October this year brought rain and a Nobel prize to Belgium,
as Francois Englert of the Free University of Brussels won the 2013 physics
prize along with Peter Higgs of the University of Edinburgh. They were
recognized for “for the theoretical discovery of a mechanism that
contributes to our understanding of the origin of mass of subatomic particles,
and which recently was confirmed through the discovery of the predicted
fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron
Collider”[1]. I think my field is witnessing the end of an era.
The first subatomic particle discovered as such was the
electron in 1895, and in the following decades many, many more particles were
discovered, including protons, neutrons, muons, pions, and neutrinos.
Different ways by which particles interact were also identified. As the
list of particles got larger, patterns in their properties and behavior began
to emerge. Theorists began to look for theories that explained these
patterns. Quark theory was the answer to describing all the hadrons,
while the description of how particles interacted was built by Sheldon Glashow
in 1960.
This Nobel-winning theory took the
electromagnetic interaction, which is responsible for pretty much all normal
particle behavior, and combined it with the weak interaction, which is
responsible for radioactive decay, to form one electroweak symmetry. The
strong interaction, which is responsible for holding the nuclei of atoms
together, was incorporated in the early 1970s after experimental confirmation
of the existence of quarks. This combined theory described every
interaction observed between every particle known at the time (and describes
the same for every particle discovered since); the only known omission was
gravity, which isn't really observed with particles anyway.
As originally written down, the glaring problem with
electroweak theory was that of particle masses. The theory does not of
itself describe masses, and mass terms (the mathematical formulas that
describes mass) cannot be added without violating the symmetries of the
theory. "Symmetries" in a particle theory is code for
fundamental conservation laws, like conservation of energy, or invariances,
like translational invariance, which says that particle behavior is independent
of location (i.e. electrons act the same in the U.S. as in Japan). The
theory therefore insisted that particles be massless or the fundamental laws of
the universe cannot exist.
Despite trying for a couple years now, I have yet to find a
good analogy to convey how almost absurd that contradiction appeared.
First, the majority of the known particles are not massless, and their masses
were known when the electroweak unification was created. Second, while
the electromagnetic and weak interactions were unified in the theory at high
energies, at the low energies the particles in our universe actually exist at,
these two interactions behave very differently and this is intrinsically related
to the masses of their force carriers. The electromagnetic interaction is
a long-range force that can connect atoms in molecules and molecules in matter
because it is carried or communicated by the massless photon; the weak
interaction being weak and short-range implied it was carried or communicated
by an extremely heavy particle, though at the time the force carrier had not
been observed.
So the brilliant theory created by Glashow needed some way
or ways to include particle masses and describe how the electroweak interaction
breaks into the electromagnetic and weak interactions that we see (called
electroweak symmetry breaking). While there are many ways of
mathematically accomplishing this, the simplest way of doing this was first
written down by Brout and Englert, Peter Higgs, and Guralnik, Hagen, and Kibble independently in 1964. When
their mechanism was merged with the rest of the theory by Steven Weinberg and
Abdus Salam, the Standard Model was born. It has been tested ever since,
and has yet to be wrong.
While
Brout, Englert, Higgs and others came up with the simplest way to deal with
electroweak symmetry breaking, there are other ways to accomplish the same
result. The defining characteristic of the mechanism these scientists
created was the existence of a scalar, massive fundamental boson. It was
commonly dubbed the Higgs boson, just as the mechanism was dubbed the Higgs
mechanism, though not at conferences held in western Europe. The Higgs boson as predicted was a bizarre particle
of a type never seen before in nature (a fundamental spin-0, while all others
are spin-1/2 or spin-1), and alternatives to this mechanism and boson have been
created ever since. In fact, as breaking electroweak symmetry and
particle masses are so fundamental, it is impossible to write a valid theory of
particle behavior without having a "Higgs sector" or something like
it that takes care of those issues, and those typically predict the existence
of a boson or bosons. The question was then, which mechanism actually
describes nature? Alternatively, where is the Higgs boson, and what
exactly are its properties?
[1] "The 2013 Nobel Prize in Physics - Press Release". Nobelprize.org. Nobel Media AB 2013. Web. 26 Oct 2013.
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