Particle physics

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Collision of 2 beams of gold atoms recorded by RHIC

Particle physics is a branch of physics that studies the elementary subatomic constituents of matter and radiation, and their interactions. The field is also called high energy physics, because many elementary particles do not occur under ambient conditions on Earth. They can only be created artificially during high energy collisions with other particles in particle accelerators.
Particle physics has evolved out of its parent field of nuclear physics and is typically still taught in close association with it. Scientific research in this area has produced a long list of particles.

Subatomic particles

Subatomic particles

The Standard Model of Physics.

Modern particle physics research is focused on subatomic particles, including atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles called baryons, made of quarks), particles produced by radioactive 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 from classical physics 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. In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. Following the convention of particle physicists, elementary particles refer to objects such as electrons and photons as it is well known that these types of particles display wave-like properties as well.

History

History

The idea that all matter is composed of elementary particles dates to at least the 6^th century BC. The philosophical doctrine of atomism and the nature of elementary particles were studied by ancient Greek philosophers such as Leucippus, Democritus and Epicurus; ancient Indian philosophers such as Kanada, Dignāga and Dharmakirti; medieval scientists such as Alhazen, Avicenna and Algazel; and early modern European physicists such as Pierre Gassendi, Robert Boyle and Isaac Newton. The particle theory of light was also proposed by Alhazen, Avicenna, Gassendi and Newton. These early ideas were founded in abstract, philosophical reasoning rather than experimentation and 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 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 (alchemy). 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.

Experimental laboratories

Experimental laboratories

In particle physics, the major international laboratories are:

• Brookhaven National Laboratory (Long Island, United States). Its main facility is the Relativistic Heavy Ion Collider (RHIC) 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, (Franco-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 now the world's most energetic collider of protons. It will also be the most energetic collider of heavy ions when it begins colliding lead ions in 2010. Earlier facilities include the Large Electron–Positron Collider (LEP), which was stopped in 2001 and then dismantled to give way for LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for LHC.
• DESY (Hamburg, Germany). Its main facility is the Hadron Elektron Ring Anlage (HERA), which collides electrons and positrons with protons.
• Fermilab, (Batavia, United States). Its main facility is the Tevatron, which collides protons and antiprotons and was the highest energy particle collider in the world until the Large Hadron Collider surpassed it on 29 November 2009.
• KEK, (Tsukuba, Japan). It is the home of a number of experiments such as the K2K experiment, a neutrino oscillation experiment and Belle, an experiment measuring the CP violation of B mesons.
• SLAC National Accelerator Laboratory (Menlo Park, United States). 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 sub-specialty nearly completely distinct from the theoretical side of the field.

Theory

Theory

Quantum field theory

(Feynman diagram)

History of... [show]Background [show]Symmetries [show]Tools [show]Equations [show]Standard Model [show]Incomplete theories [show]Scientists

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 interrelated efforts in theoretical particle physics today. One important branch attempts 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's building blocks. These efforts are made challenging by the difficulty of calculating quantities in quantum chromodynamics. Some theorists working in this area 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".

The future

The future

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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. The Large Hadron Collider (LHC) was completed in 2008 to help 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. In August 2004, a decision for the technology of the ILC was taken 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 Unified Theories at energy scales much higher than collider experiments will be able to probe any time soon.