A visit to the incredible particle zoo

In this dossier Particle quest - from atoms to quarks The elementary building blocks of nature Giant particle smashers More than just another 'theory of everything'

Discover the odd characteristics of a menagerie or sub atomic particles

Theorists think they understand how all the particles so far discovered are related, using a mathematical description called the Standard Model of particles and forces.
There are two kinds of matter particles: quarks and leptons.

Quarks
come in six "flavours" - up, down, strange, charm, bottom and top - and are never seen alone. Two ups and one down make a proton, two downs and one up make a neutron. All combinations are called hadrons, from the Greek for heavy.
A quark has an electric charge of plus or minus one-third or plus or minus two-thirds; so when two are combined they add up to plus one, minus one, or zero. This explains why protons have an electric charge of one, and neutrons have no electric charge at all.
Strange, charm and bottom quarks are heavier than up and down (quarks and exist fleetingly in the high-energy realms found in accelerators and cosmic ray events. They too are thought to have existed in the very first moments of the universe.
The top quark has yet to be found. It is by the far the heaviest and making it would require a very great amount of energy.

Proof at last! The blue tracks are an electron positron pair produced by a Z particle's decay How we know it's true Three theorists, Abdus Salam, Sheldon Glashow and Steven Weinberg, won the Nobel physics prize in 1979 for proposing that at sufficiently high energies the electromagnetic and weak forces dissolve into a single force - the so-called electroweak force. It was a momentous achievement, not only because it provided the theory that underlies the Standard Model, but also because it was the first step towards a unified description of nature. Electroweak theory predicted that the weak force is carried by two charged W particles and a neutral Z particle - the then-unknown "intermediate vector bosons" It also suggested values for their masses. With something to aim for, experimenters in Geneva converted CERN's largest accelerator so that it could make protons and antiprotons collide at incredibly high energies - equivalent to smashing two bags of oppositely charged quarks together. This event temporarily releases Wand Z bosons, which immediately decay into other particles. The experimenters looked for the pattern of this decay - and found it in 1983. It was thus that the Standard Model was verified.

Leptons
(from the Greek for light) include the electron, and the less familiar muon and tau (both of which are heavier, unstable versions of the electron). All leptons have an electric charge of plus or minus one.
For each of these three there is a neutrino partner. Neutrinos are bizarre entities: in fact they're hardly there at all, having no electric charge and little or no mass. But their role in the universe is extremely important (see the article on neutrinos in the May 1993 edition of Focus).

The Standard Model
All 12 leptons and quarks are split up into three "generations", each comprising a pair of quarks and a pair of leptons. In increasing order of mass, these are: the up and down quarks, and the electron and electron neutrino; the strange and charm quarks, and the muon and muon neutrino; the top and bottom quarks, and the tau and tau neutrino (the tau neutrino is yet to be found).
Scientists have identified four basic "forces of nature" that explain why these building blocks of matter stick together.

Electromagnetic force
This is the cement that holds atoms and molecules together and is responsible for their chemical behaviour. It is "felt" by all charged particles and can be either positive or negative; like charges repel, unlike charges attract.
In the tried and trusted theory of how electromagnetic force works - known as quantum electrodynamics - a "field" is created by charged particles exchanging "virtual photons ".

The strong force
A similar field theory explains what holds the central nucleus together. The strong nuclear force is felt only by quarks and has a very short range.
This force acts like a piece of stretched elastic, getting stronger with distance, which explains why quarks are never alone. The strong force comes in three "colours" - red, green and blue. (These are not real colours, just another physicists' tease.)
Colour is the "strong" analogue of electric charge and is carried by a particle called a gluon. Different colour gluons are attracted to each other and are thought to cluster together to form "glueballs", although no one has ever seen these.

Heroes of the particle revolution Sheldon Glashow (top),Steven Weinberg (middle) and Abdus Salam (left): their theory points to a unified theory

The weak force
This much weaker nuclear force allows quarks to change their "flavour". (So up quarks can become down quarks, for example.) This explains a kind of radioactivity in which a neutron becomes a proton, emitting an electron and a neutrino. The weak force is carried by three different particles: the W⁺, W^- and neutral Z^o. These particles differ from other force particle in that they have mass.

Gravity
Theorists think even gravity may be mediated by particles, dubbed gravitons. But in practice gravity is explained by general relativity and not by the weird quantum theory used in particle physics.

Matter Particles LEPTONS QUARKS All ordinary matter belongs to this group First Family Electron Responsible for elctricity and chemical reactions it has a charge of -1 Electron neutrino Particle with no electric charge,and possibly no mass; billions fly through your body every second Up Has an electric charge of plus two-thirds; protons have two, neutrons have one Down Has an electric charge of minus one-third; protons have one, neutrons have two These particles existed just after the Big Bang. Second Family Muon A heavier relative of the electron;it lives for two millionths of a second Muon neutrino Created along with muons when some particles decay Charm A heavier relative of the up; found in 1974 Strange A heavier relative of the down; found in 1964 Now they are only found in cosmic rays and accelerators Third Family Tau Heavier still; it is extremely unstable,it was discovered in 1975 Tau neutrino not yet discovered but believed to exist Top Heavier still; not yet discovered but believed to exist Bottom Heavier still; measuring bottom quarks is an important test of eletroweak theory Force Particles Gluons Carriers of the strong force between quarks Felt by: quarks Photons Particles that make up light; they carry the electromagnetic force Felt by: quarks an dcharged leptons These particles transmit the four fundamental forces of nature- although gravitons have so far not been discovered The explosive release of nuclear energy is the result of the strong force Electricity,magnetism and chemistry are the result of electromagnetic force Intermediate vector bosons Carriers of the weak force Felt by: quarks and charged leptons Gravitons Carriers of the gravity Felt by: all particles with mass Electricity,magnetism and chemistry are the result of electromagnetic force All the weight we experience is the result of the gravitational force

Mar94 p54