Chapter 36 Elementary particles - E

36.1 Basic forces
Fundamental forces
Particles interact by exerting forces on each other. There are four known types of force in the Universe
(see chart above). As electric and magnetic forces are closely related, they are regarded as different
varieties of one force, the electromagnetic.
Grand unified theories (GUTS) seek to link the strong, weak, and electromagnetic forces. Gravitational
force has yet to be linked with the others. It is insignificant on an atomic scale.
Classifying particles
Ordinary matter is made up of protons, neutrons, and electrons. However, in high-energy collisions, many
other particles can be created. Most are very short-lived.
Matter particles can be divided into two main groups:
(1) Hadrons (see below) These feel the strong force. They can be subdivided into
a. baryons (which include protons, neutrons, and heavier particles that these),
b. mesons (which are generally lighter than protons).
(2) Leptons (see below) These do not feel the strong force. They have no size and, in most cases, low or
no mass. There are three generations of leptons, but only the first (the electron and its neutrino) occurs in
ordinary matter.
• All leptons and most hadrons have corresponding antiparticles.
• The neutrino, v, produced by beta decay is the electronantineutrino ve.
Conservation laws
There are conservation laws for momentum and total energy. However, as mass and energy are
equivalent, the total energy must include the rest energy (see G6).
Particles have various quantum numbers assigned to them. These are needed to represent other
quantities which may be conserved during interactions.
For example:
Charge In any interaction, this is conserved: it balances on both sides of the equation (see G1 for
Lepton number This is +1 for a lepton, -1 for an antilepton, and 0 for any other particle.
For example, a ‘free’ neutron decays, after about 15 minutes, like this:
neutron -~ proton + electron + antineutrino (0) (0) (+1) (-1)
The numbers (in brackets) have the same total, 0, on both sides of the equation, so lepton number is
conserved. This applies in any type of interaction.
Baryon number This is +1 for a baryon, -1 for an antibaryon, and 0 for any other particle. It is conserved in
all interactions. Strangeness This is needed to account for the particular combinations of ‘strange
particles’ (certain hadrons) produced in some collisons. It is conserved in strong and electromagnetic
interactions, but not in all weak ones. Charm relates to the likelihood of certain hadron decays. Spin
relates to a particle’s angular momentum.
Topness and bottomness are further quantum numbers.
36.2 Quarks
The properties and quantum numbers of hadrons can be accounted for by assuming that each particle is
a combination of others, called quarks. These have a fractional charge of +3 e or -3 e. Symmetry theory
predicts that there should be three generations of quarks to match the three generations of leptons. (For
each type of quark, there is also a corresponding antiquark.)
• Ordinary matter contains only the first generation of quarks. Very high energies are needed to make
hadrons of other quark generations. These hadrons quickly decay into first generation particles.
• Individual quarks have never been detected.
All known hadrons can be constructed from different quark (and antiquark) combinations. Examples are
given above.
• Baryons are each made up of three quarks.
• Mesons are each made up of a quark and an antiquark. Kaon
The kaon K+ has a strangeness of +1. The kaon consists of an u quark and an antistrange particle
(charge + e, strangeness +1). Using the table, this gives the K+ the following quantum numbers: charge
strangeness +1
Force carriers
Like other particles, nucleons need not be in contact to exert forces on each other. To explain how the
strong force is ‘carried’ from one nucleon (e.g. neutron) to another, the idea of exchange particles is used:
Each nucleon is continually emitting and reabsorbing virtual pions, which surround it in a swarm. When
close, two nucleons may exchange a pion. The momentum transfer produces the effect of a force
(attractive or repulsive).
• The emitting nucleons lose no mass, so virtual pions are only allowed their brief existence by the
uncertainty principle. To create ‘real’ pions, the missing mass must be supplied by the energy of a
All the fundamental forces are believed to be carried by exchange particles. For example, electrons repel
each other by exchanging virtual photons. This process can be represented by a Feynman diagram as
below. For ‘real’ photons to exist, energy must be supplied.
The particles that carry the fundamental forces are known as gauge bosons. They are listed in the chart
•Quarks are bound together by gluons. As nucleons and pions are made of quarks, the gluon would seem
to be the basic force carrier for all strong interactions.
• The existence of the graviton is speculation only.
36.3 Neutrinos
Neutrinos are produced in radioactive decay which emit R-radiation. Neutrinos possess light speed, zero
rest mass, are uncharged and do not react with matter.
Neutron à Proton + Electron + Antineutrino
Proton à Neutron + Positron + Neutrino