Up to this point, the detectors have told us only about charged particles. But what
about neutral particles? Instead of a track that shows you where the particle went and
how it responded to the magnetic field, the calorimeter
gives you a measurement
of the total energy of the particles hitting it in any direction, regardless of
whether they are neutral or charged.
The calorimeter is so large that it didn't make sense to try to make a seamless detector.
So when we talk about the “calorimeter,” we are referring to a series of wedges that
are arranged into circles along both sides of the detector, fitting snugly around the COT and
magnet. Each wedge is about eight feet long and is actually two detectors in one.
First is the electromagnetic calorimeter
, which measures the energy of
lighter particles such as electrons and photons. This detector is made of
sheets of a plastic scintillator
that absorb energy and emit light, sandwiched between
3/4-inch layers of lead. Behind it is the much larger hadronic calorimeter,
measures the energy of hadrons
—more massive particles made of
. This piece
of detector uses steel instead of lead in its scintillator sandwich.
A close-up of a calorimeter wedge, with
the electromagnetic calorimeter at the
bottom and the hadronic calorimeter at
the top. The green plastic at the bottom
is scintillator; the top of the wedge (seen
in upper right image) is a forest of photomultiplier tubes. (Click image for larger
How do particles get "caught" by the calorimeter? A high-energy particle collides
with a layer of lead or steel and “showers,” creating a cascade of lower-energy, charged
particles. These in turn hit other particles that make up the steel or lead and
create their own showers of even lower-energy particles. In this way, one
100-GeV electron could become one hundred 1-GeV electrons by the end of the ride.
Every time the remaining particles
pass through a layer of scintillator, they deposit energy in it, which is converted to light and carried
away to be measured by fast electronics.
Why is the hadronic calorimeter so much larger than
the EM calorimeter?
It isn't only direct collisions with atoms in the steel and lead that
cause incoming particles to "shower." The electromagnetic attraction of protons
and electrons in the metals can also cause charged incoming particles to shower
merely by changing their direction. Since electrons are much lighter than hadrons,
they yield to the electromagnetic force more easily, and shower more often.
But hadrons have to wait until they have a head-on collision with the nucleus
of an atom in the lead or steel. This means hadrons usually penetrate deeper
into the machine before they are converted into lower-energy particles, so a
hadronic calorimeter needs more space to contain the energy from its cascading particles.
Uses of the calorimeter
Calorimeters are especially good for telling you about the neutral particles that are
coursing through your detector. Remember that neutral particles (such as neutrons and high-energy
photons) were invisible up to this point, since the inner detectors rely on
to see particles and neutral particles do not ionize atoms.
But charged particles also deposit energy in the calorimeters. So how do you tell the difference
between the two?
If you see a track leading straight into a hit in the calorimeter,
you're seeing a charged particle. If you see nothing and then suddenly
you see a hit in the calorimeter, chances are it was a neutral particle.
The ability to measure the energy of each particle helps physicists determine
the masses of particles such as the top quark
or W and Z bosons
. But the energy
that is "missing" can be just as important as the energy they "catch."
Physicists know that according to the law of conservation of energy, what they put into
the system should equal what they get out of it. Missing energy means the collision
created some particles that cannot be seen by the detector. These could be particles
such as neutrinos
that don't interact with matter very often. Or they could
be particles that haven't been discovered yet. Many searches for
postulate the existence of particles that the detector
wouldn't be able to "see" directly. Looking for missing energy is the only way to
validate the existence of these particles.
Look at a live event display
produced by the calorimeter.