Obligatory Post on the Large Hadron Collider

The recently completed mural of the ATLAS detector at CERN. Photo from CERN Media Archive.



Although it seems every physics blogger has discussed the LHC in great detail, it still serves as the most natural starting point for a blog on accelerators. The Large Hadron Collider in Switzerland has gotten a fair amount of press over the past few years, being billed as everything from the largest machine ever built to the largest collaboration of scientists on a single project. But what is a hadron, why are we colliding large ones, and why do we need such an enormous machine to do it?

To answer the first question, “what is a hadron?”, we don't need to look very far. Almost everything you see around you, from your computer to your neighbor's cat to the pizza you had for lunch last week, is comprised of atoms. Most of us learned in high school chemistry that atoms can also be broken into pieces: a nucleus with protons and neutrons, with tiny electrons whizzing around the nucleus. But the story doesn't end there. Although electrons are accepted to be “point particles” (that is, you can't split an electron no matter how hard you try), protons and neutrons can be broken down into pieces called quarks. In both protons and neutrons there are three quarks held together in a bound state.

Any particle comprised of a combination of quarks is called a hadron, therefore both protons and neutrons are classified as hadrons. There are many other ways to put quarks together besides our familiar protons and neutrons. So far only combinations of two quarks (called mesons) and three quarks (called baryons) have been observed, though it has been hypothesized that larger bound-states of quarks could exist.

Don't fret if this has left you a little confused. Baryons and mesons are both hadrons, as the following flow chart illustrates:



You might be asking yourself, why we don't see quarks floating around by themselves, like electrons? It turns out that the force holding quarks together is so incredibly strong that if you were to try and pull a hadron apart, it would take so much energy that it would simply create a quark-antiquark pair from the vacuum (as Einstein says, energy and matter are equivalent) and zoom off with one of the two, leaving the other in the original hadron.



The force holding quarks together is aptly named the strong force for this very reason.

What about the second question: why are we colliding large hadrons? Well, the “large” actually refers to the size of the machine. It turns out accelerator physicists have egos too, and they like to tack on qualifiers to their names. Words like “Large” and “Super” have a habit of showing up a lot. Large Hadron Collider. Superconducting Super-Collider. Everything is in units of Mega-this or Giga-that. It makes everything sound more impressive.

Okay, so if we aren't colliding large hadrons, what hadrons are we colliding and why? As it turns out, the LHC has two modes of operation. The LHC can accelerate either two proton beams or two beams of lead atoms to near the speed of light and smash them together. When the two beams intersect, a particle from each beam may interact. Again, as Einstein tells us, energy and matter are equivalent. All of the kinetic energy we've built up from traveling at near the speed of light is turned into matter which we hope will take the form of interesting, unique, or bizarre particles we haven't seen before.

Finally, why do we need such an enormous project to push a couple protons together? It all comes down to statistics. Keep in mind we have no control over exactly what particles come out of the collisions. The best we can do is make certain states more favorable by changing the collision energy. The higher-energy the collision is, the heavier particles we can make. At certain energies we are more likely to create some particles than others. We have theories that say where we think we'll find interesting new particles, but in the end all we can do is throw a bunch of protons together and see what happens. Sometimes we simply don't see anything new, this upsets us experimentalists:



Comic courtesy of Saturday Morning Breakfast Cereal. As a warning, some other comics on that site may be NSFW.



In the case of the LHC, the energy required to produce particles of interest is so high that you need a tunnel 27km (16.8 miles) in circumference just to store the proton beams. What could possibly warrant such an enormous experiment? That question is worth another post in of itself, but one of the main targets is a particle called the Higgs Boson, long thought to be the carrier of gravitational interaction between masses. The Higgs boson is also the only particle missing from the "Standard Model," a particle physics construct that has accurately described every particle discovered so far.

I'll leave you with perhaps my favorite introduction to the LHC, done by a friend of mine from Michigan State, Katie McAlpine. “The LHC Rap” is an entirely accurate, and catchy, description of the experiments and science being done at CERN today: