New experimental data, which show that neutrinos have mass,

are forcing theorists to revise the Standard Model of particle physics

The origin of neutrino mass

Hitoshi Murayama
AAO/DAVID MALIN

IF WE look deep into the universe, we

it incorporates massive neutrinos in a
see stars and galaxies of all shapes and
natural way will require far-reaching
sizes. What we do not see, however,
changes. For example, some theorists
is that the universe is filled with partiargue that extra spatial dimensions
cles called neutrinos. These particles
are needed to explain neutrino mass,
which have no charge and have little
while others argue that the hitherto
or no mass were created less than
sacred distinction between matter and
one second after the Big Bang, and
antimatter will have to be abandoned.
large numbers of these primordial
The mass of the neutrino may even
low-energy neutrinos remain in the
explain our existence.
universe today because they interact
very weakly with matter. Indeed, every
Birth of neutrinos
cubic centimetre of space contains
Neutrinos have been shrouded in mysabout 300 of these uncharged relics.
tery ever since they were first suggesTrillions of neutrinos pass through
ted by Wolfgang Pauli in 1930. At the
our bodies every second almost all of
time physicists were puzzled because
these are produced in fusion reactions
nuclear beta decay appeared to break
in the Suns core. However, neutrino
the law of energy conservation. In
production is not just confined to our
beta decay, a neutron in an unstable
galaxy. When massive stars die, most
nucleus transforms into a proton and
of their energy is released as neutrinos
emits an electron at the same time.
in violent supernova explosions. Even
After much confusion and debate, the
though supernovas can appear as
energy of the radiated electron was
bright as galaxies when viewed with
found to follow a continuous spectelescopes, like the Anglo-Australian
optical telescopes, this light represents Ground-based
trum. This came as a great surprise to
Observatory, saw the light from supernova 1987A several
only a small fraction of the energy re- hours after the Kamiokande and IMB experiments had
many physicists because other types
leased (see figure).
of radioactivity involved gamma rays
already detected the neutrinos that were emitted.
Physicists detected the first neutrinos
and -particles with discrete energies.
from a supernova in 1987 when a star collapsed some 150 000 The finding even led Niels Bohr to speculate that energy may
light-years away in the Large Magellanic Cloud, the galaxy not be conserved in the mysterious world of nuclei.
nearest to the Milky Way. Two huge underground experiPauli also struggled with this mystery. Unable to attend a
ments the Kamiokande detector in Japan and the IMB physics meeting in December 1930, he instead sent a letter
experiment near Cleveland in Ohio, USA detected neut- to the other radioactive ladies and gentlemen in which
rinos from supernova 1987A a full three hours before light he proposed a desperate remedy to save the law of energy
from the explosion reached Earth.
conservation. Paulis remedy was to introduce a new neutral
The event marked the birth of neutrino astronomy. New particle with intrinsic angular momentum or spin of h/2,
neutrino telescopes were built soon after, including the where h is Plancks constant divided by 2. Dubbed the
AMANDA experiment in Antarctica, and plans are under neutron by Pauli, the new particle would be emitted toway to build an even larger experiment called ICECUBE to gether with the electron in beta decay so that the total energy
detect neutrinos from gamma-ray bursters billions of light- would be conserved.
Two years later, James Chadwick discovered what we now
years away.
However, neutrinos are still the least understood of the fun- call the neutron, but it was clear that this particle was too
damental particles. For half a century physicists thought that heavy to be the neutron that Pauli had predicted. However,
neutrinos, like photons, had no mass. But recent data from Paulis particle played a crucial role in the first theory of
the SuperKamiokande experiment in Japan overturned this nuclear beta decay formulated by Enrico Fermi in 1933 and
view and confirmed that the Standard Model of particle which later became known as the weak force. Since Chadphysics is incomplete. To extend the Standard Model so that wick had taken the name neutron for something else, Fermi
PHYSICS WORLD

