332
The Physics Teacher
◆
Vol. 50, S
eptember
2012
DOI: 10.1119/1.4745683
Each of these forces is mediated by bosons. The strong
nuclear force is mediated by the gluon, while the electromag-
netic force is governed by the photon. The weak nuclear force,
which is responsible for beta decay, is caused by the exchange
of three bosons: the neutral Z boson and the charged W
+
and
W
–
bosons.
While there is much more one can say about the Stan-
dard Model,
3
the story of the Higgs boson is intimately tied
to work in the early 1960s, during which physicists Sheldon
Glashow, Stephen Weinberg, and Abdus Salam attempted
to unify the weak and electromagnetic forces.
4
Building on
earlier work by Yang and Mills, it was possible to express
the electromagnetic and weak nuclear forces in a common
theoretical formalism. This success came with a price, spe-
cifically that the force-carrying bosons for the weak nuclear
force were massless. This claim was obviously nonsense as the
behavior of the weak force was inconsistent with that conjec-
ture. However, the unification was sufficiently elegant so as to
be of interest even in light of this obvious failure.
What is now called the Higgs mechanism resolved the
problem. While the Higgs mechanism has an explicit math-
ematical formulation in which the addition of the Higgs field
minimizes the energy of the situation, the effect of the Higgs
field is often described in terms of analogies. For instance,
like how a barracuda experiences less friction in water than
a sumo wrestler and can thus move more quickly in that
medium, some particles experience greater interactions with
the Higgs field and thus have a larger mass. Another popular
The Higgs Boson: Is the End in Sight?
Don Lincoln,
Fermi National Accelerator Laboratory, Batavia, IL
T
his summer, perhaps while you were lounging around
the pool in the blistering heat, the blogosphere
was buzzing about data taken at the Large Hadron
Collider
1
at CERN. The buzz reached a crescendo in the first
week of July when both Fermilab and CERN announced
the results of their searches for the Higgs boson. Hard data
confronted a theory nearly half a century old and the theory
survived.
The Higgs boson is the missing piece of the Standard
Model of particle physics. The boson was proposed in 1964 by
Peter Higgs, although Higgs cannot (and does not) take all of
the credit. Nearly simultaneous ideas by many gifted theorists
means that the model governing the behavior of this boson
could fairly be called the Anderson-Englert-Brout-Higgs-
Guralnik–Hagen-Kibble theory,
2
but luckily for journalists
and students the world over, we just call it the Higgs boson.
To understand the significance of the announcements of July
2012, we need to know some back story.
The late 1940s and 1950s were the heyday of the “particle
zoo,” in which physicists discovered myriad particles with all
sorts of properties: different masses, lifetimes, charges, inter-
actions, spin, and so on. While it was a delightfully confusing
era, it was really a time for some simplifying ideas that could
consolidate the observations as being different manifestations
of a few underlying principles. The 1960s brought with them
the unifying ideas of quarks, quantum chromodynamics, and
the unification of the weak and electromagnetic forces. The
modern Standard Model was born.
The Standard Model
The Standard Model (see Fig. 1) postulates that the fermi-
on quarks and leptons are the particles of matter, while several
bosons mediate the forces that hold them together. Quarks
are found inside the protons and neutrons (collectively called
nucleons). Physicists have discovered six different “flavors”
of quarks, with the names up, down, charm, strange, top, and
bottom. Only up and down quarks are found inside the nucle-
ons, while the others are more massive and unstable. The most
massive quark is the top quark; with a mass of 173.5 GeV, it
weighs about as much as an entire atom of gold.
The most familiar lepton is the electron, although there ex-
ist two other charged leptons (the muon and the tau), as well
as three neutral leptons called neutrinos. These 12 quarks and
leptons are the complete set of known fundamental fermions.
These building blocks of matter are insufficient to describe
our universe. After all, without some forces to hold them to-
gether, the particles would wander around without interacting
with one another, and matter as we understand it wouldn’t
exist. The forces that are relevant in the quantum realm are
electromagnetism and the strong and weak nuclear forces. We
have no quantum theory of gravity.
Fig. 1. The Standard Model is the most successful description
of the behavior of matter ever devised. The quark and lepton
fermions, combined with the force-carrying bosons, can explain
all data taken to date. The ghostly Higgs boson is the final piece
remaining to be discovered. (Figure courtesy of Fermilab.)
