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  • 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 youit 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.

The Large Hadron Collider is a machine which collides protons at a very high energy. Now

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B2 detector particle muon energy collision proton

Catching Collisions in the LHC

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    richardwang posted on 2015/11/19
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