MAY 2002

physicsweb.org

35

had to invent a new name. Being Italian, 1 Rounding up neutrinos

particle with opposite electric charge. In
neutrino was the obvious choice: a litthe case of neutrinos, the antineutrino is
tle neutral one.
neutral but right-handed.
Because neutrinos interact so weakly
The Standard Model also includes
with matter, Pauli bet a case of chama set of particles that carry the forces
pagne that nobody would ever detect
between these elementary particles.
one. Indeed this was the case until 1956,
Photons mediate the electromagnetic
when Clyde Cowan and Fred Reines
force; the massive W + and W particles
detected antineutrinos emitted from a
carry the weak force, which only acts on
nuclear reactor at Savannah River in
left-handed particles and right-handed
South Carolina, USA. When their result
antiparticles; and eight gluons carry the
was announced, Pauli kept his promise.
strong force.
Two years later, Maurice Goldhaber,
All the particles that make up matter
Lee Grodzins and Andrew Sunyar
have mass from the lightest, the elecmeasured the handedness of neuttron, to the heaviest, the top quark and
rinos in an ingenious experiment at
can be left- or right-handed. Although
the Brookhaven National Laboratory
the Standard Model cannot predict their
in the US. The handedness of a partimasses, it does provide a mechanism
cle describes the direction of its spin
whereby elementary particles acquire
along the direction of motion the spin
mass. This mechanism requires us to acview of the SNO detector located 2000 metres
of a left-handed particle, for example, Aunderground
cept that the universe is filled with partiin the Creighton mine near Sudbury,
always points in the opposite direction Canada. The vessel is 12 metres across and is
cles that we have not seen yet.
lled with 1000 tonnes of heavy water. A few of
to its momentum.
No matter how empty the vacuum
neutrinos that pass through the detector
Goldhaber and co-workers studied the
looks,
it is packed with particles called
interact to produce electrons that travel faster
what happened when a europium-152 than the speed of light in the heavy water. These
Higgs bosons that have zero spin (and are
nucleus captured an atomic electron. electrons create ashes of Cerenkov light that are therefore neither left- or right-handed).
The europium-152 underwent inverse detected by the 9600 photomultiplier tubes
Quantum field theory and Lorentz inbeta decay to produce an unstable surrounding the vessel.
variance show that when a particle is
samarium-152 nucleus and a neutrino.
injected into the vacuum, its handedThe samarium-152 nucleus then decayed by emitting a ness changes when it interacts with a Higgs boson (figure 2a).
gamma ray. When the neutrino and the gamma ray were For example, a left-handed electron will become right-handed
emitted back-to-back, the handedness of the two particles after the first collision, then left-handed following a second
had to be the same in order to conserve angular momentum. collision, and so on. Put simply, the electron cannot travel
By measuring the handedness of the gamma ray using a po- through the vacuum at the speed of light; it has to become
larized filter made of iron, the Brookhaven team showed that massive. Similarly, muons collide with Higgs bosons more
neutrinos are always left-handed.
frequently than electrons, making them 200 times heavier
This important result implies that neutrinos have to be than the electron, while the top quark interacts with the Higgs
exactly massless. To see why this is, suppose that neutrinos do boson almost all the time.
have mass and that they are always left-handed. According
This picture also explains why neutrinos are massless. If a
to special relativity, a massive particle can never travel at the left-handed neutrino tried to collide with the Higgs boson,
speed of light. In principle, an observer moving at the speed it would have to become right-handed. Since no such state
of light could therefore overtake the spinning massive neut- exists, the left-handed neutrino is unable to interact with the
rino and would see it moving in the opposite direction. To Higgs boson and therefore does not acquire any mass. In this
the observer, the massive neutrino would therefore appear way, massless neutrinos go hand in hand with the absence of
right-handed. Since right-handed neutrinos have never been right-handed neutrinos in the Standard Model.
detected, particle physicists concluded that neutrinos had to
be massless.
Evidence for neutrino mass
I was at the conference in Takayama, near Kamioka, in 1998
The Standard Model
when the SuperKamiokande collaboration announced the
We now know that all the elementary particles six quarks first evidence for neutrino mass. It was a moving moment.
and six leptons are grouped into three families or genera- Uncharacteristically for a physics conference, people gave the
tions. Indeed, precision experiments at the Large Electron speaker a standing ovation. I stood up too. Having survived
Positron (LEP) collider at CERN in Switzerland have demon- every experimental challenge since the late 1970s, the Stanstrated that there are exactly three generations. Everyday dard Model had finally fallen. The results showed that at the
matter is built from members of the lightest generation: the very least the theory is incomplete.
up and down quarks that make up protons and neutrons; the
The SuperKamiokande collaboration looked for neutrinos
electron; and the electron neutrino involved in beta decay. that were produced when cosmic rays bombarded oxygen or
The second and third generations comprise heavier versions nitrogen nuclei in the atmosphere. These atmospheric neutof these particles with the same quantum numbers. The anal- rinos are mostly muon neutrinos and interact very weakly
ogues of the electron are called the muon and the tau, while with matter. Filled with 50 000 tonnes of water, however, the
the muon neutrino and tau neutrino are equivalent to the SuperKamiokande detector located deep in the Kamioka
electron neutrino. Each particle also has a corresponding anti- mine in Japan is so large that it can detect atmospheric neut36

