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 do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.
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 are described by state vectors in a Hilbert space; see quantum field theory). Following the convention of particle physicists, we will use "elementary particles" to 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 is 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.
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 19th 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 was 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
Main article: 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, W- and W+ and Z bosons, and the photons, respectively. 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.
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  (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 Category:Experimental particle physics for a partial list of the ideas required for such experiments.
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.
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 divide of efforts in particle physics is reflected in the names of categories on the preprint archive : hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).
Throughout the development of particle physics, there have been many objections to the extreme reductionist (or greedy reductionist) approach of attempting to explain everything in terms of elementary particles and their interaction. These objections have been raised by people from a wide array of fields, including many modern particle physicists, solid state physicists, chemists, biologists, and metaphysical holists. While the Standard Model itself is not challenged, it is contended that the properties of elementary particles are no more (or less) fundamental than the emergent properties of atoms and molecules, and especially statistically large ensembles of those. Some critics of reductionism claim that even a complete knowledge of the underlying elementary particles will not lend a thorough understanding of more complicated natural processes, while others doubt that a complete knowledge of particle behavior (as part of a larger process) could even be attained, thanks to quantum indeterminacy.
Reductionists typically claim that all progress in the sciences has involved reductionism to some extent. 
Experimental results in particle physics are often obtained using enormous particle accelerators which are very expensive (typically several billion US dollars) and require large amounts of government funding. Because of this, particle physics research involves issues of public policy.
Many have argued that the potential advances do not justify the money spent, and that in fact particle physics takes money away from more important research and education efforts. In 1993, the US Congress stopped the Superconducting Super Collider (SSC) because of similar concerns, after US$2 billion had already been spent on its construction. Many scientists, both supporters and opponents of the SSC, believe that the decision to stop construction of the SSC was due in part to the end of the Cold War which removed scientific competition with the Soviet Union as a rationale for spending large amounts of money on the SSC.
Some within the scientific community believe that particle physics has also been adversely affected by the aging population. The belief is that the aging population is much more concerned with immediate issues of their health and their parents' health and that this has driven scientific funding away from physics toward the biological and health sciences. In addition, many opponents question the ability of any single country to support the expense of particle physics results and fault the SSC for not seeking greater international funding.
Proponents of particle accelerators hold that the investigation of the most basic theories deserves adequate funding, and that this funding benefits other fields of science in various ways. They point out that all accelerators today are international projects and question the claim that money not spent on accelerators would then necessarily be used for other scientific or educational purposes.
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.
* Atomic physics
* ARXIV.ORG preprint server
* CERN Courier - International Journal of High-Energy Physics