The Large Hadron Collider is a machine which
collides protons at a very high energy.
Now with Run 2 we are going to reach an energy
level twice as big as the previous run, which
gave us the Higgs boson.
Scientists on the four major Large Hadron
Collider experiments, ATLAS, CMS, ALICE, and
LHCb, are colliding protons and collecting
data at a record-breaking energy: 13 trillion
electronvolts, or TeV.
Claudia Fruigele, a theoretical physicist,
describes what happens when protons collide
in the LHC.
It’s important to think about them not as
protons but in terms of the constituents of
a proton, and indeed, a proton is made of
a bunch of particles and those are called
quarks and gluons, so really we have to measure
collisions between this bunch of particles.
Maybe I should have warned you– it can be
a little bit of a messy subject.
Most of these particles, most of these events
are known physics, so what we are really doing
is like looking for rare events.
We are looking for a needle in a haystack.
Something like 100 particles or more can come
out of a collision, and we want to understand
the trajectory of all those particles, where
each particle went, and we want to know how
much energy each particle had.
When we do that, we can reconstruct what happened
in the collision, and in doing so, we can
learn something about our theories about how
physics works on the lowest level.
That’s Jim Hirschauer, and what he’s talking
about is potentially 100 particles resulting
from a single proton collision.
This isn’t magic, but happens because the
energy generated by a collision is converted
into a slew of new particles, including electrons
and photons and less familiar particles like
muons.
So the protons collide right in the center
of our detector.
He’s talking about the Compact Muon Solenoid,
or CMS.
At Fermilab, U.S. researchers like Jim are
studying data recorded in the CMS detectors.
The detector is pretty much a big barrel,
about five stories tall, that weighs about
14,000 tons.
Different parts of the detector measure the
trajectory of the particles and other parts
of the detector measure the energy of the
particles produced.
It’s arranged in a number of layers, and
I guess you could think of the layers as roughly
three groups.
There’s the tracker in the very center of
the barrel, and just outside that are the
calorimeters, and just outside that is the
muon system.
The trackers are made of silicon- silicon
as in the element used to make computer chips-
so the particles moving through the tracker
are recording electronic signals not unlike
the pixels in a digital camera.
Particles move through this detector without
being disturbed much, so it’s great at observing
their initial trajectory.
So by connecting the dots between the layers
of the silicon we can understand the trajectory
of the particle, and from that, we can measure
the momentum of each particle and we know
exactly where it’s going.
The outer layers are more destructive, and
in order to measure the energy of the particles
they need to stop the movement of the particles.
After the particles go through the tracker,
they might- they will strike the calorimeter.
By slowing down particles and absorbing their
energy, calorimeters help physicists observe
how different particles interact with matter.
Some particles are quickly absorbed while
others penetrate further into the calorimeter.
Basically, you can tell a lot about a particle
by the way it treats matter, and physicists
look for key patterns that give away a particle’s
identity and its origin.
As a particle like an electron strikes the
calorimeter it starts within the calorimeter
a little shower of more particles, which we
call an electromagnetic shower.
As those particles go through the crystal
of the calorimeter, they produce light, and
they produce light, an amount of light in
proportion to the energy of the incoming electron.
And so by calibrating the detector, we can
understand that a certain amount of light
that we get out of the calorimeter corresponds
to a certain energy of the particle that struck
the calorimeter in the first place.
At this point, the CMS detector has absorbed
most of the particles that have come out of
the collision.
But there’s one final layer: the muon system.
The muon particle is just like an electron
except heavier.
And we know if we see some dots to connect
in the muon system it must have been a muon
because nothing else will make it through
that far.
But of course, particles darting through the
tracker, calorimeters, and muon system are
moving way too fast for scientists to watch
in real time.
The proton collisions are occurring in our
detector about 40 million times a second,
and that’s too much data for us to record
all of the information from all the subdetectors
for every event, so we need to decide which
ones are the most interesting, which collisions
are the most interesting, and we do this with
a trigger system.
And the trigger decides very quickly, in a
few microseconds, which events to record and
which to ignore.
So at the end, we might be collecting a few
hundred hertz, so a few hundred collisions
per second will come out of our detector out
of the 40 million collisions per second that
we know is occurring in the LHC.
But even with the trigger, a few hundred collisions
per second is a tremendous amount of data.
During Run 1, the CMS detector produced about
5 petabytes of data per year- roughly equivalent
to the data used to stream 2 million HD movies.
And that’s just CMS!
The ATLAS, ALICE, and LHCb detectors are also
packing in data.
The data from those events are written to
computer discs, and eventually, they are sent
all over the world for analysis.
In the first run of the LHC, we discovered
the Higgs boson, so now we hope to discover
a new massive particle.
This can be maybe dark matter; it can be-
we can discover a new symmetry like supersymmetry;
discover bonds with new objects; or maybe
we can discover something that we didn’t
think about.
