Particle theory is a field studying fundamental particles in
the universe and their interactions. It is physics at the
shortest distance scale from which one can derive all physical
laws in principle. The quest to know the ingredients of the
universe started from as early as Greek days, but it is fair
to say that modern particle physics started in the 20th
century. If one ventures into this field now in the hope of unveil
its inner workings, possession of all the knowledge in the world
might not be enough. One's imagination might be an excellent guide.
Nature exhibits great surprises, many of them never dreamed of
before, as the old ether idea was blown up by the Michelson-Morley
experiment.
In the first half of the 20th century, we discovered
a fundamental particle 'muon'. Of course, the lighter particle
'electron' was known since late 19th century. Thirties were
the period setting ground stones in particle physics. Then
it was also known that there exists a difference between
proton and neutron even though both are strongly bound in
nucleons. Yukawa invented 'meson' to represent the strong
interactions in nuclei. It is now called 'pi' meson.
In thirties, we had the basic rules for the current day
quantum field theory, and the correct theory QED. During World
War II, a Japanese and two Americans showed how to calculate
physical quantities in QED under the name of renormalization.
But particle physics really started from the late fifties when
people began to discover a lot of strongly interacting
particles which we call hadrons (mesons for bosonic hadrons
and baryons for fermionic hadrons). Also during this
period, the parity symmetry is known to be broken in
weak interactions. When particle physics started, nuclear
physics the forerunner until that time left behind for studyng
just the physical phenomena of nuclei and mesons.
Sixties were the period of confusion in particle physics, but
the most important concept 'symmetry' was born.
Because of the profusion of hadrons, the old
idea of S-matrix model dominated in that
period. But a small group of people sticked to quantum field
theory with extended symmetries and its
violation through spontaneous symmetry breaking mechanism.
The standard model (SM), which we believe today that it describes
physical phenomena correctly down to 10^{-16} cm scale,
was discovered during the late sixties by this group of people.
Current day particle theory starts with the SM. Particle theory is the
field studying the most fundamental particles in the universe and
their interactions theoretically. Namely, in particle physics
category those who do not perform experiments are called particle
theorists. Because of not doing experiments, their method of study is
only limited to theory. But they have the tendency of not considering
any theory which is far from possible description of physical
phenomena. Students who are more interested in logic rather than
physical world are advised to study mathematics, since physics is
an experimental science, meaning any physical law must be proven by
experiments.
The SM is a kind of gauge theory, quantum field theory obeying the
gauge symmetry, with the gauge group SU(3)xSU(2)xU(1). The first two
factor groups are called nonabelian and the last is called abelian.
The SM gauge group defines interactions and reveals force
mediating particles: gluons, W, Z, and photon. These force mediating
particles are spin-1 bosons. On the other hand, the building blocks
of the universe in the SM are spin-1/2 fermions which come in three
families. The 'electron'-family, the 'muon'-family,
and the 'tau'-family. The SM also needs a spin-0 boson toward
spontaneous symmetry breaking, which is one of the unsettled
problems under the name of Higgs search, technicolor and
supersymmetry.
The SM has four particles in one family.
One family consists of two leptons ('electron' and 'electron-type
neutrino') and two quarks ('up' and 'down' quarks).
Quarks possess the strong interaction property, the original
source making nucleons compact by strong binding force.
Leptons do not possess the strong interaction
property. In total there are twelve
fermions in the SM. Out of these, two ('electron' and 'muon') were
discovered before 1950. The 'electron-type neutrino' was discovered
in 1956 and the 'muon-type neutrino' in 1961. We can attribute
the discovery of three light quarks to sixties. Thus, before 1970
we have known seven fermions out of twelve. One can say that four
('charm', 'bottom', 'tau', and 'tau-type neutrino') out of the
remaining five were discovered in seventies. The last one, the
heaviest 'top' quark, waited a long time (seventeen years after 'bottom'
quark) to be finally discovered in the middle of nineties.
By now, we found all fermions in the SM. But the spin-0 boson,
the Higgs particle, is to be discovered.
If the SM is the final story, then there is not much left
to reasearch in particle
theory. But the SM houses a lot of unsolved problems which are awaited
to be understood by future particle theorists. Some of them are:
Why are there three families? Why is 'electron' so light compared
to the 'top' quark the heaviest particle in the 'tau'-family?
Why is W boson mass so small compared to the scale defining the
gravitational interaction? Is it possible to construct a composite
structure of quarks? Is it possible to make quarks in terms
of extented objects like strings? Of course, there are many
standard workable problems as in any research field.
One promising theme in particle theory is "Beyond the standard model",
in which any prospective theory including the SM is considered. The simpest
among these is the neutrino oscillation. The next generalization is
the inclusion of the very light axion. But these do not change the SM
very much. One radical idea comes from supersymmetry. The N=1
supersymmetry requires a superpartner to any known particle in the SM.
With the discovery of any superpartner, we will move into a new
era of particle theory, probably leading to a superstring
standard model.
The universe we live in evolves according to the fundamental laws of
nature. In the very early universe, the fundamental laws applicable
to the universe are the equations of motion with (matter)
elementary particles considered in particle
theory. In some cases, the universe creates very massive particles
very easily compared to the difficulty of making them at telestrial
accelerators. Therefore, both cosmology (studying the very early Universe)
and particle physics (dealing with the fundamental laws and
elementary particles in nature) have many common interests and a new
field called particle cosmology or cosmo-particle physics has emerged.
Nowadays,
a new direction in particle theory is always reflected in the
cosmological mirror to see whether it is feasible. Some examples are
the inflationary scenario in the early Universe, dark energy of 73%, and
cold dark matter of 23%. In particular, identifying cold dark matter
is of the prime importance now.
Sometimes the methods developed toward the study of particle
physics were the key ingredients toward new civilization. We know
that the atomic bomb and peaceful use of nuclear energy were the
byproducts in the study of fundamental interactions. In this
information age, your life must be fun with internet,
but remember that it was started by particle physicists.
The reason I believe that particle physics introduce so many new
ideas is that it is a very competitive field where you can survive only
by having new ideas. Because of their good preparation in general
science and the idea oriented education, some
of them trained in particle theory work very successfully even at
Wall Street. Think about this challenge also!!
One can glimpse this field by looking at the names, leading
to the current day particle theory: Galileo Galilei, Issac Newton,
James Maxwell, Max Planck, Albert Einstein, Niels Bohr, Sir
Rutherford, Werner Heisenberg, Erwin Schroedinger,
H. Goudsmidt, Uhlenbeck, Wolfgang Pauli, Paul Dirac,
Hideki Yukawa, Enrico Fermi, Richard Feynman, Julian Schwinger,
Shinichiro Tomonaga, T. D. Lee and C. N. Yang,
Murray Gell-Mann, M. Y. Han and Yoichiro Nambu, Sheldon Glashow,
Abdus Salam, Steven Weinberg, Gerard 't Hooft and Tiny Veltman,
M. Kobayashi and T. Maskawa, H. D. Politzer,
David Gross and Frank Wilczek, Howard Georgi,
and Edward Witten, and many more. With the discovery of
superparticles and axions, many more names would join in the
discovery of particles and interactions. As these names show,
the pure theoretical activity in particle physics has been
the leading scientific mind in the science history.
Someone like you, sometime, somewhere, may fill in
these unfinished list of names, or even rewrite a whole
new idea. This is the field that particle theory aims.