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From Wikipedia, the free encyclopedia

Collision of 2 beams of gold atoms recorded by RHIC

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. Research in this area has produced a long list of particles.


Subatomic particles

An image showing 6 quarks, 6 leptons and the interacting particles, according to the Standard Model

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, made up 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 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, it is well known 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 17 species of elementary particles (12 fermions (24 if you count antiparticles separately), 4 vector bosons (5 if you count antiparticles separately), and 1 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 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.[1][2][3][4]


The idea that all matter is composed of elementary particles dates to at least the 6th 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. 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

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 photons. The model also contains 24 fundamental particles, which are the constituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which is yet to be discovered.

Experimental Laboratories

In particle physics, the major international laboratories are:

  • Brookhaven National Laboratory, located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Collider 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, 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 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 LEP, the Large Electron Positron collider, 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 was the highest energy particle collider in the world until the Large Hadron Collider surpassed it on 29 November, 2009.
  • 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-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 Category:Experimental particle physics for a partial list of the ideas required for such experiments.


Quantum field theory
Feynmann Diagram Gluon Radiation.svg
Feynman diagram
History of...

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".

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 [1]: hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).

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, 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, 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


Further reading

General readers

  • Frank Close (2004) Particle Physics: A Very Short Introduction. Oxford University Press. ISBN 0-19-280434-0.
  • --------, Michael Marten, and Christine Sutton (2002) The Particle Odyssey: A Journey to the Heart of the Matter. Oxford Univ. Press. ISBN 0-19-850486-1.
  • Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press.
  • Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume.
  • Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ. Press. ISBN 0-8018-7971-X.

Gentle texts

  • Frank Close (2006) The New Cosmic Onion. Taylor & Francis. ISBN 1-58488-798-2.
  • Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.


A survey article:

  • Robinson, Matthew B., Karen R. Bland, Gerald Cleaver, and J. R. Dittmann (2008) "A Simple Introduction to Particle Physics" - Part 1, 135pp. and Part 2, nnnpp. Baylor University Dept. of Physics.


  • Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4. 
  • Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. 
  • Perkins, Donald H. (1999). Introduction to High Energy Physics. Cambridge University Press. ISBN 0-521-62196-8. 
  • Povh, Bogdan (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-Verlag. ISBN 0-387-59439-6. 

External links

Study guide

Up to date as of January 14, 2010

From Wikiversity

At present, we understand that there are two categories of fundamental particles: bosons and fermions.


Elementary Particles


Fermions are things that we normally associate with matter. Examples of fermions are quarks, which combine to make up protons and neutrons, and electrons. There are other more exotics particles that are fermions.


Quarks are the basis of most elements in the present world, at the present day there are 6 known (non-anti-) quarks, these are:

  • Up
  • Down
  • Strange
  • Charm
  • Bottom
  • Top

Each of these have their own respective anti-quark; this adds up to a total of 12 quarks.


Leptons do not experience strong interaction such are that of a nuclear force. The leptons form a family of elementary particles that are distinct from the other known family of fermions: the quarks. There are 6 different Leptons in total, consisting of:

  • Electron
  • Muon
  • Tau lepton
  • Electron Neutrino
  • Muon neutrino
  • Tau neutrino

The leptons have Anti-Leptons just like that of the Quarks although their names are different, the Anti-Leptons consist of;

  • Positron
  • Muon (no change)
  • Tau Lepton (no change)
  • Electron antineutrino
  • Muon antineutrino
  • Tau antineutrino


Bosons are particles that mediate forces, such as light and gravity. Light is mediated by photons and gravity by gravitons. All Bosons have an integer spin, unlike Fermions. The Higg's Boson is a theoretical particle that has yet to be discovered. Scientists hope the Higg's Boson explain why some particles are heavy and others, like the photon, are massless.

Composite Particles


Hadrons are strongly interacting composite subatomic particles. They are classified into two seperate classes, which are:

  • Baryons
  • Mesons

Both of these consist of of quarks.


Baryons are strongly interacting Fermions, such as a Proton or a Neutron, both of which are made up 3 Quarks. Baryons have a Baryon No. of 1. The list of Baryons consist of:

  • Proton
  • Neutron
  • Delta
  • Lambda
  • Sigma
  • Xi
  • Omega


Mesons are strongly interacting Bosons which consist of a quark and an antiquark. Mesons have baryon number 0. They are classified according to their quark content and their angular momenta.

Neutral mesons can spontaneously change their quark content in a process known as mixing. This process was first discovered in the neutral kaon system in 1964 and has since been observed in the neutral B system, the Bs system and the neutral D system.

The ground state mesons are:

Name Symbol Quark content
Charged pion π + u\bar{d}
Neutral pion π0 \frac{1}{\sqrt{2}}\left(u\bar{d}-d\bar{u}\right)
Charged kaon K + u\bar{s}
Neutral kaon K0 d\bar{s}
Charged D meson D + c\bar{d}
Neutral D meson D0 c\bar{u}
Strange D meson D_s^+ c\bar{s}
Charged B meson B + u\bar{b}
Neutral B meson B0 d\bar{b}
Strange B meson B_s^0 s\bar{b}
Charmed B meson B_c^+ c\bar{b}

Mesons consisting of a quark and an antiquark of the same flavor do not have a unambiguous quark content. For example, the neutral pion is a superposition of flavor state u\bar{u} and d\bar{d}. Other neutral states of this kind are shown below:

Name Symbol Quark content
Phi φ s\bar{s}
Eta-c ηc c\bar{c}
Eta-b ηb b\bar{b}

The top quark decays too rapidly form bound mesonic states.

Simple English

Particle physics is a category of physics that studies really tiny particles. These particles are the really small pieces that build up the world around us.


Fundamental Forces

Particles carry fundamental forces. For example, the electromagnetic force is carried by photons.

Standard Model

One of the central concepts of particle physics is called the Standard Model. The Standard Model is a theory which tries to explain the fundamental forces. The Standard Model combined with General Relativity is currently the best explanation of how the Universe works.

The Standard Model is known to have problems. For example, there is nothing in the standard model that explains gravity. This is why General Relativity, a different theory, needs to be included in order for physicists to explain the Universe. There is a lot of work to improve the theory and/or find a better theory is being done. This work is often called theoretical particle physics, a category of physics. Theoretical particle physicists make theories that improve the Standard Model. One example of this is how there are many theories that predict undiscovered particles.


Physicists find out about particles by studying collisions between different particles. A good analogy of how physicists study particles through colliding is Car Crash example. Imagine a person wanted to look inside cars. By crashing two cars together at very high speeds, we can break the cars apart and see inside. In the same way, Physicists crash two particles together in order to break them and study the inside.

If particles moving at very high speeds, they will break apart when they collide. When they break, they create new smaller particles. These particles are very hard to find and detect because they decay (change into a lighter particle) very quickly. Modern particle physics involves colliding particles together very energetically to create new particles inside a particle accelerator. this it is called high-energy physics, due to the large amount of energy needed.


Particle physics can help us learn about the early Universe, because conditions that are similar to the early Universe (which was a much more energetic place than it is now) can be made in a small volume of space using the collisions of these particles. The biggest particle accelerator in the world is the Large Hadron Collider at CERN in Europe.


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