physicsweb.org

PHYSICS WORLD

MAY 2002

rinos. These neutrinos interact with atomic nuclei in the

water to produce electrons, muons or tau leptons that travel
faster than the speed of light in water to produce a shock wave
of light called Cerenkov radiation. This radiation can be
detected by sensitive photomultiplier tubes surrounding the
water tank.
From these signals, the SuperKamiokande team could also
determine the directions from which the neutrinos came.
Since the Earth is essentially transparent to neutrinos, those
produced high in the atmosphere on the opposite side of the
planet can reach the detector without any problems. The
team discovered that about half of the atmospheric neutrinos
from the other side of the Earth were lost, while those from
above were not. The most likely interpretation of this result is
that the muon neutrinos converted or oscillated to tau neutrinos as they passed through the Earth. SuperKamiokande is
unable to identify tau neutrinos. The particles coming from
the other side of the Earth have more opportunity to oscillate
than those coming from above. Moreover, if neutrinos convert to something else by their own accord, we conclude that
they must be travelling slower than the speed of light and
therefore must have a mass.
SuperKamiokande was also used to monitor solar neutrinos. The fusion reactions that take place in the Sun only
produce electron neutrinos, but these can subsequently oscillate into both muon and tau neutrinos. Though the experiment was able to detect the solar neutrinos, it was unable
to distinguish between the different neutrino types. In contrast, the Sudbury Neutrino Observatory (SNO) in Canada
can identify the electron neutrinos because it is filled with
heavy water, which contains hydrogen nuclei with an extra
neutron. Small numbers of electron neutrinos react with the
heavy-hydrogen nuclei to produce fast electrons that create
Cerenkov radiation (figure 1).
By combining the data from SuperKamiokande and its own
experiment, the SNO collaboration determined how many
muon neutrinos or tau neutrinos were incident at the Japanese detector. The SNO results also provided further evidence
for neutrino mass and confirmed that the total number of
neutrinos from the Sun agreed with theoretical calculations.
The implications of neutrino mass are so great that it is
not surprising that particle physicists had been searching
for direct evidence of its existence for over four decades. In
retrospect, it is easy to understand why these searches were
unsuccessful (figure 3). Since neutrinos travel at relativistic
speeds, the effect of their mass is so tiny that it cannot be
determined kinematically. Rather than search for neutrino
mass directly, experiments such as SuperKamiokande and
SNO have searched for effects that depend on the difference in
mass between one type of neutrino and another.
In some respects these experiments are analogous to interferometers, which are sensitive to tiny differences in frequency
between two interfering waves. Since a quantum particle can
be thought of as a wave with a frequency given by its energy
divided by Plancks constant, interferometry can detect tiny
mass differences because the energy and frequency of the
particles depend on their mass.
Interferometry works in the case of neutrinos thanks to the
fact that the neutrinos created in nuclear reactions are actually mixtures of two different mass eigenstates. This means,
for example, that electron neutrinos slowly transform into
tau neutrinos and back again. The amount of this mixing is
PHYSICS WORLD

MAY 2002

2 Neutrinos meet the Higgs boson

a

eL

tL
t

tR
t

R

L

eR

tR

e
L

eR

1/M

(a) According to the Higgs mechanism in the Standard Model, particles in the
vacuum acquire mass as they collide with the Higgs boson. Photons () are
massless because they do not interact with the Higgs boson. All particles,
including electrons (e), muons () and top quarks (t), change handedness
when they collide with the Higgs boson; left-handed particles become
right-handed and vice versa. Experiments have shown that neutrinos () are
always left-handed. Since right-handed neutrinos do not exist in the Standard
Model, the theory predicts that neutrinos can never acquire mass. (b) In one
extension to the Standard Model, left- and right-handed neutrinos exist.
These Dirac neutrinos acquire mass via the Higgs mechanism but
right-handed neutrinos interact much more weakly than any other particles.
(c) According to another extension of the Standard Model, extremely heavy
right-handed neutrinos are created for a brief moment before they collide with
the Higgs boson to produce light left-handed Majorana neutrinos.

