Particle physics 23259 224095971 2008-07-07T08:08:49Z Vishnava 6847159 Reverted edits by [[Special:Contributions/76.103.206.11|76.103.206.11]] to last version by LX (using [[WP:HG|Huggle]]) {{Morefootnotes|date=July 2008}} [[Image:First Gold Beam-Beam Collision Events at RHIC at 100 100 GeV c per beam recorded by STAR.jpg|thumb|right|400px|Thousands of particles explode from the collision point of two [[relativistic particle|relativistic]] (100 [[GeV]] per nucleon) [[gold]] ions in the [[STAR detector]] of the [[Relativistic Heavy Ion Collider]]. Electrically charged particles are discernible by the curves they trace in the detector's magnetic field.]] '''Particle physics''' is a branch of [[physics]] that studies the [[elementary particle|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 [[collision]]s of other particles, as is done in [[particle accelerator]]s. Research in this area has produced a long [[list of particles]]. ==Subatomic Particles== Modern particle physics research is focused on [[subatomic particle]]s, which have less structure than [[atom]]s. These include atomic constituents such as [[electron]]s, [[proton]]s, and [[neutron]]s (protons and neutrons are actually composite particles, made up of [[quark]]s), particles produced by [[radiative process|radiative]] and [[scattering]] processes, such as [[photon]]s, [[neutrino]]s, and [[muon]]s, 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 [[mathematical formulation of quantum mechanics|state vectors]] in a [[Hilbert space]]; see [[quantum field theory]]). Following the convention of particle physicists, "elementary particles" refer to objects such as [[electron]]s and [[photon]]s, 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 [[fermion]]s, 12 [[vector boson]]s, and 4 [[scalar boson]]s), 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.<ref>{{cite web|url=http://pdg.lbl.gov/|title=Review of particle physics}}</ref><ref>{{cite web|url=http://www.interactions.org/|title=Particle Physics News and Resources}}</ref><ref>{{cite web|url=http://cerncourier.com|title=CERN Courier - International Journal of High-Energy Physics}}</ref><ref>{{cite web|url=http://www.symmetrymagazine.org/cms/?pid=1000345|title=Particle physics in 60 seconds}}</ref> ==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 [[Hellenic civilization|ancient Greek]] [[philosopher]]s such as [[Leucippus]], [[Democritus]], and [[Epicurus]]. In the 19<sup>th</sup> 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 [[chrysopoeia|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== {{main|Standard Model}} The current state of the classification of elementary particles is the [[Standard Model]]. It describes the [[strong nuclear force|strong]], [[weak nuclear force|weak]], and [[electromagnetism|electromagnetic]] [[fundamental force]]s, using mediating [[gauge boson]]s. The species of gauge bosons are the [[gluon]]s, [[W boson|{{SubatomicParticle|W boson-}} and {{SubatomicParticle|W boson+}}]] and [[Z boson]]s, and the [[photons]]. The model also contains 24 [[fundamental particle]]s, 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 [http://www.bnl.gov/RHIC/ Relativistic Heavy Ion Collider] which collides [[heavy ion]]s such as [[gold]] ions and polarized [[proton]]s. It is the world's first heavy ion collider, and the world's only polarized proton collider. *[[Budker Institute of Nuclear Physics]] [http://www.inp.nsk.su] ([[Novosibirsk]], [[Russia]])] *[[CERN]], located on the French-Swiss border near [[Geneva]]. Its main project is now [[Large Hadron Collider|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 [[Hadron Elektron Ring Anlage|HERA]], which collides [[electron]]s or [[positron]]s and [[proton]]s. *[[Fermilab]], located near Chicago, USA. Its main facility is the [[Tevatron]], which collides [[proton]]s and [[proton|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 [http://neutrino.kek.jp/ K2K], a neutrino oscillation experiment and [http://belle.kek.jp/ 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 [[electron]]s and [[positron]]s. Many other [[particle accelerator]]s 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. ==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 '''[[Particle physics phenomenology|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 building (particle physics)|'''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 [[Arxiv|preprint archive]] [http://www.arxiv.org]: 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 accelerator]]s 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]], [[SUSY|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, [[physical cosmology|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 [[proton decay|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. ==See also== *[[Atomic physics]] *[[Beyond the Standard Model]] *[[Introduction to quantum mechanics]] *[[Fundamental particle]] *[[List of accelerators in particle physics]] *[[Standard model (basic details)]] *[[Subatomic particle]] *[[High pressure physics]] *[[ICHEP|Rochester conference]] ==Notes== {{reflist|2}} ==External links== *[http://www.arxiv.org/ ARXIV.ORG preprint server] *[http://particleadventure.org/ The Particle Adventure] - educational project sponsored by the [[Particle Data Group]] of the [[Lawrence Berkeley National Laboratory]] (LBNL) *[http://www.symmetrymagazine.org ''symmetry'' magazine] *''Introduction to Particle Physics'' by Matthew Nobes (published on [[Kuro5hin]]): **[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] {{Physics-footer}} <!-- [[Category:Physics]] redundant supercat --> [[Category:Particle physics|Particle physics]] {{Link FA|sl}} [[ar:فيزياء الجسيمات]] [[bn:কণা পদার্থবিজ্ঞান]] [[bs:Fizika elementarnih čestica]] [[bg:Физика на елементарните частици]] [[ca:Física de partícules]] [[cs:Fyzika částic]] [[da:Partikelfysik]] [[de:Teilchenphysik]] [[el:Σωματιδιακή Φυσική]] [[es:Física de partículas]] [[eo:Partikla fiziko]] [[eu:Partikulen fisika]] [[fa:فیزیک ذرات]] [[fr:Physique des particules]] [[gl:Física de Partículas]] [[ko:입자물리학]] [[hr:Fizika elementarnih čestica]] [[id:Fisika partikel]] [[ia:Physica de particulas]] [[is:Öreindafræði]] [[it:Fisica delle particelle]] [[he:פיזיקת חלקיקים]] [[ka:ელემენტარული ნაწილაკების ფიზიკა]] [[ku:Fîzîka keriyan]] [[la:Physica particularum elementariarum]] [[lt:Dalelių fizika]] [[hu:Részecskefizika]] [[ml:കണികാഭൗതികം]] [[ms:Fizik zarah]] [[nl:Deeltjesfysica]] [[ja:素粒子物理学]] [[no:Partikkelfysikk]] [[pl:Fizyka cząstek elementarnych]] [[pt:Física de partículas]] [[ro:Fizica particulelor]] [[ru:Физика элементарных частиц]] [[simple:Particle physics]] [[sk:Fyzika častíc]] [[sl:Fizika osnovnih delcev]] [[fi:Hiukkasfysiikka]] [[sv:Partikelfysik]] [[th:ฟิสิกส์พลังงานสูง]] [[vi:Vật lý hạt]] [[tr:Parçacık fiziği]] [[uk:Фізика елементарних частинок]] [[ur:ذراتی طبیعیات]] [[zh:粒子物理學]]