Understanding Bosons and Fermions
The Elusive Higgs Boson and the Large Hadron Collider (LHC)
Oct 18, 2008
The Pauli exclusion principle states that no two electrons in an atom can occupy the same quantum state. Physicists completely describe which shell or energy level the electron occupies by its quantum state. Hence an easily understandable, but oversimplified, interpretation of the exclusion principle might be that two electrons cannot be at the same place at the same time.
Bosons and Fermions
Electrons and other subatomic particles that obey the exclusion principle are called fermions. Not all subatomic particles, however, follow the exclusion principle. Those that do not are called bosons. In addition to other classification schemes, such as hadrons vs. leptons, all subatomic particles are classified as either fermions or bosons.
In large numbers, fermions obey a set of statistical laws that were discovered by Enrico Fermi and Paul Adrien Maurice Dirac. These laws are called Fermi-Dirac statistics. Bosons, in large numbers, obey a different set of statistical laws, called Bose-Einstein statistics, that were discovered by Satyendranath Bose and Albert Einstein.
Elementary particle physicists describe subatomic particles by their spin, among other properties. The spin of elementary particles is poorly understood but might be thought of as their intrinsic angular momentum in integer or half integer units which is called the spin quantum number or simply spin. Bosons all have integer (0, 1, etc.) spin values, while fermions have half integer (1/2, 3/2, etc.) spin values.
The familiar protons, electrons, and neutrons are all examples of fermions. Photons, mesons, alpha particles, and the Higgs boson are examples of bosons.
Higgs Boson
In 1964 Peter Higgs suggested the possibility of a field (now called the Higgs field) that permeates space and gives subatomic particles (and by extension macroscopic objects) their mass. The Higgs field interacts with particles creating a drag type force on the particles when they change their velocity. This drag gives particles their inertia, and mass is a measure of the inertia of an object. The Higgs field therefore gives particles their mass by causing their inertia.
Elementary particle physicists think that fields all have associated particles. For example electromagnetic fields are associated with photons, which are particles of light. The Higgs boson is the postulated particle associated with the Higgs field. As the particle that physicists think gives objects their mass, the Higgs boson has been referred to by physicist, Leon Lederman, as the god particle. Particle physicists think that Higgs bosons exist, but they have not yet discovered the Higgs boson because the energies required are too large.
The Large Hadron Collider
Elementary particle physicist hope that the CERN Large Hadron Collider (LHC) will produce collisions with enough energy to allow them to observe the Higgs boson. Theoretical calculations suggest that energies of at least 1e12 electron volts (1 Tev) are needed to find the Higgs boson. The Large Hadron Collider will produce collisions between protons in that energy range.
The elusive Higgs boson is the only elementary particle predicted by the standard model of particle physics that has not yet been discovered. If particle physicists using the Large Hadron Collider finally discover the Higgs boson, it will provide strong evidence that their standard model of elementary particles is correct.
Further Reading
Beiser, A., Concepts of Modern Physics, 3rd ed. McGraw-Hill, 1981.
Serway, R.A., Moses, C.J., and Moyer, C.A., Modern Physics, 3rd ed. Thomson, 2005.
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