The ILC, Part I: The Motivation

Artist's rendition of the ILC. Image from University of Colorado experimental HEP website.


In my last post I wrote about the Large Hadron Collider, the largest and most complex scientific experiment in history. One of the primary objectives for the LHC is to search for the elusive Higgs boson, the carrier for the gravitational force, by smashing together protons at nearly the speed of light.

Unfortunately accelerator physicists can't just stop and celebrate where we are now. Colliders are so complicated and expensive that we have to be thinking forward to the next big experiment, even before we've seen any results out of the latest machine. The LHC alone took almost thirty years between initial concept and first beam, so if we are to have any hope of building a complimentary accelerator we need to get the ball rolling... well... about fifteen years ago.

"Wait a minute," you might say. "You just told me the LHC is going to search for the origins of gravity, the single elementary particle missing from the framework of particle physics. What more can you possibly want?" Well, although the LHC is an exceptional machine that will no doubt do groundbreaking research, it leaves a lot to be desired.

The LHC collides protons, which are composite particles made of three quarks. However when two protons collide at the LHC, the collision doesn't occur between protons. It occurs between individual quarks inside the protons*. Right away this means only 1/3 of the beam energy is available for generating particles in the collision (since there are three quarks, and each has roughly 1/3 of the total energy of the proton). The quarks inside protons are spinning around each other very quickly, and it is extremely difficult to know exactly what state individual quarks are in when they collide. As a result, the initial conditions of the event are not known very well.

This is a problem we encounter any time we want to collide composite particles. But what if we could collide individual point particles instead? We would know the exact energy of the collisions, and the entire beam energy would be available for producing particles. We could have a complimentary collider: the LHC would do a broad search for anything interesting and a second, precision accelerator would pin down the parameters of any new particles.

As it turns out, we've been doing exactly that for as long as there have been accelerators. Instead of protons, we can use electrons and anti-electrons (positrons). Electrons and positrons are point particles (so far as we can tell), so it is comparatively easy to know their exact states right before collision, leading to precision measurements on particles generated in the interaction.

It sounds like positron/electron (or e+/e-, in physicist shorthand) colliders are an obvious choice. So why isn't the LHC an e+/e- collider? Well, electrons aren't without their complications. In fact some of the best features of electron beams are also the biggest technical problems. To better understand the differences between electron and proton accelerators, we need a little more background.

Accelerators control their beams through use of magnets. When a charged particle such as an electron or proton enters a magnetic field it is deflected. This is how we steer and focus the beam. The particle also emits synchrotron radiation, so named because the radiation was first observed in a synchrotron accelerator. (I'm going to get a little technical for a second, so bear with me). The energy a particle loses to synchrotron radiation is proportional to γ⁴, where γ (gamma) is the ratio of total particle energy to the particle's rest mass. In short, if all other things are equal, the more energy a particle has the more energy it radiates. The heavier a particle is, the less it radiates. And the exponent tells us there is a very strong dependence.



How does this fit in with our discussion? In addition to being point particles, electrons are extremely light, only 1/2000th the mass of a proton. Under identical conditions, electrons will radiate (2000)⁴ = 16,000,000,000,000 times more energy than protons. Now it looks like electrons are a terrible choice! How are we going to deal with losing that much energy? Why would anyone ever suggest such an idea?

Well, hold on a sec. Remember collisions with electrons are far more efficient-- unlike protons, all of the energy is available for particle production. That means we only need a fraction of the energy to make the same particles. Also note the disclaimer: under identical conditions. What if we were to accelerate the beam under different conditions? In particular, could we get away with accelerating the beam and NOT bending it through strong magnetic fields? Then we would have all of the benefits of using electrons-- all of their energy is available for generating exotic particles-- without the drawback of massive synchrotron radiation.

The solution is to use a linear collider. In a circular collider like the LHC, we ramp up energy through use of RF (radio-frequency) cavities (we'll elaborate on those another day). The nice thing about going in a circle is that you get to reuse the same accelerating structures every time you go around-- a very cost-effective operating mode. In a linear collider you can only pass through each element once on your one-way trip to the collision point. In order to achieve higher energies, you need to put a lot of accelerating RF cavities in a row. I mean, a lot. Try 7,000 of them. For each beam.

Once again this sounds like an unrealistic option, but it really isn't much worse than the LHC**. Even though the LHC is huge (and therefore requires weaker bending magnets than a smaller ring would), it still takes an enormous amount of energy to keep the beam going in a circle. For a linear collider we don't need nearly as strong of beam-steering magnets since we want the beams to go in a straight line anyway. The amount of power that would be used to drive an electron beam in a straight line is comparable to storing a proton beam in a circular ring.

The scenario I've just described is exactly what we would do at the International Linear Collider, or ILC. It has been under development for quite some time, and we are nearing the final proposal stages. Over the next few posts I'll focus on the ILC, its design, and why the physics community wants to build it.



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*There is also some possibility that a collision will occur between a quark and a gluon, the force-carrier of the strong force binding quarks together. There is an even smaller possibility that two gluons may interact. Discussing these possibilities only complicates things, though, and doesn't change my argument about the complexity of using composite particles in a collider.

** Total power consumption for the entire CERN facility in the middle of summer is about 180MW. The ILC Reference Design Report from 2007 quotes about 230MW total AC power consumption.

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:

First Light

When I go to social events with non-scientists, I'm often asked what I do. It's gotten to the point that I'm almost too embarrassed to respond. If I say, "I'm a graduate student in particle accelerator physics," their reaction is almost always the same. Those three words are like kryptonite for the brain, and they shut down. "Oh wow, you must be really smart!" they say. "There's no way I could ever understand that." And I feel very self-conscious, because everyone at the table now thinks I must be some sort of genius when in reality I'm just an average graduate student. This reaction is so strong that when I try to further explain what I do, they will often remain in this "it must be over my head" state and not even listen when I say I mostly write "for" loops in Fortran.

There's a stigma that associates particle accelerators with people like Albert Einstein. Particle Physics is the new Rocket Science, the new occupation society uses to say someone is really smart. It used to be Far Side comics, and now it's Big Bang Theory-- an entire sitcom devoted to theoretical physicists being socially awkward and making Star Trek references. Don't get me wrong, I am a huge nerd, however I don't think we need Sheldon Cooper as our spokesman.

The thing is, people got over their fear of rocket science. You know how? Better PR. You find a science writer who can convey complex ideas in terms of what ordinary people understand (Carl Sagan), throw in some pretty pictures (thank you Hubble), and put a good spin on it, and people will often stop being so scared. The accelerator community has been trying for the past decade or so, but so far it's been difficult. I blame it on the lack of pretty pictures.

So that's the goal of this blog. For those who will have the patience to stay with me, I will try to explain what I find to be some of the most interesting and exciting areas of particle accelerator physics in ways the general public can appreciate. I'll even try to find some pretty pictures to help. It may be difficult at first to find the right balance, so feedback will be greatly appreciated. Feel free to introduce yourself, help me learn my audience, and let me know what you'd like to hear about.

Thanks for reading, and stay tuned for the first real post!