From: James R (Avatar) 23/11/2000 19:06:59 Subject: Tutorial: Particles and Forces post id: 175076 This tutorial aims to give a little background information about the particles and interactions which physicists consider to be fundamental. Fundamental Particles Our current understanding of matter at the smallest scales is encompassed by the Standard Model of fundamental particles and their interactions. In this model, there are three families of particles, each of which contains two leptons and two quarks. These particles are listed below. Leptons Quarks Flavor Mass (MeV/c2) Charge (e) Flavor Mass (MeV/c2) Charge (e) ne <0.001 0 u 3 +2/3 e- 0.511 -1 d 6 -1/3 nm <0.2 0 c 1300 +2/3 m- 106 -1 s 100 -1/3 nt- <2 0 b 4300 -1/3 t 178 -1 t 175000 +2/3 The first family consists of the electron neutrino, the electron, and the up and down quarks. The second family consists of the mu neutrino, the muon, and the charm and strange quarks. The final family consists of the tau neutrino, the tauon, and the top and bottom (or truth and beauty) quarks. The are good reasons to suspect that their are no further families waiting to be discovered. Every fundamental particle has an antiparticle, which has the same mass as the particle but the opposite sign charge (except for the antineutrinos, which have no charge). Thus, the table should technically be twice as big. The masses here are quoted in energy equivalent units. For example, an electron can be created from 511 kilo-electron volts of energy. We are not certain of the neutrino masses yet, so at the moment we can only place limits on their maximum possible masses. The charges here are quoted as multiples of the charge on an electron. [continued ...] From: James R (Avatar) 23/11/2000 19:10:15 Subject: re: Tutorial: Particles and Forces post id: 175080 The Particle Zoo All the particles we see around us are built from the fundamental particles listed above (and their antiparticles). Isolated quarks are never found in nature. They always come in either pairs or triplets, which are known collectively as hadrons. A meson is a hadron made up of a quark and an antiquark. A baryon is a hadron made up of three quarks or antiquarks. The following table lists a few examples: Mesons Baryons Quarks Particle Quarks Particle ud us ud bu p+ K- r+ B+ uud ddu uds ddd uss udc p n S0 D- X0 Lc+ The underlined letters above indicate an antiquark. Thus, an up quark and an anti-down quark combine to form a pi meson. Notice that the charges on the quarks add up when they combine to form a particle. For example, a proton (charge +e) consists of two up quarks (each +2/3 e) and one down quark (-1/3 e). [continued...] From: James R (Avatar) 23/11/2000 19:12:51 Subject: re: Tutorial: Particles and Forces post id: 175083 Fundamental Forces There are four fundamental forces which can act on particles. These are the Strong, Weak, Electromagnetic and Gravitational forces. Here is a summary table: Strong (fundamental) Strong (residual) Weak Electromagnetic Gravity Acts on Quarks Hadrons Quarks, Leptons Charged particles All particles Mediated by Gluons Mesons W, Z bosons Photons Gravitons Strength*(1) 25 N/A 0.8 1 10-41 Strength*(2) N/A 20 10-7 1 10-36 Range Quarks Nuclear distances Nuclear distances Infinite Infinite * The strengths given here are based on the force between (1) two up quarks separated by 10-18 metres, and (2) two protons in an atomic nucleus. The attraction between protons and neutrons in a nucleus is due to the residual effects of the strong force operating directly on quarks. The standard model describes forces between particles as being due to the exchange of virtual mediating particles, which are listed in the table. For example, when two electrons repel each other via the electromagnetic force, one electron emits a photon, which travels to the other electron, creating the repulsion. The fundamental forces of limited range have mediating particles with mass, whilst the forces with infinite range have massless mediating particles. [continued...] From: James R (Avatar) 23/11/2000 19:15:06 Subject: re: Tutorial: Particles and Forces post id: 175086 Building atoms Everything we commonly see around us is made of atoms. Atoms are built from the fundamental particles by the interplay of fundamental forces. Atoms consist of protons and neutrons in a small, compact nucleus, surrounded by clouds of electrons. The nucleus has a net positive electrical charge due to the protons (and, in turn, the quarks) comprising it. The negatively charged electrons are held in place by the electromagnetic force between themselves and the protons. Electrons are not affected by the strong nuclear force. The electromagnetic force in the nucleus tends to repel protons from each other, but this is dominated by the effects of the strong force, which binds the protons and neutrons together. In an atom, the gravitational force is negligible compared to the other forces, and the weak force is only important in some types of radioactive decay. Atoms combine together to make molecules, as well as other structures such as metals and crystal lattices. All of these are bonded together by the fundamental forces (mostly electromagnetic). Gravity Gravity is not fully included in the standard model yet. This is because the model is fundamentally a quantum mechanical model, and so far we do not have a quantum theory of gravity which works. Nobody has yet detected the graviton, the particle thought to mediate the gravity force. At the nuclear scale, gravity is very weak compared with the other fundamental forces. This is why it is so hard to detect. However, gravity becomes very important at larger scales (like the ones we're familiar with). Apart from electromagnetism, gravity is the only force which acts at infinite distance. Since many things in the macroscopic world have no net electrical charge, gravity is often the only force which affects them. Gravity is the force which keeps the planets in orbit around the sun. Where to from here? This has been a brief summary of the Standard Model. Whilst I have given a broad overview here, I have missed several things out, such as a discussion of spin, particle symmetries and quark colour. If you have any questions, please reply in the accompanying Questions thread (or start a new thread). JR This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. 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