The Physics Teacher
◆
Vol. 50, S
eptember
2012
333
quark/antiquark pair that can annihilate and make a Higgs
boson, which then decays.
While this production mechanism is the dominant one,
it is not the only one (see Fig. 2). Further, the contribution
from other sources of Higgs boson production depends on
the beam energy. At the lower energy accessible using the
Fermilab Tevatron, another production mechanism is signifi-
cant. At a rate of approximately a third that of the production
of Higgs boson through virtual quarks, Higgs bosons can
be produced in association with a W or Z boson. Investigat-
ing this “associated production” is favored at the Tevatron
because the backgrounds (i.e., events indistinguishable from
Higgs boson production, but frommore ordinary sources)
are much smaller.
When a Higgs boson is produced, it exists for on order
10
-22
seconds if its mass is low (i.e., on the order of 120 GeV).
Thus we never observe the Higgs boson directly, but rather
need to look for its decay products. Prior to 2010, it was nec-
essary to look for the Higgs boson over a large mass range,
from 114 GeV to about 600 GeV. Because of the Higgs boson’s
affinity for particles of heavy mass, it preferentially decays
into the heaviest particles it can, again consistent with energy
conservation. The heavier denizens of the subatomic realm
are the charm quark (1.3 GeV), tau lepton (1.8 GeV), bot-
tom quark (4.5 GeV), W boson (80 GeV), Z boson (91 GeV),
and top quark (173.5 GeV). In order to conserve energy and
momentum, the Higgs boson must decay into at least two
daughter particles, one matter and one antimatter. If the
Higgs boson had a mass near 114 GeV, it would decay about
70% of the time into a bottom quark/antiquark pair, with the
remainder of the decays into gluons, charm quarks, and tau
leptons. In the mass range of 114–160 GeV, these fractions
drop quasi-linearly as it becomes possible to make a W
+
/W
–
pair. At a mass of about 160–165 GeV (twice the mass of the
W boson), Higgs bosons decay more than 95% of the time
into pairs of W bosons. Above that mass, it becomes possible
for the Higgs boson to decay into pairs of Z bosons, and from
a mass of about 200–350 GeV, Higgs bosons decay into pairs
of W bosons 70% of the time and pairs of Z bosons about 30%
of the time. Above that mass, it becomes possible to decay
analogy invokes two people, a celebrity and a nobody, try-
ing to walk through a crowd. Because of the crowd’s desire to
interact with the well-known person, the celebrity has a more
difficult time moving and can be imagined to have more iner-
tia and to have effectively gained more mass.
Returning to a more technical description, while the
theoretical electroweak bosons were massless, the Higgs
mechanism “broke the symmetry” between the photon and
the weak bosons, resulting in the following physical particles:
a massless photon, a massive neutral weak boson (Z), two
massive charged weak bosons (W
±
), and a new particle, now
called the Higgs boson. The photon has long been known and
was understood to be a quantized particle with Einstein’s pro-
posal of the photoelectric effect, and the weak bosons were
discovered in 1983 at the CERN laboratory in Switzerland.
However, the existence of the Higgs boson remains to be es-
tablished. That’s what this summer’s news was about.
The Higgs boson was originally postulated to break the
theoretical symmetry between the electromagnetic and weak
forces, by giving mass to the weak bosons. This mechanism is
called electroweak symmetry breaking or EWSB. While the
scope of the original proposal of the Higgs boson was limited
to EWSB, it turned out to be easy to extend the theory to the
fermion sector and to give mass to the quarks and leptons.
Within this theoretical paradigm, there is only one unknown
and that is the mass of the Higgs boson. Once the mass of the
Higgs boson is determined, one can precisely calculate the
production rates for it and its various decay modes. Because
of this theoretical certainty, it is possible to optimize detectors
and search strategies to maximize the chances for discovery.
Additional firm predictions for the Higgs boson are that it is
electrically neutral, is fundamental (i.e., contains no constitu-
ents), its quantummechanical spin is zero, and its parity is
positive.
Production
Higgs bosons are created inside particle accelerators from
the transformation of beam kinetic energy into the mass en-
ergy of the boson. For recent searches, the Tevatron, located
at Fermilab just outside Chicago, collided a beam of protons
with a beam of antiprotons at a center of mass energy of
1.96 TeV, and the LHC collided two beams of protons at a
center of mass energy of 8 (7) TeV in 2012 (2011). Because
the Higgs boson generates the mass of fundamental particles,
it preferentially interacts with the most massive particles it
can, consistent with energy conservation. Since the heaviest
known particle is the top quark, it is this particle that inter-
acts most strongly with the Higgs boson. Since the top quark
has a mass of 173.5 GeV, and the mass of the proton is
0.938 GeV, top quarks are not generally found inside the
beam particles. However, it is possible to make “virtual” top
quarks in the proton. These quarks do not have the “right”
mass and can only exist because of the Heisenberg uncer-
tainty principle. In order to generate the requisite interac-
tion, gluons from the two colliding beam protons make a top
Fig. 2. While the production of Higgs boson is dominated by vir-
tual top quarks originating from gluons in the beams, there are
other ways in which it can be created. In this example, two vir-
tual electroweak bosons are initially produced, which annihilate
and create a Higgs boson.
