Everything about Particle Physics totally explained
Particle physics is a branch of
physics that studies the
elementary constituents of
matter and
radiation, and the interactions between them. It is also called
high energy physics, because many elementary particles don't occur under normal circumstances in
nature, but can be created and detected during energetic
collisions of other particles, as is done in
particle accelerators. Research in this area has produced a long
list of particles.
Subatomic particles
Modern particle physics research is focused on
subatomic particles, which have less structure than
atoms. These include atomic constituents such as
electrons,
protons, and
neutrons (protons and neutrons are actually composite particles, made up of
quarks), particles produced by
radiative and
scattering processes, such as
photons,
neutrinos, and
muons, as well as a wide range of
exotic particles.
Strictly speaking, the term
particle is a misnomer because the dynamics of particle physics are governed by
quantum mechanics. As such, they exhibit
wave-particle duality, displaying particle-like behavior under certain experimental conditions and
wave-like behavior in others (more technically they're described by
state vectors in a
Hilbert space; see
quantum field theory). Following the convention of particle physicists, "elementary particles" refer to objects such as
electrons and
photons, with the understanding that these "particles" display wave-like properties as well.
All the particles and their interactions observed to date can almost be described entirely by a
quantum field theory called the
Standard Model. The Standard Model has 40 species of elementary particles (24
fermions, 12
vector bosons, and 4
scalar bosons), which can combine to form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s. The Standard Model has been found to agree with almost all the experimental tests conducted to date. However, most particle physicists believe that it's an incomplete description of nature, and that a more fundamental theory awaits discovery. In recent years, measurements of
neutrino mass have provided the first experimental deviations from the Standard Model.
Particle physics has had a large impact on the philosophy of science. Some particle physicists adhere to
reductionism, a point of view that has been criticized and defended by philosophers and scientists. Part of the debate is described below.
History
The idea that all
matter is composed of elementary particles dates to at least the 6th century BC. The philosophical doctrine of
atomism was studied by
ancient Greek philosophers such as
Leucippus,
Democritus, and
Epicurus. In the 19
th century John Dalton, through his work on
stoichiometry, concluded that each element of nature was composed of a single, unique type of particle. Dalton and his contemporaries believed these were the fundamental particles of nature and thus named them atoms, after the Greek word
atomos, meaning "indivisible". However, near the end of the century, physicists discovered that atoms were not, in fact, the fundamental particles of nature, but conglomerates of even smaller particles. The early 20th century explorations of
nuclear physics and
quantum physics culminated in proofs of
nuclear fission in 1939 by
Lise Meitner (based on experiments by
Otto Hahn), and
nuclear fusion by
Hans Bethe in the same year. These discoveries gave rise to an active industry of generating one atom from another, even rendering possible (although not profitable) the transmutation of
lead into gold. They also led to the development of
nuclear weapons. Throughout the 1950s and 1960s, a bewildering variety of particles were found in scattering experiments. This was referred to as the "particle zoo". This term was deprecated after the formulation of the
Standard Model during the 1970s in which the large number of particles was explained as combinations of a (relatively) small number of fundamental particles..
The Standard Model
The current state of the classification of elementary particles is the
Standard Model. It describes the
strong,
weak, and
electromagnetic fundamental forces, using mediating
gauge bosons. The species of gauge bosons are the
gluons,
and and
Z bosons, and the
photons. The model also contains 24
fundamental particles, which are the constituents of
matter. Finally, it predicts the existence of a type of
boson known as the
Higgs boson, which has yet to be discovered.
Experiment
In particle physics, the major international laboratories are:
- Brookhaven National Laboratory, located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Collider
which collides heavy ions such as gold ions and polarized protons. It is the world's first heavy ion collider, and the world's only polarized proton collider.
- Budker Institute of Nuclear Physics (External Link) (Novosibirsk, Russia)]
- CERN, located on the French-Swiss border near Geneva. Its main project is now LHC, or the Large Hadron Collider, which is currently under construction. The LHC will be in operation in 2008 and will be the world's most energetic collider upon completion. Earlier facilities include LEP, the Large Electron Positron collider, which was stopped in 2001 and which is now dismantled to give way for LHC; and SPS, or the Super Proton Synchrotron.
