The Nobel Prize in Physics 1990
Jerome I. Friedman, Henry W. Kendall, Richard E. Taylor
17 October 1990
The Royal Swedish Academy of Sciences has decided to
award the 1990 Nobel Prize in Physics jointly to Professors
Jerome I. Friedman and Henry W. Kendall both of the
Massachusetts Institute of Technology, Cambridge, MA, USA, and
Richard E. Taylor of Stanford University, Stanford, CA,
USA, for their pioneering investigations concerning deep
inelastic scattering of electrons on protons and bound neutrons,
which have been of essential importance for the development of
the quark model in particle physics.
Professors Jerome I. Friedman and
Henry W. Kendall, both of the Massachusetts Institute of
Technology (MIT), and Richard E. Taylor, of the Stanford
Linear Accelerator Center (SLAC), share this year's Nobel Prize
in Physics. The value of the prize is 4 million Swedish crowns.
The three prizewinners were key persons in a research team which
in a series of investigations found clear signs that there exists
an inner structure in the protons and neutrons of the atomic
nucleus. What has become known as the "SLAC-MIT experiment" paved
the way for further investigations of the innermost structures of
matter. Ever since the beginning of this century, researchers
have studied the inner structure of atoms. Our knowledge has
increased successively, among other ways through the discovery
(around 1910-1930) of the nucleus of the atom and its nucleons.
During the 1950s there arrived on the scene a large number of
what were termed hadrons, whose properties resembled those of
nucleons. To reduce these to order, the concept of quarks was
introduced, at the beginning of the 1960s. Yet it was impossible
to see any traces of quarks in nature until the SLAG-MIT
experiment itself.
The discovery was made when protons and neutrons were illuminated
with beams from a giant "electron microscope" - a two-mile-long
accelerator at SLAC in California, USA. The inner structure was
interpreted to mean that quarks form the fundamental
building blocks of protons and neutrons. The electrically neutral
"glue" binding the quarks together is called gluons. All
matter on earth, including our human bodies, consists to more
than 99% of quarks with associated gluons. The little that
remains is electrons.
Background information
The prizewinners' contributions
The work now rewarded was carried out at the end of the 1960s and
the beginning of the 1970s by a group of researchers from MIT and
SLAC. The work was a continuation of earlier investigations in
which, using the electron as a probe, the structure of nucleons
(protons and neutrons) was studied. Unlike in earlier
investigations, electron beams of record-high energies were now
available. These beams were supplied by a two-mile-long linear
accelerator at SLAC, which afforded a "microscope" of higher
resolution than earlier. No new phenomenon was expected: the
experiment was fairly generally regarded as routine. Electron
scattering against nucleons, but at lower electron energies, had
been performed over two decades, and it was thought that enough
was known about the structure of nucleons - a view that proved to
be entirely false.
The essence of the SLAG-MIT experiments was to observe how a beam
of electrons at high velocities (energy from 4 GeV to 21 GeV) is
affected when it is led through a target consisting either of
liquid hydrogen or of deuterium. The scattered electrons were
recorded using two large magnetic spectrometers. One of these was
used for observing electrons scattered at 6 and 10 degrees, and
the other for greater scattering angles (18, 26 and 34 degrees).
As well as the scattering angle, the energy of the electrons was
measured with the spectrometers.
Collaboration between SLAC and MIT started at the beginning of
1967, with the study of so called elastic scattering against
protons (the process e + p - > e + p, in which the electron
bounces against the proton as if they were both rubber balls).
Similar experiments at lower electron energies had shown that the
nucleons behaved like "soft" structures which were only able to
scatter the electrons at small angles. The new results from
elastic scattering confirmed the earlier measurements. The
probability of obtaining a large scattering angle was found to be
very small. Following this conventional initial phase, it was
decided to have a look also at what was termed inelastic electron
scattering, e + p -> e + X, where X is not necessarily a
proton. Such processes were known from experiments at lower
energies, and nothing fundamentally new was to be expected.
However, the researchers found to their amazement that the
probability of deep inelastic scattering - where the incident
electron loses a large part of its original energy and emerges at
a large angle in relation to the original direction - was
considerably greater than expected. At first they believed the
result was incorrect or misinterpreted. One suspected source of
error was so-called radiation corrections - the incident or
departing electron could radiate away part of its energy in the
form of light, which they had not observed, and which could
therefore, they thought, have caused them to misinterpret what
had happened. But after careful work on the part of the research
group it gradually became clear that an inner nucleonic
structure, termed hard scattering centres, had been observed.
Here was a repetition, although at a deeper level, of one of the
most dramatic events in the history of physics, the discovery of
the nucleus of the atom.
History
At the beginning of our century Hans Geiger and Ernest Marsden
performed a series of experiments in which they measured the
scattering of alpha particles passing through a thin metal foil.
