
mountains west of Tokyo, exploits electron antineutrinos (e) arriving from 22 nuclear power plants within a few hundred kilometers as surrogates for neutrinos from the Sun.
The three neutrino mass eigenstates are presumed to be different coherent superpositions of the three flavor eigenstates (ne, nm, and nt)
associated with the three charged leptons: the electron, the muon, and
the tau. There is good evidence that only two of the three mass
eigenstates contribute significantly to ne. In that approximation, one can write
) = 1 - A
for the oscillating probability that a neutrino born as a ne is still a ne after a journey of length L through vacuum.
The characteristic vacuum oscillation length in kilometers is given by
l = E/1.27Dm2,
where E is the neutrino's energy in GeV and Dm2 is the difference between the squared masses of the two relevant mass eigenstates in eV2. The amplitude of the probability oscillation is
A = sin22q,
where the "mixing angle" q is the angle by which the state ne is rotated from the lighter of the two solar-neutrino mass states in the Hilbert space spanned by the mass eigenstates.
Because there are, as yet, no data good enough to reveal the very small contribution of the third neutrino mass eigenstate to ne, it is customary to summarize solar-neutrino oscillation results in terms of the two parameters q and Dm2.
Prior to the Kamland result, the region of parameter space favored by
the existing solar-neutrino data was the so-called large-mixing-angle
(LMA) solution, with q ranging from about 25° to 40° and Dm2 near 5 x 10-5 eV2.
However, several alternatives to the LMA solution had also
survived the experimental gauntlet with statistically respectable,
albeit larger, chi-squares. Some, like the LMA solution, had large
mixing angles; but they had much smaller Dm2, ranging from 10-7 eV2 all the way down to 5 x 10-12 eV2. Another solution still in play last year gave a q
of only about 1°. So small a mixing angle would suit the theoretical
prejudice that neutrino mixing angles ought not to be much bigger than
those that describe the mixing of quark flavors among the quark-mass
eigenstates. There were also several solutions that
hypothetical fourth neutrino mass eigenstate that would be "sterile" in
the sense that it did not participate in the usual weak interactions.
The highest energy of neutrinos produced in the solar core is
only 20 MeV. The third neutrino mass eigenstate, though it contributes
little, if anything, to solar-neutrino oscillation, is important for
the well-attested oscillation of GeV muon neutrinos produced in the
atmosphere by cosmic rays. For atmospheric nm oscillation, the relevant Dm2 is a few times 10-3 eV2 (see Physics Today, August 1998, page 17).
Twenty-two surrogate Suns
principal goal of the Kamland experiment is to test and refine the LMA
solar-neutrino solution with controlled terrestrial antineutrino
sources free of astrophysical complications. The heart of the detector
is a kiloton of liquid scintillator in a transparent spherical vessel
monitored by a surrounding array of almost 2000 large photomultiplier
tubes. It sits, shielded from cosmic-ray muons by a kilometer of rock
overhead, in the same cavern of the Kamioka mine that previously housed
the first-generation Kamiokande water-Ù
solar-neutrino detector. The Japan-US Kamland collaboration, headed by
Atsuto Suzuki of Tohuko University in Sendai, began building the
detector in 1998.
The beta decay of fission fragments in a power reactor
produces a continuous, penetrating flux of electron antineutrinos. The
22 reactor sites in Japan and Korea that contribute significant e
flux at the Kamland detector range in distance from 80 to 900 km, with
the strongest concentration between 160 and 180 km. If the neutrino
oscillation parameters do indeed lie within the LMA region, a
significant fraction of the reactor antineutrinos will change flavor
along the way and become invisible to the detector. Kamland detects
arriving neutrinos when they instigate inverse beta decay,
e + p ® e+ + n,
in the liquid scintillator. And that reaction can only be accomplished by electron antineutrinos.
The energy distribution of the neutrinos recorded by Kamland
peaks at about 4 MeV. For a neutrino of that energy, the LMA solution
gives an oscillation length l
of about 100 km. That's why the Kamland complex is so well suited to
probe the LMA region of the parameter space. All of the alternative
solutions to the solar-neutrino data have much smaller Dm2 and thus correspondingly longer l.
Therefore, only the LMA solution predicts that Kamland will record a
significant shortfall of reactor neutrinos. And previous
reactor-neutrino detectors, none of which was farther than a kilometer
from its source reactor, could only have seen a shortfall if Dm2 were much larger than 10-4 eV2.
"When Kamland was first proposed," recalls Stuart Freedman
(University of Calfornia, Berkeley), a spokesman for the
collaboration's US contingent, "we had a hard time selling it to the
community, because most of them were betting on the small-mixing-angle
solution, to which Kamland wouldn't be sensitive."
In the first five months of data taking, the Kamland group found a total of 53 e
events above an estimated background of only one event. That's only 61%
of the 87 events one would expect in the absence of neutrino
oscillation. But it's in good agreement with what the LMA solution
predicts. The figure on page 14 shows the Kamland shortfall, together
with the null results from the earlier reactor-neutrino detectors.
