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.
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