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.