GUEST COMMENT
Fireworks on the 4th of July
The February editions of The Physics Teacher and Ameri-
can Journal of Physics include a poster by the Contemporary
Physics Education Project with the title: “The Higgs Boson
– Born on the 4th of July,” covering the Higgs boson news
reported on July 4, 2012. For an extensive description of the
discovery of the Higgs-like particle, see Don Lincoln’s arti-
cle in The Physics Teacher.
After half a century of waiting, the drama was intense.
Physicists slept overnight outside the auditorium to get seats
for the seminar at the CERN lab in Geneva, Switzerland. Ten
thousand miles away on the other side of the planet, at the
world’s most prestigious international particle physics confer-
ence, hundreds of physicists from every corner of the globe
lined up to hear the seminar streamed live from Geneva (see
Fig.
). And in universities from North America to Asia physi-
cists and students gathered to watch the streaming talks.
In 1964, six theoretical physicists hypothesized a new field
(like an electromagnetic field) that would permeate all of
space and solve a critical problem for our understanding of
the universe. Independently, other physicists were construct-
ing a theory of the fundamental particles, eventually called
the “Standard Model,” that would prove to be phenomenally
accurate. These otherwise unrelated efforts turned out to be
intimately interconnected. The Standard Model needed a
mechanism to give fundamental particles mass. The field
theory devised by Peter Higgs, Robert Brout, Franc¸ois Eng-
lert, Gerald Guralnik, Carl Hagen, and Thomas Kibble did
just that.
Peter Higgs realized that, in analogy with other quantum
fields, there would have to be a particle associated with this
new field. It would have intrinsic spin of zero and therefore
be a boson, a particle with integer spin (unlike fermions,
which have half-integer spin: 1/2, 3/2, etc.), and indeed it
soon became known as the Higgs boson. The only drawback
was that no one had seen it.
Unfortunately, the theory that predicted its existence
didn’t specify the mass of the Higgs boson. Everyone hoped
it would be fairly light so that existing accelerators could dis-
cover it. But as the years went by it became clear that the
Higgs boson would be extremely massive, and most likely
beyond the reach of all machines built prior to the Large
Hadron Collider (LHC).
By the time the LHC started collecting data in 2010,
experiments at other accelerators had shown that the mass of
a Higgs boson had to be greater than about 115 GeV. The
LHC experiments planned to search for evidence anywhere
in the mass range 115–600 GeV or even up to 1,000 GeV.
On July 4, the leaders of the ATLA
and CM
experi-
ments were presenting their latest results on the search for
the Higgs boson. Rumors were flying that they were going to
report more than search results, but what was it? Indeed,
when the talks were presented, both experiments reported
that they had evidence for a “Higgs-like” boson with a mass
around 125 GeV. There was definitely a particle there, and if
it wasn’t the Higgs it was a very good mimic. The evidence
was far from weak; they were five sigma results, meaning
less than one chance in a million of the data being only a sta-
tistical fluctuation.
The data were convincing but not perfect, and there were
significant shortcomings. For one thing, the limited statistics
collected by July 4 couldn’t establish if the rate at which this
Higgs candidate decays to various collections of less massive
particles (the “branching ratios”) are those predicted by the
Standard Model
How does one know when one sees a collision event if it
is a candidate for a Higgs boson? There are unique character-
istics that make these events stand out.
Higgs bosons decay into other particles almost instantly
after they are produced, so we only see the products of the
decay. The most common decays (among those we are capa-
ble of seeing) are those to:
•
a
b
-quark and its antiquark (
bb
),
•
a tau lepton and its antiparticle (
s
þ
s
),
•
two photons (
cc
),
•
two W bosons (
W
þ
W
),
•
two Z bosons (
Z
0
Z
0
).
A technicality: For a 125-GeV Higgs boson, the decay to
two
Z
bosons is not possible because
Z
bosons have a mass
of 91 GeV so the pair has a mass of 182 GeV, which is more
than 125 GeV. However, what we do observe is the decay to
a
Z
boson and a virtual
Z
boson (
Z
) whose effective mass is
much less.
This
Z Z
decay mode is quite easy for the ATLAS and
CMS experiments to detect because the
Z
boson sometimes
decays into an electron/antielectron pair or a muon/antimuon
pair. So in the collision of two protons, one of the many
Fig. 1. Physicists applaud the Higgs boson news at the International Confer-
ence on High Energy Physics in Melbourne (July 4, 2012).
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Am. J. Phys.
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(2), February 2013
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2013 American Association of Physics Teachers
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