Big questions require big answers, and occasionally,
very big scientific facilities.
Among the very largest of the scientific facilities
ever built is the Large Hadron Collider or
LHC.
The LHC is a particle accelerator, fully 27
kilometers in circumference.
It accelerates two beams of protons to nearly
the speed of light and collides them together.
I made a different video that tells about
this incredible piece of technology.
In 2012, my colleagues and I announced that
we discovered the Higgs boson using the LHC.
This particle, which was the last unconfirmed
prediction of the Standard Model of particle
physics and an accomplishment that took nearly
half a century to achieve.
If you’re watching this video, you’ve
almost certainly heard reporters say things
like “LHC scientists discover the Higgs
boson.”
However, even as amazing as the LHC is, it’s
not the entire story.
Being able to accelerate particles with the
LHC is a crucial capability, but until you
can record the outcome of the collisions,
you really aren’t doing science the science
that the accelerator was built to do.
To capture these collisions, you need to use
particle detectors.
I made an earlier video about how particle
detectors work, but in this video I want to
talk about the specific detectors at the LHC.
There are four main detectors at the LHC,
each with unique capabilities.
Each one is basically a huge camera that can
study millions or billions of particle collisions
per second.
The four detectors are called ALICE, ATLAS,
CMS and LHCb.
ATLAS and CMS are the ones involved in the
discovery of the Higgs boson, while the other
two are specialty detectors.
So let me tell you a little bit about each
one in turn.
ALICE (sometimes pronounced Alice) stands
for A Large Ion Collider Experiment and is
designed to study what happens when you collide
two nuclei of lead together.
It is 26 meters long and 16 meters high and
weighs 10,000 tonnes.
It takes about 1,500 physicists from 154 institutes
and 37 countries to perform.
Its purpose is to study a new state of matter,
called a quark gluon plasma, in which matter
is heated to 100,000 times hotter than the
center of the sun- hot enough to literally
melt protons and neutrons at the center of
atoms.
Conditions like these were last common in
the universe about a millionth of a second
after the Big Bang.
The LHCb experiment is quite different.
Its core purpose is to understand why the
universe is made of 100% matter when our best
understanding of the early moments of the
universe say that matter and antimatter were
made in equal quantities.
It weighs about 4,500 tonnes.
About 700 scientists, representing 69 universities
and laboratories from 17 countries are involved.
The name LHCb means LHC beauty, as beauty
is an older name of a heavy and unstable quark.
While that name is, well, beautiful, it is
now more commonly called the bottom quark.
It is thought that studying this ephemeral
particle is the most promising way to shed
light on this puzzling asymmetry between matter
and antimatter.
The ATLAS and CMS experiment are more similar
in their fundamental goal.
These two detectors are designed to record
collisions between beams of protons at a collision
energy of fourteen trillion electron volts.
The energy densities in these collisions haven’t
been common since the universe was less than
a trillionth of a second old.
Collisions occur at a rate of a billion times
a second and these detectors inspect every
single collision before recording only about
a thousand select collisions each second.
You may have heard that the LHC is colliding
beams at 13 trillion electron volts, but that’s
just to ensure that we can run the accelerator
reliably.
It also means that when we have more experience
with the machine that we might still squeeze
out another 7% or so more energy.
The ATLAS experiment is physically the largest
particle experiment ever built.
Its name is an acronym for A Toroidal LHC
ApparatuS.
It is 45 meters long, more than 25 meters
high and weighs about 7,000 tonnes.
It’s about half as big as the Notre Dame
Cathedral in Paris and it weighs the same
as the Eiffel Tower.
It involves over 3,000 scientists from about
180 universities and laboratories from 38
countries.
More than a thousand Ph.D. students do their
research using this detector.
The Compact Muon Solenoid Experiment or CMS
has some significant commonalities with ATLAS,
but looks different in detail.
For one thing, it is more massive, weighing
14,000 tonnes or about twice the weight of
the Eiffel Tower.
It is 22 meters long and about 15 meters in
diameter.
The detector consists of about 100 million
individual elements.
Like ATLAS, CMS requires 3,000 scientists
to design, build and operate, from 182 institutions
drawn from 42 countries all around the world.
The solenoid from which the experiment draws
its name is one of the biggest and strongest
magnets in the world, at 6 meters wide and
13 meters long, with a field of 80,000 times
that of the Earth and containing energy equivalent
to about half a ton of TNT.
These four experiments are crucial to the
mission of the LHC.
The Large Hadron Collider may provide collisions,
but without these four experiments, there
would be no LHC science.
So you might be asking yourself which of the
experiments is the best and most important.
Unfortunately- or perhaps fortunately- there
is no answer to that question.
Each of the four detectors is exquisitely
designed to answer difficult questions and
each is truly a marvel of technology.
It would be totally inappropriate of me to
indicate that I have a favorite.
Totally inappropriate.
Not gonna do it.
The research program at the LHC will fascinate
science enthusiasts for the next couple of
decades.
The discovery of the Higgs boson was just
the beginning of an ambitious research program,
but the LHC experiments will make many more
groundbreaking measurements.
It may well be that these experiments will
make discoveries that completely overturn
what we think we know about matter and energy,
teaching us something new and fundamental
about the universe.
As in all things, time will tell.