The estimated background of only about one event in the final
sample of 54 is impressively low by the standards of neutrino detectors
in underground settings rife with radioactive contaminants. It points
up a considerable advantage of looking for reactor antineutrinos rather
than solar neutrinos. Each inverse beta-decay reaction produces not
just one, but a correlated pair of signals that makes it much easier to
spot spurious events. First, the slowing and annihilation of the
positron produces a "prompt" scintillation signal whose intensity
provides a measure of the incident neutrino's energy. Then, typically a
few hundred microseconds later, the neutron can be captured by a proton
to create a deuteron and a telltale 2.2 MeV photon.
learns about the oscillation parameters not simply from the overall
fraction of neutrinos that have become invisible to the detector, but
also from the energy dependence of the shortfall. The figure above
shows the distribution of prompt positron-annihilation energies for the
54 events in the final sample. To good approximation, the prompt energy
measured by the phototubes is the incident neutrino's energy minus a
kinematic correction of 0.8 MeV.
The Kamland group's best-fit oscillation parameters from the observed shortfall and energy distribution are q = 45° and Dm2 = 6.9 x 10-5 eV2.
The 32 events in the plot with prompt energies below 2.6 MeV were
excluded from the analysis because, below this cutoff, antineutrinos
from radioactive uranium and thorium in Earth's crust contribute
significantly to the tally of inverse-beta-decay events. At the 95%
confidence level, the Kamland data already exclude all the previously
surviving alternatives to the LMA solution.
The posting of the first Kamland results on the Web triggered
a torrent of analyses by the theorists. (One wag commented that the
appearance of five of those papers hours before the Kamland paper
constituted evidence of causality violation.) The first order of
business was to extract the oscillation parameters from a global fit to
all the existing solar-neutrino data together with the Kamland results.
A typical fit of this kind, carried out by John Bahcall (Institute for
Advanced Study) and coworkers,2 is shown in the lower figure on page 15.
"The Kamland results decisively shrink the allowed range of the
solar-neutrino parameters," says Bahcall. "And they limit a possible
sterile component to less than 9%."
The Kamland result is also good evidence that antineutrinos
and neutrinos share the same oscillation parameters, as required by
fundamental theory. But the theory does allow the neutrino mixing
matrix to include a complex phase that might engender a subtle
neutrino-antineutrino asymmetry (leptonic CP
violation) strong enough to explain the upsetting of the
matter-antimatter balance in the early cosmos. (See the article by
Helen Quinn in Physics Today, February 2003, page 30.)
The triumph of the LMA solution raises the prospect that future
long-baseline neutrino experiments may find evidence for leptonic CP violation.
The MSW effect
For neutrinos coming from the solar core, the LMA solution implies
that vacuum oscillation on the journey to Earth plays second fiddle to
an irreversible flavor change that takes place in high-density regions
of the Sun: the Mikheyev-Smirnov-Wolfenstein (MSW) effect. If vacuum
oscillation with a l
very much shorter than our distance from the Sun were the dominant
process, the energy dependence of the solar-neutrino shortfall would be
almost completely washed out. But that's not the case; the
solar-neutrino detectors do see a clear energy dependence.
Because of its association with the electron, a ne
passing through matter feels an extra interaction energy, proportional
to the ambient electron density, beyond the matter-interaction energy
common to all the neutrino flavors. In 1986, Stanislav Mikheyev and
Alexei Smirnov at the Institute for Nuclear Research in Moscow, using a
formalism developed by Lincoln Wolfenstein (Carnegie Mellon
University), pointed out that this extra energy term should produce a
flavor metamorphosis when a ne
produced in the core passes through a critical-electron-density region
of the Sun. The critical density, and thus the distance from the core
at which it's encountered, depends on the neutrino's energy. When the
neutrino finally emerges from the Sun it is, to good approximation, in
a coherent superposition of flavor states that constitutes a mass
eigenstate. A pure mass eigenstate would experience no vacuum
oscillation on the rest of the journey to Earth. There is now good
evidence that this mass eigenstate, the heavier of the two whose
splitting is given by Dm2, is roughly an equal superposition of all three neutrino flavors. The MSW mechanism was originally invoked to explain how a small mixing angle might cause a large solar-neutrino shortfall.
"I'm sometimes asked whether our first Kamland result is a
discovery or just a confirmation of what we already believed," says
Giorgio Gratta (Stanford University), the US contingent's other
spokesman. "I like to compare it to the first creation of spectral
lines in the laboratory in the 19th century. Frauenhofer had already
found lines in the solar spectrum. But until they were also made on
Earth, one couldn't be sure that they were more than just something
that happened only in stars."
1. K. Eguchi et al. (Kamland collaboration), Phys. Rev. Lett. 90, 021802 (2003).
2. J. Bahcall, M. Gonzalez-Garcia, C. Peña-Garay, J. High Energy Phys. 02, 009 (2003).
© 2003 American Institute of Physics