A major trend in the development of physics in the twentieth
century was the search for the ultimate structure of matter.
Having found the structures of atomic nuclei, physicists continued
to study those of nucleons (protons and neutrons) through
collision experiments. The diameter of a nucleon is approximately
 m(1)
. They therefore need to use particles with much higher energy
as "bullets", so that they can break up the nucleons.
In Rutherford's experiment, when low energy  particles
were collided with the atomic nucleus, they could only interact
with the whole nucleus as one particle. It was as if the -particles
were short-sighted and could not 'see' the nucleons inside
the atomic nucleus. Physicists had to raise their energy above
the binding energy(2)
of the nucleus before they could 'sense' the individual nucleons.
The higher the particle energy is, the better the spatial
resolution becomes. That's why physicists build high energy
particle accelerators (Fig. 1) and use them to probe the microscopic
world. These giant particle accelerators are really powerful
microscopes.
 
(Photo Courtesy Stanford Linear Accelerator
Center. www.slac.stanford.edu/
Fig. 1: Stanford Linear Accelerator. Left: Aerial view of
the accelerator. Right: Inside the accelerator tunnel.)
Between 1966 and 1978, Jerome Friedman, Henry Kendall and
Richard Taylor(3)
led a group of American physicists to carry out a series of
high energy electron-nucleon collision experiments. They used
the Stanford Linear Accelerator (shown in Fig. 1) to produce
a beam of electrons with energy of about 10 billion electron
volts(4) (or
GeV), and then they directed these electrons to bombard nucleons
(Fig. 2)!
(Fig. 2: The target area of the high
energy electron-nucleon collision experiment. The white box
represents a typical person¡¦s size. Photo courtesy Stanford
Linear Accelerator Center.)
How much energy is 10 GeV? If you could watch the motion
of air molecules at room temperature, you might be really
surprised, because they move randomly with an average speed
of several hundred meters per second. An enormously large
number of air molecules hit you every second, and that's why
you feel the atmospheric pressure(5).
However, the average kinetic energy(6)
of air molecules at room temperature is only about 1/40 eV.
Let's consider the particles at the surface of the sun, which
has a temperature of about 6,000K. Imagine touching the surface
of the sun with your hand. Feel good huh? The average kinetic
energy of the particles hitting your palm is a mere 1/2 eV.
That's too low in energy. How about we reach all the way to
the center of the sun? Before your hand vaporizes in the hot
furnace there, which has a temperature of about 15 million
K, you may feel particles hitting you with an average kinetic
energy of about 25,000 eV. We need to multiply this by 400,000
times further to reach the energy of 10 GeV!
What would happen when electrons with such a high energy
crash onto a nucleon? Many physicists at the time thought
that the nucleon could hardly deflect the electrons, and so
the electrons should just go straight through. Surprisingly,
there were always some electrons being scattered by a large
angle, just like what Rutherford saw in his experiment. When
the famous physicist Professor Richard Feynman saw the data,
he immediately concluded that there must be some small but
hard objects inside a nucleon! Further experiments showed
that these point-like particles have the same properties as
the hypothetical particles called quarks , the existence of
which was proposed by Professor Murray Gell-Mann(7)
and his student George Zweig in 1964. It is now commonly accepted
that nucleons are made up of quarks(8).
There are six kinds of quarks, called u (up), d (down), s
(strange), c (charm)(9)
, b (bottom), and t (top), and they differ in their masses
as well as their electric charges: u, c, and t quarks carry
a charge of +2/3 |e|, while d, s, and b quarks carry -1/3
|e|, with e being the charge of an electron. The various combinations
of these quarks made up different kinds of particles(10).
For example, a proton is made up of two u and one d quarks.
The reader must have noticed the remarkable similarities
between Rutherford's -atom
collision experiment and high energy electron-nucleon collisions,
in terms of methodology, experimental results, and even the
final discoveries. There was actually another even stronger
resemblance. Recall that Rutherford hypothesized the existence
of neutrons by noting that the atomic number of an atomic
nucleus only accounts for about half of its mass number. Well,
history has a way of repeating itself. Physicists soon realized
that the quarks that scatter with the electrons only account
for about half of the mass of a nucleon, and so naturally,
they deduced that another kind of particles, called gluons,
must also exist inside the nucleon. Just like the neutrons,
these gluons do not carry electrical charges, and so the electrons
cannot 'feel' their presence.