quantified by a mixing angle, . We can only detect interference between two eigenstates with small mass differences if
the mixing angle is large enough. Although current experiments have been unable to pin down the mass difference and
mixing angle, they have narrowed down the range of possibilities (figure 4).
Implications of neutrino mass
Now that neutrinos do appear to have mass, we have to solve
two problems. The first is to overcome the contradiction between left-handedness and mass. The second is to understand
why the neutrino mass is so small compared with other particle masses indeed, direct measurements indicate that electrons are at least 500 000 times more massive than neutrinos.
When we thought that neutrinos did not have mass, these
problems were not an issue. But the tiny mass is a puzzle, and
there must be some deep reason why this is the case.
Basically, there are two ways to extend the Standard Model
in order to make neutrinos massive. One approach involves
new particles called Dirac neutrinos, while the other approach involves a completely different type of particle called
the Majorana neutrino.
The Dirac neutrino is a simple idea with a serious flaw. According to this approach, the reason that right-handed neutrinos have escaped detection so far is that their interactions are
at least 26 orders of magnitude weaker than ordinary neut-

physicsweb.org

37

3 Fermions weigh in
fermion masses
d

s
c

u
2 3

1
eV

meV

e
eV

keV

MeV

GeV

TeV

A comparison of the masses of all the fundamental fermions, particles with

spin h
/2. Other than the neutrino, the lightest fermion is the electron, with a
mass of 0.5 MeV c2. Neutrino-oscillation experiments do not measure the
mass of neutrinos directly, rather the mass difference between the different
types of neutrino. But by assuming that neutrino masses are similar to this
mass difference, we can place upper limits on the mass of a few hundred
millielectron-volts.

rinos. The idea of the Dirac neutrino works in the sense that
we can generate neutrino masses via the Higgs mechanism
(figure 2b). However, it also suggests that neutrinos should have
similar masses to the other particles in the Standard Model. To
avoid this problem, we have to make the strength of neutrino
interactions with the Higgs boson at least 1012 times weaker
than that of the top quark. Few physicists accept such a tiny
number as a fundamental constant of nature.
An alternative way to make right-handed neutrinos extremely weakly interacting was proposed in 1998 by Nima
Arkani-Hamed at the Stanford Linear Accelerator Center,
Savas Dimopoulous of Stanford University, Gia Dvali of the
International Centre for Theoretical Physics in Trieste and
John March-Russell of CERN. They exploited an idea from
superstring theory in which the three dimensions of space
with which we are familiar are embedded in 10- or 11-dimensional spacetime. Like us, all the particles of the Standard
Model electrons, quarks, left-handed neutrinos, the Higgs
boson and so on are stuck on a three-dimensional sheet
called a three-brane.
One special property of right-handed neutrinos is that they
do not feel the electromagnetic force, or the strong and weak
forces. Arkani-Hamed and collaborators argued that righthanded neutrinos are not trapped on the three-brane in the
same way that we are, rather they can move in the extra
dimensions. This mechanism explains why we have never
observed a right-handed neutrino and why their interactions
with other particles in the Standard Model are extremely
weak. The upshot of this approach is that neutrino masses
can be very small.
The second way to extend the Standard Model involves
particles that are called Majorana neutrinos. One advantage
of this approach is that we no longer have to invoke righthanded neutrinos with extremely weak interactions. However, we do have to give up the fundamental distinction
between matter and antimatter. Although this sounds bizarre,
neutrinos and antineutrinos can be identical because they
have no electric charge.
Massive neutrinos sit naturally within this framework.
Recall the observer travelling at the speed of light who overtakes a left-handed neutrino and sees a right-handed neutrino. Earlier we argued that the absence of right-handed
neutrinos means that neutrinos are massless. But if neutrinos
and antineutrinos are the same particle, then we can argue
that the observer really sees a right-handed antineutrino and
that the massive-neutrino hypothesis is therefore sound.
38

