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The Physics Teacher
◆
Vol. 50, S
eptember
2012
photons precisely enough, this decay chain becomes attrac-
tive due to its relatively small backgrounds.
Backgrounds
One of the greatest challenges in finding the Higgs boson
is the backgrounds. For instance, if the mass of the Higgs
boson is low (say, 125 GeV), it preferentially decays into a
bottom quark/antiquark pair. The problem is that producing
bottom quarks/antiquarks via more ordinary physical pro-
cesses totally swamps the Higgs boson signal by a huge factor.
It is literally impossible to find the Higgs boson via this decay
pattern. However the background for decays into pairs of Z
bosons is much lower, as is the diphoton decay mode. The
background for decay into W boson pairs is also relatively
low, but to identify the W boson involves decays into unde-
tectable neutrinos. Further, the ability of detectors to identify
photons and the decay products of Z bosons is excellent. For
these reasons, the Z and photon decay chains are the most
powerful methods to identify events in which Higgs bosons
are created.
Higgs hunting history
In 1989, the Large Electron Positron (LEP) accelerator be-
gan operations at the CERN laboratory by circulating beams
under the French and Swiss countryside. This accelerator
collided electrons and positrons at an energy precisely tuned
to copiously produce Z bosons. By 1994, the CERN Council
decided that the LEP tunnel would ultimately house the LHC.
In 1995, the LEP accelerator physicists raised the collision
energy to 140 GeV. Further improvements culminated in a
collision energy of 208 GeV, when the LEP accelerator was
turned off for good in 2000 to make way for the LHC.
The LEP data were studied for hints of the Higgs boson,
and, after an exciting false lead or two, the physicists finally
concluded that they had not observed it and set a lower limit
on the mass of the boson of 114.4 GeV. This was the state of
affairs when the Fermilab Tevatron turned on in 2001 with a
refurbished accelerator that raised the collision energy from
1.8 to 1.96 TeV and increased the instantaneous beam lumi-
nosity by a factor of 10. After accumulating data for nearly a
decade, the two Tevatron collaborations (DZero and CDF)
5
made our first announcement restricting the allowed mass of
the Higgs boson in July of 2010, when we had excluded the
mass range of 158–175 GeV.
In September of 2008, the LHC began operations in front
of the world, only just a few days later to have an improperly
soldered electrical connection fail, which caused the cryo-
genic system to release a vast amount of liquid helium. The
resultant damage took about a year and a half to repair and
the LHC began running again in March of 2010 at half the
design energy (e.g., 7 TeV as opposed to 14 TeV). Even with
this reduced capacity, the writing was on the wall for the Fer-
milab Tevatron. By July of 2011, the LHC experiments were
able to rule out the range of about 150–200 GeV as a possible
into pairs of top quarks. For masses above about 400 GeV, the
Higgs boson decays into pairs of (W/Z/top) particles about
(54/25/20)% of the time (see Fig. 3).
Given the expected Higgs boson decay modes, it is im-
perative that any detector have excellent capabilities to detect
bottom quarks and W and Z bosons. Good ability to identify
tau leptons and top quarks is also beneficial.
There is one decay mode of the Higgs boson that is ex-
tremely powerful and entirely counterintuitive. This is its de-
cay into two photons (see Fig. 4). Because photons are mass-
less, they do not interact with the Higgs boson directly, but
occur in instances when the Higgs boson decays into an inter-
mediate state of a pair of top quark/antiquarks that have tem-
porary masses which are far from their “right” mass. (Again,
this is possible under the aegis of the Heisenberg uncertainty
principle.) This quark pair then annihilates into a pair of pho-
tons. This process is rather rare, occurring about 0.1% of the
time. However if the detector is able to measure the energy of
Fig. 3. The decay fractions of the Higgs boson are completely
specified by the theory for each value of the mass of the Higgs
boson. This plot shows the range of possible decay modes.
Fig. 4. While the Higgs boson preferentially decays into the
heaviest particles consistent with energy conservation, through
the intermediate decay into top quarks or W bosons, the Higgs
can convert into two photons. This is an especially clear pro-
duction mechanism and is therefore a dominant method to
search for Higgs bosons at the LHC. The decay percentages for
this mechanism are too small to show up in Fig. 3.
