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(Redirected from Particle theory)
'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 began in the early 20th century as an exploration into the structure of the atom. The discovery of the atomic nucleus in the gold foil experiment of Geiger, Marsden, and Rutherford was the foundation of the field. The components of the nucleus were subsequently discovered in 1919 (the proton) and 1932 (the neutron). In the 1920s the field of quantum physics was developed to explain the structure of the atom.
The binding of the nucleus could not be understood by the physical laws known at the time. Based on electomagnetism alone, one would expect the protons to repel each other. In the mid-1930s, Yukawa proposed a new force to hold the nucleus together, which would eventually become known as the strong nuclear force. He speculated that this force was mediated by a new particle called a meson. Also in the 1930s, Fermi postulated the neutrino as an explanation for the observed energy spectrum of beta decay, and proposed an effective theory of the weak force. Separately, the positron and the muon were discovered by Anderson. Yukawa's meson was discovered in the form of the pion in 1947. Over time, the focus of the field shifted from understanding the nucleus to the more fundamental particles and their interactions, and particle physics became a distinct field from nuclear physics.
Throughout the 1950s and 1960s, a bewildering variety of additional 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.
Main articles: 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 photon, respectively. The model also contains 24 fundamental particles (12 particle/anti-particle pairs), 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.
The major laboratories carrying research in the field of particle physics, listed in alphabetical order, are:
★ Brookhaven National Laboratory, located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Colliderwhich collides heavy ions such as gold ions (it is the first heavy ion collider) and protons.
★ Budker Institute of Nuclear Physics [1] (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.
'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 division of efforts in particle physics is reflected in the names of categories on the preprint archive [2]: hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).
Throughout the development of particle physics, there have been some 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. Approached in several distinct ways, the overarching goal 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. Theoretical hints also imply 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.[1]
New collider experiments are a central part of the effort to discover physics beyond the standard model. A near term goal is the completion of the Large Hadron Collider 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 on the accelerator technology of the ILC was made in August 2004 . A decision has not been made concerning the site for the ILC.[2]
Additionally, there are important non-collider experiments that 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
★ Beyond the Standard Model
★ Introduction to quantum mechanics
★ Fundamental particle
★ List of particles
★ List of accelerators in particle physics
★ Standard model (basic details)
★ Subatomic particle
★ Timeline of particle physics
★ High pressure physics
★ Rochester conference
1. Executive summary of the report ''Quantum Universe''
2. Executive summary of the report ''Discovering the Quantum Universe''
★ Particle Physics News and Resources
★ ARXIV.ORG preprint server
★ The Particle Adventure - educational project sponsored by the Particle Data Group of the Lawrence Berkeley National Laboratory (LBNL)
★ Hands-on-Cern - Award-winning website with introduction to particle physics
★ ''symmetry'' magazine
★ ''Introduction to Particle Physics'' by Matthew Nobes (published on Kuro5hin):
★
★ Part 1
★
★ Part 2
★
★ Part 3a
★
★ Part 3b
★ SPIRES: High-Energy Physics Literature Database
★ CERN Courier - International Journal of High-Energy Physics
★ Particle physics in 60 seconds
Thousands of particles explode from the collision point of two 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 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.
| Contents |
| History |
| The Standard Model |
| Experiment |
| Theory |
| Reductionism |
| Public policy |
| The future |
| See also |
| References |
| External links |
History
Modern particle physics began in the early 20th century as an exploration into the structure of the atom. The discovery of the atomic nucleus in the gold foil experiment of Geiger, Marsden, and Rutherford was the foundation of the field. The components of the nucleus were subsequently discovered in 1919 (the proton) and 1932 (the neutron). In the 1920s the field of quantum physics was developed to explain the structure of the atom.
The binding of the nucleus could not be understood by the physical laws known at the time. Based on electomagnetism alone, one would expect the protons to repel each other. In the mid-1930s, Yukawa proposed a new force to hold the nucleus together, which would eventually become known as the strong nuclear force. He speculated that this force was mediated by a new particle called a meson. Also in the 1930s, Fermi postulated the neutrino as an explanation for the observed energy spectrum of beta decay, and proposed an effective theory of the weak force. Separately, the positron and the muon were discovered by Anderson. Yukawa's meson was discovered in the form of the pion in 1947. Over time, the focus of the field shifted from understanding the nucleus to the more fundamental particles and their interactions, and particle physics became a distinct field from nuclear physics.
Throughout the 1950s and 1960s, a bewildering variety of additional 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 articles: 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 photon, respectively. The model also contains 24 fundamental particles (12 particle/anti-particle pairs), 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
The major laboratories carrying research in the field of particle physics, listed in alphabetical order, are:
★ Brookhaven National Laboratory, located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Colliderwhich collides heavy ions such as gold ions (it is the first heavy ion collider) and protons.
★ Budker Institute of Nuclear Physics [1] (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.
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 [2]: hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).
Reductionism
Throughout the development of particle physics, there have been some 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.
Public policy
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.
The future
Particle physicists internationally agree on the most important goals of particle physics research in the near and intermediate future. Approached in several distinct ways, the overarching goal 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. Theoretical hints also imply 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.[1]
New collider experiments are a central part of the effort to discover physics beyond the standard model. A near term goal is the completion of the Large Hadron Collider 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 on the accelerator technology of the ILC was made in August 2004 . A decision has not been made concerning the site for the ILC.[2]
Additionally, there are important non-collider experiments that 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.
See also
★ Atomic physics
★ Beyond the Standard Model
★ Introduction to quantum mechanics
★ Fundamental particle
★ List of particles
★ List of accelerators in particle physics
★ Standard model (basic details)
★ Subatomic particle
★ Timeline of particle physics
★ High pressure physics
★ Rochester conference
References
1. Executive summary of the report ''Quantum Universe''
2. Executive summary of the report ''Discovering the Quantum Universe''
External links
★ Particle Physics News and Resources
★ ARXIV.ORG preprint server
★ The Particle Adventure - educational project sponsored by the Particle Data Group of the Lawrence Berkeley National Laboratory (LBNL)
★ Hands-on-Cern - Award-winning website with introduction to particle physics
★ ''symmetry'' magazine
★ ''Introduction to Particle Physics'' by Matthew Nobes (published on Kuro5hin):
★
★ Part 1
★
★ Part 2
★
★ Part 3a
★
★ Part 3b
★ SPIRES: High-Energy Physics Literature Database
★ CERN Courier - International Journal of High-Energy Physics
★ Particle physics in 60 seconds
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