So how is neutrino mass generated? In this scheme, it is

possible for right-handed neutrinos to have a mass of their
own without relying on the Higgs boson. Unlike other quarks
and leptons, the mass of the right-handed neutrino, M, is not
tied to the mass scale of the Higgs boson. Rather, it can be
much heavier than other particles.
When a left-handed neutrino collides with the Higgs boson,
it acquires a mass, m, which is comparable to the mass of
other quarks and leptons. At the same time it transforms into
a right-handed neutrino, which is much heavier than energy
conservation would normally allow (figure 2c). However, the
Heisenberg uncertainty principle allows this state to exist for a
short time interval, t, given by t ~ h/Mc2, after which the
particle transforms back into a left-handed neutrino with
mass m by colliding with the Higgs boson again. Put simply,
we can think of the neutrino as having an average mass of
m2/M over time.
This so-called seesaw mechanism can naturally give rise to
light neutrinos with normal-strength interactions. Normally
we would worry that neutrinos with a mass, m, that is similar
to the masses of quarks and leptons would be too heavy. However, we can still obtain light neutrinos if M is much larger
than the typical masses of quarks and leptons. Right-handed
neutrinos must therefore be very heavy, as predicted by grandunified theories that aim to combine electromagnetism with
the strong and weak interactions.
Current experiments suggest that these forces were unified
when the universe was about 1032 m across. Due to the uncertainty principle, the particles that were produced in such
small confines had a high momentum and thus a large mass.
It turns out that the distance scale of unification gives righthanded neutrinos sufficient mass to produce light neutrinos
via the seesaw mechanism. In this way, the light neutrinos that
we observe in experiments can therefore probe new physics at
extremely short distances. Among the physics that neutrinos
could put on a firm footing is the theory of supersymmetry,
which theorists believe is needed to make unification happen
and to make the Higgs mechanism consistent down to such
short distance scales.
Why do we exist?
Abandoning the fundamental distinction between matter and
antimatter means that the two states can convert to each
other. It may also solve one of the biggest mysteries of our universe: where has all the antimatter gone? After the Big Bang,
the universe was filled with equal amounts of matter and antimatter, which annihilated as the universe cooled. However,
roughly one in every 10 billion particles of matter survived
and went on to create stars, galaxies and life on Earth. What
created this tiny excess of matter over antimatter so that we
can exist?
With Majorana neutrinos it is possible to explain what
caused the excess matter. The hot Big Bang produced heavy
right-handed neutrinos that eventually decayed into their
lighter left-handed counterparts. As the universe cooled, there
was insufficient energy to produce further massive neutrinos.
Being an antiparticle in its own right, these Majorana neutrinos decayed into left-handed neutrinos or right-handed
antineutrinos together with Higgs bosons, which underwent
further decays into heavy quarks. Even slight differences in the
probabilities of the decays into matter and antimatter would
have left the universe with an excess of matter.

physicsweb.org

PHYSICS WORLD

MAY 2002

4 Limits on neutrino properties

100
LSND

103

SuperKamiokande

m2 (eV)

It is encouraging that we have seen

such a phenomenon recently. In the
past three years, the KTeV experiment
at Fermilab near Chicago and the
NA48 experiment at CERN have established that the neutral kaon a
bound state of a down quark and antistrange quark and its antiparticle
decay in a slightly different manner. At
only one part in a million, this difference is very small. However, we only
need one part in 10 billion for us to
exist. If a similar difference in the decay
probabilities exist in right-handed neutrinos, which is quite likely, it could have
produced a small excess of primordial
matter from which all the other particles have been formed.

106

e or

there are hoping to establish that electron neutrinos do indeed convert to

other types of neutrinos.
In the longer term, there are serious
discussions about sending neutrinos
thousands of kilometres. Beams produced at Fermilab or Brookhaven, for
example, could be fired towards experiments in Japan or Europe. Also, a serious effort is being made to observe
the conversion of matter and antimatter using a rare process in nuclei called
neutrinoless double beta decay. In this
reaction, which is forbidden by the
Standard Model, two neutrons decay
into two protons and two electrons
without emitting any antineutrinos. Recently Hans Klapdor-Kleingrothaus
and co-workers at the Max Planck Institute for Nuclear Physics in Heidelberg claimed to have observed such a
process, but the evidence is far from
conclusive (see Physics World March p5).

