particle physics - Subatomic particles, History, The Standard Model, Experiment, Theory, Reductionism, Public policy, The future
The material world is composed of atoms. Each atom in turn comprises a central nucleus surrounded by electrons, and the nucleus is composed of protons and neutrons. These protons, neutrons, the particles from which they are made, and other related objects are the entities studied in particle physics. Subatomic particles thought to be indivisible into smaller particles are known as fundamental particles: these are the matter particles (quarks, neutrinos, electrons, muons, and taus) and the force particles (gluons, photons, and W and Z bosons). The important forces acting between these particles are the electromagnetic, strong nuclear, and weak nuclear forces. Gravity is ignored.
By the late 1930s, protons, neutrons, and electrons were all known; nuclear fission had been observed; and the subject of nuclear physics was established. Particle physics explores the structure of matter at one level beneath nuclear physics. The earliest particle physics experiments involved measuring tracks left by cosmic rays in photographic emulsions. In this way muons (1937) and pions (1947) were discovered. Originally the pion was thought to be the fundamental carrier of nuclear force, in line with an early theory of nuclear force proposed by Hideki Yukawa in 1935. This view has now been superseded, although in low energy nuclear physics the force between protons and neutrons can be discussed in terms of mediation by pions.
During the 1950s, further cosmic ray studies and early accelerator experiments revealed other particles of various masses. Some exhibited unusual or ‘strange’ behaviour. Such particles were produced easily, suggesting they formed via strong interactions, but decayed slowly via weak interactions. A new quantum number called strangeness, conserved in strong but not in weak interactions, was invented to explain these results. Other similar quantum numbers having equally unlikely names have subsequently been introduced to account for observed particle interactions. During the 1950s and 1960s, many particles and resonances were discovered, including the antiproton (confirming that antiparticles exist) and the neutrino.
A scheme of particle classification based on symmetry, in which particles were labelled by quantum numbers such as isospin and strangeness, was introduced in 1961 by Murray Gell-Mann and Yuval Ne'eman. In 1964 Gell-Mann and US physicist George Zweig (1937– ) proposed quarks as abstract entities underlying symmetry patterns. However, experiments (1968) at the Stanford Linear Accelerator Center in the USA, which involved firing electrons at protons, suggested that objects within protons have the properties of quarks. Although no quarks have ever been observed directly, it is widely assumed they are the ultimate components of protons, neutrons, and most other subatomic particles.
The full theory of strong interaction, in which the strong force between quarks is carried by gluons, dates from 1973, and is called quantum chromodynamics. Weak interactions, governing radioactive beta decay, are understood in terms of the decay of individual quarks. For a neutron decaying to a proton (plus electron and antineutrino), a single u quark in the proton decays to a d quark plus electron and antineutrino. The weak force is carried by W and Z particles, and is well described by Glashow–Weinberg–Salam theory (1968). Purely electromagnetic interactions are described by quantum electrodynamics. Current research focuses on resolving difficulties in existing theories, constructing unified theories of strong, weak, and electromagnetic forces, and incorporating gravity to give a complete theory of the physical universe.
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.
Subatomic 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 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. 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 be described 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 scalars), which can combine to form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s. However, most particle physicists believe that it is an incomplete description of Nature, and that a more fundamental theory awaits discovery.
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 by philosophers and scientists.
History
The idea that all matter is composed of elementary particles dates to at least the 6th century BC. Although Isaac Newton in the 17th century thought that matter was made up of particles, it was John Dalton who formally stated in 1802 that everything is made from tiny atoms.
Dmitri Mendeleev's first periodic table in 1869 helped cement the view, prevalent throughout the 19th century, that matter was made of atoms.
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.
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
The current state of the classification of elementary particles is the Standard Model. The model also contains 24 fundamental particles, which are the constituents of matter.
Experiment
In particle physics, the major international collaborations are:
Brookhaven National Laboratory, located on Long Island, USA. Its main facility is the Tevatron, which collides protons and antiprotons and is presently the highest energy particle collider in the world.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.
Theory
Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments. There are several major efforts in theoretical particle physics today and each includes a range of different activities.
One of the major activities in theoretical particle physics is the attempt to better understand the standard model and its tests.
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).
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.
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).
Reductionism
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.
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.
Some within the scientific community believe that particle physics has also been adversely affected by the aging population. 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.
The future
Particle physicists internationally agree on the most important goals of particle physics research in the near and intermediate future.
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 2007 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.
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.
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