particles produced on rare occasions is a Higgs boson
H
,
which occasionally decays as
H
!
Z
þ
Z
. Each of these
Z
bosons then (occasionally) decays as either
Z
!
e
þ
e
þ
or
Z
!
l
þ
l
þ
. The end result is that we will sometimes
see (in addition to some unrelated particles) four muons, or
four electrons, or two muons and two electrons.
Figure
shows a neon sign of an actual event recorded by
ATLAS in which four muons (shown in neon) were pro-
duced. The other particles are shown in the background.
While decays of this kind had been observed for the new
particle by July 4, the rates at which they occur were still
uncertain. It was not even known if the newly discovered
particle has the right quantum numbers—that is, whether it
has the spin and parity required of a Higgs boson.
In other words, the July 4 particle looks like a duck, but
we need to make sure it swims like a duck and quacks like a
duck. This work is continuing. There is a major conference
in March 2013, which comes after the conclusion of this
year’s proton-proton collision run at the LHC. Physicists
look forward with great anticipation to seeing what the
experiments report about branching ratios, spin, and parity,
the properties essential to confirm that this really is the Higgs
boson.
Most physicists believe it is; it is difficult to create a
theory with a massive particle having significantly different
properties. However, the confirmation—or not—will eventu-
ally come from the data.
The discovery of the Higgs boson is an enormous clue
about the mechanism for giving mass to fundamental par-
ticles, as conceived by Higgs, Brout, Englert, Guralnik,
Hagen, and Kibble. What is this mechanism? It is a mathe-
matical theory for which an overly simplified cartoon (see
Fig.
) can be used to demonstrate its essential nature.
Fundamental particles get their masses from the Higgs
mechanism. However, most of the ordinary mass of the
Fig. 2. A neon sign showing an actual event recorded by ATLAS, which
might reflect the decay of a Higgs boson into four muons, shown in neon.
The other particles are shown in the background.
Fig. 3. A cartoon helps to understand the Higgs mechanism. (a) Imagine that a room full of physicists chattering quietly is like space filled with the Higgs field
(top left). A well-known scientist walks in (top center), creating a disturbance as he moves across the room and attracting a cluster of admirers with each step
(top right). This cluster of admirers increases his resistance to movement; in other words, he acquires mass, just like a particle moving through the Higgs field.
(b) On the other hand, if a rumor crosses the room (bottom left), it creates the same kind of clustering, but this time among the scientists themselves (bottom
right). In this analogy, these clusters are the Higgs particles.
#
1996 CERN. We thank CERN for the use of these images and text; the concept was inspired by
Professor David J. Miller of University College London.
86
Am. J. Phys., Vol. 81, No. 2, February 2013
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