Outlook
e
9
10
It is an exciting time for neutrino physe
ics. Many experiments are currently
under way or are being constructed
or planned to put the evidence for
neutrino mass on a more solid footing.
Conclusion
1012
Physicists prefer to use man-made
We are at an amazing moment in the
4
2
0
2
10
10
10
10
neutrinos produced by accelerators or
history of particle physics. The Higgs
tan2
in nuclear reactors because these neut- Previous experiments have failed to detect neutrino
boson, the mysterious object that fills
due to a lack of sensitivity. The lack of a
rinos can be controlled, unlike atmo- oscillations
our universe and disturbs particles, will
signal, however, can be interpreted as a limit on the
spheric or solar neutrinos.
be found sometime this decade, and
mass difference m2 between types of neutrinos and
The difficulty is that neutrinos only the mixing angle, . This plot of m2 as a function of
evidence for neutrino mass appears
2
appear to oscillate over long distances, tan shows the regions inside the lines that are
very strong. The Standard Model,
The grey region is excluded by
thereby motivating a series of so-called excluded.
which was established in late 1970s and
SuperKamiokande. The solid lines are from searches
long-baseline experiments. The K2K for electron neutrinos (e) transforming into any other has withstood all experimental tests,
experiment in Japan has already been type of neutrino. The limits on oscillations specically has finally been found to be incomrunning for a few years. It involves between muon neutrinos () and tau neutrinos ()
plete. To incorporate neutrino mass
are indicated by the dotted line, while the dashed line
firing a beam of muon neutrinos pro- shows the results for e to oscillations. The
into the theory and to explain why it
duced in an accelerator at the KEK dot-dashed line highlights the limits on e to
is so small requires major changes
laboratory towards the SuperKamio- oscillations. For experiments that are able to detect
to the Standard Model. We may need
oscillations, the blue and yellow areas
kande detector, some 250 km away. So neutrino
to invoke extra dimensions or we may
highlight the preferred values of m2 and tan2 with
far the experiment has detected the 90% and 99% condence. The LSND experiment at
need to abandon the sacred distinction
disappearance of muon neutrinos due the Los Alamos National Lab also reported evidence between matter and antimatter. If the
to neutrino oscillations, which is com- for neutrino oscillations, but this is unconrmed.
latter is the case, neutrino mass may
pletely consistent with what we have
reveal the very origins of our existence.
learned from atmospheric neutrinos. An even better ex- One thing is certain, we are sure to learn a lot more about
periment called MINOS will extend the search for neutrino neutrinos in the coming years.
oscillations. Currently under construction, the neutrinos produced at Fermilab will be sent a distance of 750 km to the Further reading
Soudan mine in Minnesota, and there are similar plans to fire S Abel and J March-Russell 2000 The search for extra dimensions Physics World
muon neutrinos produced at CERN towards detectors at the November pp3944
Gran Sasso Laboratory in Italy. Particle physicists there are Q R Ahmad et al. 2001 Measurement of the rate of e + d p + p + e
also hoping to detect tau leptons produced by the oscillation interactions produced by 8B solar neutrinos at the Sudbury Neutrino
Observatory Phys. Rev. Lett. 87 071301
of muon neutrinos into tau neutrinos.
Last year the SNO collaboration upgraded its detector in an Y Fukuda et al. 1998 Evidence for oscillation of atmospheric neutrinos Phys. Rev.
effort to detect muon neutrinos or tau neutrinos directly. On Lett. 81 15621567
the rare occasions when these neutrinos interact in the detec- H Quinn and J Hewett 1999 CP and T violation: new results leave open questions
tor, they break up the deuterium nuclei in the heavy water to Physics World May pp3742
release neutrons. In order to count the muon neutrinos and The ultimate neutrino page cupp.oulu./neutrino
tau neutrinos, the SNO team added purified sodium chloride, The history of the neutrino wwwlapp.in2p3.fr/neutrinos/aneut.html
which captures the neutrons. And another experiment called
KamLAND in Japan is studying antineutrinos from commer- Hitoshi Murayama is in the Department of Physics, University of California,
cial nuclear-power plants some 175 km away. Researchers Berkeley, CA 94720, USA, e-mail murayama@hitoshi.berkeley.edu
PHYSICS WORLD

MAY 2002

physicsweb.org

39