Strangely enough, neither the quarks nor the
gluons like to be seen. Nobody has yet observed a free quark
or a free gluon. It seems that the strong force(11)
among the quarks and gluons somehow confines them within the
nucleons. This confinement of quarks and gluons is still not
understood and remains an outstanding problem in modern physics.
You may be wondering - you should be - how come
we believe in the existence of quarks and gluons based on
only some indirect evidences? Indeed, physicists have been
working diligently to search for free quarks. One of the most
interesting series of experiments began recently in Brookhaven
National Laboratory, where high energy heavy-ion collisions(12)
are being carried out (Fig. 3). In principle, when the temperature
reaches a critical point, the quarks and gluons in matter
will be freed from the confinement. This is analogous to boiling
water; at the boiling point, water transforms from liquid
state to gas state(13),
and water molecules are "freed". According to theoretical
calculation, this critical temperature for quarks and gluons
is about 
K, or 100,000 times the temperature at the center of the sun!
Using a giant particle accelerator, physicists accelerate
the nuclei of some heavy elements such as Uranium - the so-called
heavy ions - to the energy of about 200 GeV per nucleon. Two
beams of these high energy heavy ions are then collided together.
During a collision, part of the kinetic energy of the heavy
ions will be converted into heat, creating a high density
microscopic(14)
fireball, with temperature reaching over
K (Fig. 4) Theoretically, this fireball is made up of free
quarks and gluons, and a new phase of matter - the quark-gluon
plasma -is formed. Unfortunately, because of the rapid expansion
of the fireball, the temperature drops quickly, and so the
quark-gluon plasma phase is maintained for only a very short
duration(15).
Physicists have to search for clues of the existence of the
quark-gluon plasma from the remnants of the fireball (Fig.
5), which consists of hundreds of thousands of various particles.
You should have noticed that this is nothing but another example
of the 'blast-it-up paradigm'.
These high energy heavy-ion collision experiments also represent
efforts to recreate the conditions in the early universe.
We simulate the Big Bang(16)
by the little bangs in the laboratory. Indeed, the extremely
high temperature of
K could only be reached during the Big Bang. With these little
bangs, physicists created the hottest spots in the universe
today, and through these fireballs they study the ultimate
structure of matter, as well as the conditions at the beginning
of the universe.
( Fig. 3: Relativistic Heavy-Ion Collider
(RHIC) in Brookhaven National Laboratory, U.S.A... Top: Aerial
view of the collider. Its circumference is about 2.4 miles.
Lower left: A section of the pipeline of the accelerator.
Right: One of particle detectors. Photos are downloaded from
http://www.bnl.gov. Photo
Courtesy Brookhaven National Laboratory.)
(Fig. 4: Schematic diagram of a high
energy heavy-ion collision.)
(Fig. 5: Remnants of heavy-ion collisions
¡V a large number of particles. Photo is downloaded from
www.bnl.gov, representing data obtained by the STAR experiment.
Photo Courtesy Brookhaven National Laboratory.)
| (1) |
Physicists define the length
of m
to be 1 fm (Fermi). |
| (2) |
The binding energy is the energy needed
to take away one nucleon from a nucleus. |
| (3) |
For these experiments, the three physicists
were awarded the 1990 Nobel Prize in Physics.
|
| (4) |
This is about 10,000 times the energy of
the ƒÑ particles used in Chadwick¡¦s experiment.
|
| (5) |
The force corresponding to the standard
atmospheric pressure at sea level is about the same as
having the weight of ten persons crushing on you.
|
| (6) |
The average kinetic energy of particles
in a gas is proportional to its temperature.
|
| (7) |
M. Gell-Mann, The Quark and the Jaguar
(W. H. Freeman and Co., New York, 1994).
|
| (8) |
He was awarded the Nobel Prize in Physics
in 1969 for his contribution to elementary particle physics.
|
| (9) |
The charm quark was discovered by Professor
Samuel Ting and Professor Burton Richter¡¦s research groups
independently. Richter and Ting shared the 1976 Nobel
Prize in Physics for this discovery.
|
| (10) |
Particles made up of quarks are called
hadrons. They all undergo strong interactions. |
| (11) |
One of the four fundamental forces in Nature
(strong, weak, gravity, and electromagnetic forces). It
governs the world of subatomic particles. |
| (12) |
See http://www.bnl.gov/rhic/ |
| (13) |
This is an example of a phase transition
in physics. |
| (14) |
The radius of this fireball is about the
same as that of a heavy ion, which is about several tens
of fm. |
| (15) |
Estimated to be  s. |
| (16) |
See www.phy.cuhk.edu.hk/astroworld
|
|
