With the LHC coming back on line, and starting on its next run of data collection, it seems like a good time to review another particle accelerator that is still in the developmental stage, and to advocate for more investment in the fundamental sciences.

Most people with a serious interest in science in general and particle physics in particular have heard of Fermilab. They were the lab that discovered many of the fundamental particles of nature. And they came close to discovering the Higgs boson in the Tevatron experiment, until they had to shut down for maintenance and upgrades and lost that race to the LHC.

Now there is a proposal to construct a new accelerator at the site, known as Project-X (which apparently had no greater meaning than no one has thought of a better title yet). And Project-X has the potential to be far more valuable to society than most particle accelerators that have come before it.

The advantage of the LHC over previous experiments is the energy levels it can explore. Each proton-proton collision carries 14TeV of energy when the LHC is at full power, where previous experiments were limited to less than a tenth of that energy. This energy is needed to perform direct probes of higher energy physics, such as generating the Higgs boson or possibly exploring theories of supersymmetry and dark matter.

However in what is sure to be a shock to many particle physicists, energy is not the only consideration. It doesn't matter if we can reach 14TeV with each collision, but can only see one rare reaction every year. There simply isn't enough data in one or two reactions, (or even a few hundred reactions) to be certain of what we are seeing.

And that is where ProjectX has a potential advantage. Using existing equipment from the Tevatron, it will only generate proton collisions at energies of 3 - 8 TeV, much lower than the LHC. However the plan is to generate a larger number of proton collisions, and to sift through the data for very rare processes. Finding and studying a one-in-a-billion reaction at a 3 TeV collider would be just as important for science as finding and studying a more common reaction at 14 TeV. Some early estimates even claim that ProjectX could detect indirect signals of new physics three times quicker than the LHC. And unlike the LHC, ProjectX would be built with a lot of existing technology and equipment so that the costs and build time are much less.

So what are these rare reactions? There are many of them, and so I will review just a handful of the most interesting. 
  • A Neutrino Factory: About fifteen years ago physicists discovered that the three types of neutrinos can oscillate from one type to another, proving that they have mass but opening questions of what mechanism allows this variation to occur. A high intensity accelerator could produce a large number of electron neutrinos, muon neutrinos, and their antiparticles, which could be directed towards a detector far away in another lab. This high intensity neutrino beam would permit physicists to pin down the properties of neutrino oscillations. 
  • A Muon Collider: Along with neutrinos, a high intensity collider will also generate a lot of 'heavy electrons' known as muons. For technical reasons that I lack space to cover here, while electron collider experiments are relatively inexpensive to build and operate, they are also inefficient because of the lightness of the electron. However a muon has very similar properties, except for a mass that is ~200 times greater than the electrons, meaning that a muon collider would be more efficient and would allow physicists to study a wide range of new reactions.
  • Muon-to-Electron Decay: Another benefit of having a large number of muons produced is to study the intriguing muon to electron decay channel. Aside from effects due to neutrino oscillations, which are a very weak effect on muon decays, muons cannot decay to electrons without generating a muon-neutrino. Add up the number of muons and muon neutrinos that go into a reaction, subtract the number of their antiparticles, and the result is constant in every reaction (except neutrino oscillations). But there is no reason for this to be true in general. While many conservation laws in physics have their basis in symmetry properties, this is not true of muon-numbers. And many proposed theories do predict a rare decay of a muon to an electron and a photon. This has never been seen, but it is potentially such a rare process that we wouldn't expect it to be detected in existing experiments. Maybe the ProjectX will generate enough muons that a few of these rare decays will be observed. (And then the theorist can debate whose theory came the closest to predicting the right decay rates)
  • Rare Kaon Decays: Kaons are interesting particles. While they are not fundamental particles, they do have one very special property. The laws of physics generally are the same for antiparticles in a mirror-image world as they are in our own world for ordinary particles (for those who are interested, this is called CP symmetry). However the decay of the neutral kaon was found to be different from the decay of its anti-particles. But these decays are still quite rare, and difficult to study. However the high intensity ProjectX could generate trillions of kaons, and study the decay of each one. For the first time, physicist would be able to see one-in-a-trillion decays which are produced by higher energy effects. An unknown force which has no direct effect on proton collisions below 10,000TeV could create large effects on these rare kaon decays. It may even be possible, in certain theories of higher energy physics, that at this intensity kaon decays could probe energy levels of over 100,000 TeV!
  • Electric Dipole Moments: Most people are aware of electric charges. A positive charge attracts a negative charge and will repel another positive charge. It is also possible though for an object to have an electric dipole moment, in which a positive and negative charge are bound together. Such an object is not attracted or repelled by other charges, but will align itself with electric fields. For obvious reasons, a point like particle should not have an electric dipole - it has no physical size and so cannot have two opposite charges held apart by some distance. However at this scale quantum mechanics does odd things to particles, and we do know that many point-like particles exhibit properties of spin and magnetic field interactions which should also be impossible for a particle with no size. So maybe they can have an EDM as well. ProjectX would be able to generate heavy nuclei, and by examining their properties the experimentalists could probe another three orders of magnitude on the electron dipole. If they find a non-zero value, even if it is tiny, it would indicate some form of new physics and CP violation. And that could have implications for models of the Universe, as the absence of antimatter in the Universe requires some as yet unknown form of CP-violating physics.
By this point I am certain that some of my readers are falling asleep, or wondering about my earlier comments about ProjectX benefiting society in general. These rare processes are interesting, and as a scientist I believe firmly that learning the laws of physics in this way is itself a valuable benefit, but others may want more practical applications. 

The biggest benefit of a high intensity accelerator such as this one is in energy production and waste cleanup. Current nuclear reactor technologies generate a lot of long-lived, highly radioactive waste materials. At present this waste has to be stored away somewhere safe, and everyone hopes the containers do not break. But a high intensity proton beam could be directed at the waste material, and convert it into other substances. It could be broken down quickly into less harmful, lighter nuclei, or converted to a slightly heavier but more stable nuclei. The technology could be a waste disposal system for radioactive waste.

But it could also generate power. In principle a high intensity proton beam can cause more stable materials to generate a fission reaction, producing energy in the same way as a traditional reactor, but with several benefits. First and foremost, these reactions are "sub-critical" meaning that when you flip the off switch, they stop. In a traditional reactor the reactions continue until some other mechanism is applied to stop them, which is why they sometimes meltdown on their own. In these reactions, there is no meltdown risk. The second benefit is of course the lack of weapon making material. Where a traditional reactor generates material that is handy for making nuclear bombs, an accelerator based reactor does not and so can be used in areas that really shouldn't have access to weapons-grade uranium/plutonium. And the final benefit is that the fuel supply is far more plentiful. Where a traditional reactor needs uranium or plutonium, which the Earth may only be able to provide for another century or two at current mining rates, the accelerator reactor can use more plentiful materials like thorium, which is so plentiful at present that it could be used for millenia. (It should be noted here by the way that ProjectX is not going to be used as a reactor, but rather to test the methods and technology so that such reactors might be built somewhere else in the future.)

So that is ProjectX in a nutshell. It is quite an interesting idea, and one that could be built and operated for a small investment from the governments involved. Hopefully they can get it approved and start exploring in the near future.