Geiger and Marsden (1909) found to their surprise that some of
the alpha particles were scattered at very large angles, such as
90°, to their original direction. The head in Manchester,
where Geiger and Marsden were working, was Ernest Rutherford, one
of the most eminent physicists of the time and winner of the
1908 Nobel Prize
in Chemistry. Rutherford undertook a systematic theoretical
investigation of Geiger's and Marsden's results and those of
similar experiments with beta particles (as electrons were called
at the time). In these, the no-less-amazing phenomenon had been
discovered that a small fraction of the incident electrons
boomeranged back after impact. Rutherford showed in a classic
paper (1911) that the observations made did not agree with the
current picture of the atom - a soft, jelly-like sphere in which
the positive and negative charges were diffusely distributed.
Such a soft target could at most produce a small deflection of
the incident particles. He also found that the probability of
many small deflections adding to achieve a large deflection was
vanishingly small. After careful comparison of the data with
theoretical expectations he concluded that "considering the
evidence as a whole, it seems simplest to suppose that the atom
contains a central charge distributed through a very small
volume, and that the large single deflections are due to the
central charge as a whole, and not to its constituents". Thus
the concept of atomic nucleus was born.
Knowledge of the structure of the nucleus of the atom increased
considerably when James Chadwick discovered the neutron in 1932.
In the same year, Werner Heisenberg realised that atomic nuclei
consist of protons and neutrons. Chadwick was rewarded with the
1935 Nobel Prize in Physics for
the discovery of the neutron and Heisenberg received the 1932 Physics Prize for "the creation of
quantum mechanics". The realization that the proton and the
neutron were building blocks of atomic nuclei represented a giant
step forwards in the systematisation of the design of matter.
Proton, neutron and electron became the three fundamental
building blocks of nature. But as early as 1933-1934 it was
suspected that the proton and the neutron were more complicated
particles than the electron. The nucleons exhibited unexpectedly
large magnetic fields ("anomalous magnetic moments") which could
be interpreted in such a way that they contained electric
currents. The magnetic properties of the nucleons were first
measured by Otto Stern and co-workers. Stern was rewarded with
the 1943 Nobel Prize in Physics
for "the molecular ray method and his discovery of the magnetic
moment of the proton. "
During the 1950s, the structure of nucleons was systematically
investigated using electron scattering. A series of interesting
phenomena were observed, among them that electrons with energies
up to 1 GeV saw nucleons as soft "spheres", implying that
electron scattering at large angles was very improbable.
Measurements were taken of how charge and magnetism are
distributed inside the nucleons. Robert Hofstadter played a
leading role in these investigations and was rewarded with a
1961 Nobel Prize in Physics for
his "pioneering studies of electron scattering in atomic nuclei
and for his thereby achieved discoveries concerning the structure
of the nucleons".
During the 1950s and 1960s the special position of the proton and
the neutron as nature's building blocks was questioned. A large
number of particles, termed hadrons, were being discovered at
this time, and showed similarities to the nucleons. It became a
matter of urgency to bring new order to physics so as to
understand how hadrons should be classified. After many attempts,
the riddle of the hadrons was successfully solved, mainly through
the work of Murry Gell-Mann (Nobel
Prize in Physics, 1969). The various hadrons were found to be
related and to behave as members of a kind of family
("representations of a symmetry-group"). This abstract
mathematical description became appreciably easier and more
comprehensible when three building blocks were introduced,
quarks. Now all the hadrons then known could be built up of these
three quarks and their antiparticles. Since this involves great
conceptual simplification, the quark concept was immediately
taken seriously. Quarks were sought both in nature, e.g. in sea
water, meteorites and cosmic rays, and in experiments using
high-energy accelerators. But no quarks were to be found. After a
time the most popular explanation of the absence of quarks was
that they were only "mathematical quantities" included in the
equations of physics.
The first traces of quarks
The SLAG-MIT experiments became the contemporary counterpart of
Geiger's and Marsden's experiment. At that time, the scattering
of alpha particles at large angles was explained by the existence
of a "hard grain" the atomic nucleus - in the middle of the atom.
In the modern version, Rutherford's role was assumed chiefly by
the theoreticians James D. Bjorken and the late Richard P.
Feynman (who received a Nobel Prize
in Physics in 1965). This time, the scattering of electrons
at large angles was explained by the existence of "hard grains" -
quarks - in the nucleons. But the results could not be fully
explained using quarks alone. The experiments indicated that
there were also electrically neutral components in the nucleons,
and there was great eagerness to discover their nature as well.
Development was rapid and the neutral components of the nucleons
were soon interpreted as gluons, the intermediaries of the strong
force. This introduced a new era in the history of physics.
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