KamLAND Physics Impact
KamLAND (Kamioka Liquid-scintillator Anti-Neutrino Detector) has demonstrated convincingly that neutrinos are massive and undergo flavour oscillations. This is a profound discovery !
The experiment has determined the associated oscillation parameter Δ m221 to unprecedented
precision, has helped constrain the neutrino mixing angle θ 12,
and has explored the potential application of neutrinos as a
geophysical probe. The detector is currently undergoing a
purification upgrade which will enable KamLAND to execute a low energy
solar neutrino program in parallel with this already highly fruitful
Many questions of fundamental significance remain open; but with a new
understanding of neutrino propagation, neutrino science is now poised
to provide illuminating answers to some of society's most probing
questions concerning the Earth, the Sun and fantastic astro-physical
events such as supernovae.
Motivation ... The Solar Neutrino Problem
The Solar Neutrino Problem was a
fascinating mystery that motivated almost 4 decades
of activity in neutrino physics. The issue was that early neutrino
detectors counted fewer neutrinos from the sun than expected.
As more experiments were performed and predictions were refined,
evidence for a paradox between the Standard Solar Model(SSM) and the
Standard Model of Particle Physics (SM) became increasingly
significant. The early detectors were flavour selective; they only
detected 1 of the 3 known neutrino flavour species. A mechanism which
allowed neutrinos to change flavour and become invisible to the
detectors was a possible way forward to explain the experimental
results. Extensions to neutrino properities in the standard model were
proposed which would accomodate the necessary flavour change. Neutrino
flavour oscillation arising from mixing through neutrino mass states
was a leading theory. KamLAND was designed to probe this family of
solutions using reactor anti-neutrinos. Meanwhile other novel
mechanisms, such as neutrino decay, also competed to explain the solar
If neutrino flavour states are indeed
non-trivial mixtures of the neutrino mass states, precise predictions
can be made about how the flavour content of a neutrino beam evolves
with time. In the two neutrino approximation these predictions depend
on only 2 parameters; a mixing angle θ, and the difference in
squared-mass of the neutrino mass states involved, Δ m2. The
mixing matrix between the flavour basis and mass basis is parametrised
by the angle θ and the equation below shows how the flavour states
(left) can be written as an admixture of the mass states (right):
For a neutrino beam with energy, E, Δ m2
governs how the relative phase of the mass states in the admixture
changes with time. A non-zero value of this parameter gives rise to an
oscillatory evolution of the flavour content of the neutrino beam. It
also implies there is at least one massive neutrino !
For an experiment such as KamLAND, which
detects electron anti-neutrinos, the flavour evolution is usually
written as the probability for an electron neutrino to again behave
like an electron neutrino after it has travelled a distance L from the source to the detector. The equation below shows how this probability, P( νe -> νe, L), known as the survival probability, depends on E, L, Δ m2 and θ :
The Reactor Neutrino Signal
The dominant sources of anti-neutrinos
for the KamLAND experiment are commercial nuclear reactors in Japan.The
average distance, L0, from the reactors to the
KamLAND detector is ~180km. KamLAND measures the survival probability
by measursing the flux at the detector and comparing it to the known
flux produced by the reactors. Anti-neutrinos are detected using the delayed coincidence
method arising from inverse beta decay. Upon entering KamLAND, an
electron anti-neutrino may capture on a free proton in the hydro-carbon
based scintillator. The following reaction, known as inverse beta
decay, then occurs:
The positron quickly (~10's ns) deposits it's energy and then annihilates. The energy associated with this prompt
event is directly related to the incoming neutrino energy. The
remaining neutron thermalises and then later ( after ~200s μ s)
proton yielding a deuteron and a ∼ 2.2MeV photon.
This neutron capture event is called the delayed event.
The delayed coincidence of the prompt and
delayed event pair is an extremely robust signature of an
anti-neutrino. Great care has to be taken in detector design,
construction and operation to accomplish and maintain the radiopurity
levels required to achieve a negligable background from accidental
pairing of uncorrelated singles events. However, the radiopurity
requirements are much less stringent that they would otherwise be for a
non-conicidence type signal.
Pernicious backgrounds arising from
processes that produce a delayed coincidence signal similar to that of
anti-neutrinos are very important and much effort has been devoted to
characterising and quantifing all such backgrounds.
