Particle physics
Particle physics, also called high energy physics, is that branch of physics which studies the elementary constituents of matter and the interactions between them. In modern Quantum Mechanics, particles and field quanta are interchangeable - for instance electrons are just the quanta of an electron field, and the electromagnetic field is carried by particles called photons. The name particle is somewhat of a misnomer. They are actually subject to Wave-Particle duality: objects that appear to be particles in some circumstances can be interpreted as waves in others.
The current "Standard Model" of particle physics
The current state of the classification of elementary particles is called the "Standard Model". It holds that all everyday matter is composed of 4 different particles, and that there are two similar families of 4 particles of higher mass and short lifespan. The fundamental forces are thought to be transmitted by 4-5 other particles. It is unknown whether there are any more classes of particles. Before the Standard Model was established, a bewildering variety of particles had been described; this was sometimes called the "particle zoo".
Fundamental particles are subdivided into bosons and fermions, the former with integer spin (for example, 0, 1, 2) and the latter with half-integer spin (for example, 1/2, 1 1/2). Fermions are subject to the Pauli exclusion principle which states that no two particles can share the same quantum numbers--that is, two identical fermions can't be in the same position, momentum, energy, and angular momentum state at the same time, whereas bosons can. At extremely low energies, fermions pair up to form pseudo-bosons which can then share the same quantum numbers. This state of matter has been achieved in Bose-Einstein condensates, superfluids and superconductors.
The field quanta of the fundamental forces are all bosons:
- photons are responsible for generating the electromagnetic interaction
- The W+ and W- bosons together with the Z0 boson are responsible for generating the weak nuclear force (weak interaction).
- The 8 colorful gluons are responsible for the strong nuclear force (strong interaction). Six of these gluons come in color pairs of colors and anti-colors (for example, a gluon can carry red and anti-green) while the other two are a complicated mix of colors and anti-colors.
- The graviton is believed to be responsible for the gravitational interaction, No gravitons have thus far been discovered. They remain theoretical objects until some evidence of their existence is found.
- Various high mass bosons are predicted by unification models, notably the Higg bosons or the higgson.
The basic constituents of matter are fermions, including the well-known proton, neutron, and electron. Of these, though, only the electron is really elementary, the other two being aggregates of smaller particles held together by the strong interaction. What appear to be elementary fermions come in four basic varieties, each of which come in three generations with different masses, for a total of twelve different "flavors":
| Symbol | Electromagnetic charge | Weak charge* | Strong charge (color) | Mass | |
| Electron | e- | -1 | -1/2 | 0 | 0.511 MeV |
| Muon | μ- | -1 | -1/2 | 0 | 105.6 MeV |
| Tau | τ- | -1 | -1/2 | 0 | 1.784 GeV |
| Up quark | u | +2/3 | +1/2 | R/G/B | ~5 MeV |
| Charm quark | c | +2/3 | +1/2 | R/G/B | ~1.5 GeV |
| Top quark | t | +2/3 | +1/2 | R/G/B | > 30 GeV |
| Down quark | d | -1/3 | -1/2 | R/G/B | ~10 MeV |
| Strange quark | s | -1/3 | -1/2 | R/G/B | ~100 MeV |
| Bottom quark | b | -1/3 | -1/2 | R/G/B | ~4.7 GeV |
| Electron neutrino | Ve | 0 | +1/2 | 0 | < 50 eV |
| Muon neutrino | Vμ | 0 | +1/2 | 0 | < 0.5 MeV |
| Tau neutrino | Vτ | 0 | +1/2 | 0 | < 70 MeV |
* - particles on the table only have a weak charge when they have left-handed spin, and their antiparticles, right-handed.
These particles can be arranged in three "generations", the first one consisting of the electron, the up and down quarks, and the electron neutrino. All ordinary matter is made from first generation particles; the higher generation particles decay quickly into the first generation ones and can only be generated for a short time in high-energy experiments.
Colorless particles (leptons) occur free this is because their interactions (weak and electromagnetic) fall off rapidly with distance. On the other hand (and for somewhat complicated reasons) the strong force, between quarks, gets stronger with distance, and so colored particles (quarks) are always found in colorless combinations called hadrons. These are either fermionic baryons composed of three quarks (for example, protons and neutrons) or bosonic mesons composed of a quark-antiquark pair (for example, pions). The total mass of such aggregates exceeds that of the components thanks to the binding energy and in fact each comes in a series of energy states.
Experimental particle physics
In Particle Physics, the major international collaborations are:
- CERN, located on the French-Swiss border near Geneva. Its main facilities are LEP, the Large Electron Positron collider (now dismantled) and the LHC, or Large Hadron Collider (under construction).
- DESY, located in Hamburg, Germany. Its main facility is HERA, which collides electrons and protons.
- SLAC, located near Palo Alto, USA. Its main facility is PEP-II, which collides electrons and positrons.
- Fermilab], located near Chicago, USA. Its main facility is the Tevatron, which collides protons and antiprotons.
- Brookhaven National Lab], located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Collider, which collides heavy ions such as Au (it is the first heavy ion collider) and protons.
Many other particle accelerators exist.
See also:
External links:
- http://library.advanced.org/10380/: An interactive walkthrough of the atom]
- http://ParticleAdventure.org: A site with more detail, and news from particle physics]