- DESY, located in Hamburg, Germany. Its main facility is HERA, which collides electrons or positrons and protons.
- Fermilab, located near Chicago, USA. Its main facility is the Tevatron, which collides protons and antiprotons and is presently the highest energy particle collider in the world.
- KEK The High Energy Accelerator Research Organization of Japan located in Tsukuba, Japan. It is the home of a number of interesting experiments such as K2K, a neutrino oscillation experiment and Belle, an experiment measuring the CP-symmetry violation in the B-meson.
- SLAC, located near Palo Alto, USA. Its main facility is PEP-II, which collides electrons and positrons.
Many other
particle accelerators exist.
The techniques required to do modern experimental particle physics are quite varied and complex, constituting a subspecialty nearly completely distinct from the theoretical side of the field. See for a partial list of the ideas required for such experiments.
Theory
Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments. See also
theoretical physics. There are several major efforts in theoretical particle physics today and each includes a range of different activities. The efforts in each area are interrelated.
There are five most important states in particle theory: one of the major activities in theoretical particle physics is the attempt to better understand the
standard model and its tests. By extracting the parameters of the standard model from experiments with less uncertainty, this work probes the limits of the standard model and therefore expands our understanding of nature. These efforts are made challenging by the difficult nature of calculating many quantities in
quantum chromodynamics. Some theorists making these efforts refer to themselves as
phenomenologists and may use the tools of
quantum field theory and
effective field theory. Others make use of
lattice field theory and call themselves
lattice theorists.
Another major effort is in model building where
model builders develop ideas for what physics may lie beyond the standard model (at higher energies or smaller distances). This work is often motivated by the
hierarchy problem and is constrained by existing experimental data. It may involve work on
supersymmetry, alternatives to the
Higgs mechanism, extra spatial dimensions (such as the
Randall-Sundrum models),
Preon theory, combinations of these, or other ideas.
A third major effort in theoretical particle physics is
string theory.
String theorists attempt to construct a unified description of
quantum mechanics and
general relativity by building a theory based on small strings, and
branes rather than particles. If the theory is successful, it may be considered a "
Theory of Everything".
There are also other areas of work in theoretical particle physics ranging from particle cosmology to
loop quantum gravity.
This division of efforts in particle physics is reflected in the names of categories on the
preprint archive (External Link): hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (
lattice gauge theory).
Public policy
Experimental results in particle physics are often obtained using enormous
particle accelerators which are very expensive and require large amounts of government funding. Because of this, particle physics research involves issues of public policy.
The future
Particle physicists internationally agree on the most important goals of particle physics research in the near and intermediate future. The overarching goal, which is pursued in several distinct ways, is to find and understand what physics may lie beyond the
standard model. There are several powerful experimental reasons to expect new physics, including
dark matter and
neutrino mass. There are also theoretical hints that this new physics should be found at accessible energy scales. Most importantly, though, there may be unexpected and unpredicted surprises which will give us the most opportunity to learn about nature.
Much of the efforts to find this new physics are focused on new collider experiments. A (relatively) near term goal is the completion of the
Large Hadron Collider (LHC) in 2008 which will continue the search for the
Higgs boson,
supersymmetric particles, and other new physics. An intermediate goal is the construction of the
International Linear Collider (ILC) which will complement the LHC by allowing more precise measurements of the properties of newly found particles. A decision for the technology of the ILC has been taken in August 2004, but the site has still to be agreed upon.
Additionally, there are important non-collider experiments which also attempt to find and understand physics beyond the standard model. One important non-collider effort is the determination of the
neutrino masses since these masses may arise from neutrinos mixing with very heavy particles. In addition,
cosmological observations provide many useful constraints on the dark matter, although it may be impossible to determine the exact nature of the dark matter without the colliders. Finally, lower bounds on the very long
lifetime of the proton put constraints on
Grand Unification Theories at energy scales much higher than collider experiments will be able to probe any time soon.
Further Information
Get more info on 'Particle Physics'.
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