All particles have an antiparticle.

Prior to the development of quantum theory it was believed that all matter was made of three fundamental particles: electron, proton, and neutron.  More recent developments in this theory predict the existence of other subatomic particles, some with very peculiar properties.  For example, quantum theory predicts that every kind of particle has a corresponding antiparticle.  The antiparticle of an electron, called the positron, has an identical magnitude charge-to-mass ratio as an electron but with a positive charge.  American physicist Carl Anderson identified the positron in particle tracks in 1932.

When a particle and its antiparticle collide, they are both annihilated and produce a pair of high-energy gamma ray photons.  The collision of an electron and a positron are part of the nuclear process in stars.  This matter-antimatter collision can be described with an equation .


Antimatter: an extension of the concept of normal matter that is made up of particles where antimatter is made up of antiparticles

Example Question 1. How much energy is released when an electron-positron pair annihilate? 

Given

 

Required

the energy released from the annihilation

 

Analysis and Solution

Remember that in the annihilation one electron and one positron's worth of mass is annihilated.

 

Paraphrase

One electron positron pair annihilation releases 1.64×10-13 J.

 

 

Example Question 2. How much energy would be released from the annihilation of 1.00 kg of electrons in an antimatter reaction?

Given

 

Required

the energy released from the annihilation of 1.00 kg of electrons

 

Analysis and Solution

Determine the amount of energy released; remember that to annihilate 1.00 kg of electrons takes 1.00 kg of positrons.

 

Paraphrase

When 1.00 kg of electrons is annihilated it will produce 1.80 × 1017 J of energy. 

Remember from page 820 in your physics textbook that 1.0 kg of uranium releases 7.10 × 1013 J/kg and that 1.0 kg of gasoline releases 4.4 × 107 J/kg.  So, antimatter reactions are extremely energy dense.


Mediating Particles

Quantum theory also predicts the existence of particles that produce fundamental forces like gravity and the strong nuclear force.  These mediating particles are thought to carry the fundamental forces and exist for such a short time that they are undetectable.  The following table summarizes the mediating particles and their relationship to the fundamental forces.



Mediating particle: a virtual particle that carries a fundamental force


Mediating Particle

Fundamental Forces

Particles Observed?

photons

electromagnetic

yes

gluons

strong nuclear

indirectly

gravitrons

gravitational

no

W + ,W - ,Z o

weak nuclear

yes

 












Read
Read "Quantum Theory and the Discovery of New Particles" on pages 836 to 838 of your textbook.

The Subatomic Zoo

More than 300 more subatomic particles have been discovered using new and more powerful particle accelerators and detectors.  These particles have been classified by family.

Leptons do not interact via the strong nuclear force and are relatively small.

Hadrons do interact via the strong nuclear force and are subdivided based on size ( meso is Greek for "middle"; barus is Greek for "heavy").  

The particles are also classified by "spin," which is analogous to describing the rotational momentum of the spinning particle.  Boson and fermion are classifications based on the spin of the particle.


Read
Read "The Subatomic Zoo" on pages 842 to 844 of the textbook.  Note "Table 17.3: An Introduction to the Subatomic Zoo," which identifies the particles, symbols, mass, and lifetime of many subatomic particles.