At lower energies, (E < 2.6 MeV),
geologically produced anti-neutrinos from uranium and thorium decays in
the earth must be considered. There are indications for a geo-neutrinos
component in the anti-neutrino spectrum measured by KamLAND. While
these are a background for the reactor anti-neutrino measurement, the
detection of these would be an important milestone.
The figure below shows the survial
probalility as a function of L/E which KamLAND has measured. Knowledge
of how the survival probability depends on neutrino energy is a
powerful discriminant of the underlying mechanism responsible for
The data are best described by the LMA
neutrino oscillation scenario and other mechanisms such as neutrino
decay are strongly disfavoured. This result provides compelling
evidence that neutrinos oscillate and that neutrinos are massive !
As mentioned previously, KamLAND has seen
indications for geologically produced anti-neutrinos arising from
uranium and thorium decays within the earth. Precision global
measurements of the geo-neutrino flux can provide important and
otherwise unattainable information about the composition and radiogenic
heating of the earth's core. While this approach is still in its
infancy, the KamLAND measurement was an important milestone in
establishing its feasibility. There is considerable interest and
excitement in the geological and physics communities about developing
this technique to its full potential.
Much of the recent progress in neutrino physics
has been motivated by questions arising from attempts to unravel the
Solar Neutrino Puzzle (SNP). KamLAND had its genesis in the desire to
test a whole family of neutrino oscillation scenarios and was uniquely
positioned to do this with electron anti-neutrinos from reactors. The
results have demonstrated that neutrinos are massive and have played a
decisive role in establishing the so-called LMA-MSW solution as the
correct explanation for the solar neutrino deficit.
Now that neutrino properties are better
understood, precision measurements of solar neutrino fluxes are inorder
so that we might rigourously test our understanding of the sun, further
probe the MSW effect and search for other possible subdominant effects
arising from heretofore unknown properties of neutrinos. The figure
below shows the predicted solar neutrino spectrum.
As the figure shows, low energy neutrinos,
E<1MeV, dominate the solar neutrino spectrum, thus sensitivity in
the low energy regieme is key to any precision measurement. KamLAND has
demonstrated effectiveness as a calorimeter for events with energies
ranging from hundreds of keV to several MeV and given its unprecedented
size is well positioned to make a timely precision measurement of 7Be
solar neutrinos. Such measurements represent an important compliment to those already underway at Borexino.
Low energy neutrino signal
Low energy neutrinos are detected by their
elastic scattering on electrons in the scintillator. This is a
singles-type signal and does not enjoy the robust delayed coincidence
signal associated with anti-neutrinos. Unfortunately it is impossible
to distinguish an energetic electron scattered from a neutrino from an
energetic electron emitted in the beta decay of scintillator
contaminants such as 85Kr and 210Bi. Given the
expected signal rate of ~100s of events per day in the detector, the
radiopurity requirements are very stringent indeed. These difficult
requirements were not met in the initial construction. However the
KamLAND collaboration is currently purifiying the scintillator volume
to reduce the offending backgrounds to acceptable levels. The figure
below shows the event spectrum expected upon successful completion of
the purification process
In this environment the 7Be solar neutrino signal (red line) can be measured.
At slightly higher energy, (0.8MeV < E <
1.2MeV) the opportunity to observe events arising from pep(black) and
CNO(cyan) solar neutrinos also exists. The dominant background in this
case comes from 11C. 11C is a long-lived (T1/2
~ 30 minutes) spallation isotope which is continually produced by
cosmic ray muons passing through the detector. In 95% of cases a
neutron is produced in association with each 11C nucleus.
By rejecting events that have the right spatial and time correlation to
muon tracks and neutron captures it is possible to veto most of this 11C
and achieve a healthy signal to background ratio for this measurement.
The success of this approach relies on very efficient neutron tagging
and precise muon tracking. The collaboration is working vigourously to
realise both of these requirements.
Unfortunately, the pp neutrino flux lies below the sensitivity of KamLAND because of the irreducible 14
C background at very low energy. Therefore, the opportunity to measure
the solar pep flux is especially tantilising as it is directly related
to the pp process. By studying these tiny flashes of light generated by
neutrinos in a cavern deep underground it may be possible to illuminate
the driving process that is fundamental to the sun and to our lives
here on earth.