The Physics Teacher
◆
Vol. 50, S
eptember
2012
335
their excluded mass range (this extended region had already
been ruled out by the LHC) but, more interestingly, an-
nounced a two standard deviation excess in their data that
suggested the existence of a particle in the mass range of
115–135 GeV.
Higgs physics heats up
This was the state of affairs as we entered July 2012. As
a collaborator on both the Fermilab DZero and the CERN
CMS experiment, I was able to see the scientific process that
precedes a big announcement. Physicists involved in smaller
scientific groups may not appreciate the magnitude of how
these measurements were vetted prior to being made public.
Each of the Tevatron experiments were conducted by about
range for the Higgs boson. While the Tevatron experiments
announced an update at the same conference, those results
were no longer competitive. Other announcements of prog-
ress were made in a seminar at CERN on Dec. 13, 2011, at
which the two large LHC experiments (ATLAS and CMS, see
sidebar) ruled out all masses outside the mass range of about
116–127 GeV (and with both experiments setting similar
limits). More interestingly, both experiments found small sta-
tistical excesses of order two standard deviations at a mass of
about 125 GeV. A statistical excess of two standard deviations
is not enough to be of real interest, although the fact that both
experiments found an excess in the same region certainly did
add some spice to the announcements.
In March of 2012, the Tevatron experiments extended
The detectors at the LHC
The detectors at the LHC are immense and must work under incredible conditions. When the LHC is operating at full capacity, the beams will
cross in the center of the detectors about 40 million times a second. During each crossing, 20 pairs of protons will collide on average. Thus each
detector must inspect of order a billion collisions per second, with the traces of those collisions often occurring in the equipment at the same
time. At most, a few hundred beam crossings can be recorded to computer tape each second. Selecting which ones are recorded is the job of
very fast custom electronics that we call a
trigger.
The two large detectors at the LHC are CMS (Compact Muon Solenoid) and ATLAS (A Toroidal Large ApparatuS). See Figs. 5 and 6. Both de-
tectors can be thought of as approximate cylinders, with the symmetry axis along the beam line. CMS is about 50 feet wide, 70 feet long, and
weighs about 14,000 tons, while ATLAS is much larger and lighter at 70 feet wide, 140 feet long, and about 8000 tons. Both detectors consist of
about 100,000,000 individual detector elements, with the majority of these elements occurring in the finely grained silicon detectors at the heart
of the experiment. Like most particle physics detectors, they contain layers of subdetectors nested like Russian matryoshka dolls. The center
of the detectors is a huge array of silicon pixels a few tens of microns across. Surrounding the precision central tracker are trackers of larger
granularity. Following the tracking detectors are first calorimeters that measure electromagnetic energy (photons and electrons) and a second
set of calorimeters that measure hadronic energy (pions and protons). The outer detectors are designed to measure muons, which are the one
electrically charged particle that can penetrate that far. Mind you, each experiment uses different technology for each layer of the detector and
the curious reader can see Ref. 1 for details. Both detectors have similar capabilities, as they are studying the same phenomena, under the same
conditions. As they say, time will tell which design choices were good and which were bad.
Both ATLAS and CMS have a solenoidal magnetic field in their tracking volume, although the CMS solenoid actually surrounds the calorim-
eters as well. This is an uncommon choice in particle physics. Also, both detectors have a second set of magnets for the muon detector region,
with ATLAS having a series of eight distinctive large toroidal magnets that give the detector its name.
Fig. 6. The ATLAS detector is the largest particle detector
at a particle collider. If you look closely, you see a person
standing near the center of the detector. (Figure courtesy
of CERN.)
Fig. 5. The CMS detector is the most massive particle
detector at a particle collider. The white dots in the yel-
low region (center bottom) are hard hats worn by people.
(Figure courtesy of CERN.)
