- Particle physics
Particle physics is a branch of
physicsthat studies the elementary constituents of matterand 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. Research in this area has produced a long list of 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 scatteringprocesses, 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, "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 theorycalled 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 masshave 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. [cite web|url=http://pdg.lbl.gov/|title=Review of particle physics] [cite web|url=http://www.interactions.org/|title=Particle Physics News and Resources] [cite web|url=http://cerncourier.com|title=CERN Courier - International Journal of High-Energy Physics] [cite web|url=http://www.symmetrymagazine.org/cms/?pid=1000345|title=Particle physics in 60 seconds]
The idea that all
matteris composed of elementary particles dates to at least the 6th century BC. The philosophical doctrine of atomismand the nature of elementary particles were studied by ancient Greek philosophers such as Leucippus, Democritusand Epicurus; ancient Indian philosophers such as Kanada, Dignāgaand Dharmakirti; medieval scientists such as Alhazen, Avicennaand Algazel; and early modern European physicists such as Pierre Gassendi, Robert Boyleand Isaac Newton. The particle theory of lightwas also proposed by Alhazen, Avicenna, Gassendi and Newton. These early ideas were founded in abstract, philosophicalreasoning rather than experimentationand empirical observation.
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 physicsand quantum physicsculminated in proofs of nuclear fissionin 1939 by Lise Meitner(based on experiments by Otto Hahn), and nuclear fusionby Hans Bethein 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 Modelduring 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, SubatomicParticle|W boson- and SubatomicParticle|W boson+ 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 bosonknown 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 Colliderwhich collides heavy ions such as goldions 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 the Large Hadron Collider(LHC), which had its first beam circulation on 10 September 2008 and is the world's most energetic collider. Earlier facilities include LEP, the Large Electron Positroncollider, which was stopped in 2001 and then dismantled to give way for LHC; and SPS, or the Super Proton Synchrotron, which is being reused as a pre-accelerator for LHC.
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 experiments such as K2K, a neutrino oscillation experiment and Belle, an experiment measuring the CP-symmetryviolation in the B-meson.
SLAC, located near Palo Alto, USA. Its main facility is PEP-II, which collides electrons and positrons.
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.
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 modeland 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 theoryand effective field theory. Others make use of lattice field theoryand 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 problemand is constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgs mechanism, extra spatial dimensions (such as the Randall-Sundrummodels), Preontheory, 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 mechanicsand general relativityby building a theory based on small strings, and branesrather 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 [http://www.arxiv.org] : hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (
lattice gauge theory).
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.
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 matterand 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
neutrinomasses 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 Theoriesat energy scales much higher than collider experiments will be able to probe any time soon.
Beyond the Standard Model
Introduction to quantum mechanics
List of accelerators in particle physics
Standard model (basic details)
High pressure physics
* [http://www.arxiv.org/ ARXIV.ORG preprint server]
* [http://particleadventure.org/ The Particle Adventure] - educational project sponsored by the
Particle Data Groupof the Lawrence Berkeley National Laboratory(LBNL)
* [http://www.symmetrymagazine.org "symmetry" magazine]
*"Introduction to Particle Physics" by Matthew Nobes (published on
** [http://www.kuro5hin.org/story/2002/5/1/3712/31700 Part 1] , [http://www.kuro5hin.org/story/2002/5/14/19363/8142 Part 2] , [http://www.kuro5hin.org/story/2002/7/15/173318/784 Part 3a] , [http://www.kuro5hin.org/story/2002/8/21/195035/576 Part 3b]
* [http://www-spires.slac.stanford.edu/spires/hep/ SPIRES: High-Energy Physics Literature Database]
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