Why is the proposed International Linear Collider going to be straight and not ringed like the LHC?
The International Linear Collider (ILC) was briefly in the Indian news on June 10, when it was announced that the machine would be 32-km long, making it longer than the LHC tunnel (27 km). This isn't an important number and definitely not a feat to be in the news for because the ILC is a linear accelerator (linac).
Thirty-two km is necessary for a linac that's going to be the Large Hadron Collider's sidekick. The distance is used to accelerate the particles through before collision. Even longer distances would result in greater collision energies, but the power consumption would be phenomenal. In fact, to optimize power and energy, a ring-accelerator would've been best... if not for a phenomenon called synchrotron radiation.
In the LHC, hundreds of giant superconducting magnets send two opposing beams of protons, which are a positively charged type of heavy particles called hadrons, in curved paths over and over again until they've been accelerated to extremely high energies. Then, the beams are brought head-on at the collision point, where the protons smash each other open to reveal a plethora of exotic, high-energy phenomena.
While this entire process lasts some microseconds, the protons must be stable enough to survive what to them must be excruciating conditions. While they're scooting around inside the underground tunnel in a curved path, their charge and the applied magnetic field makes the particles lose some energy called synchrotron radiation.
The precise amount of energy lost is inversely proportional to the fourth power of their mass, i.e. for a small decrease in mass, the energy lost is quadruply exponential.
Because protons are the relatively heavier particles in the particle physics zoo, they can be handled with ring-accelerators. The ILC, on the other hand, handles electrons and their antimatter counterpart - positrons. These are both much lighter particles, almost 55,000 times lighter than protons. Thus, their energy loss through synchroton radiation will be almost 91 quadrillion times higher. After losing this much energy, the eventual collision will be hardly strong enough to be of academic value.
So, the ILC accelerates the particles in a straight line and initiates a collision.
Other differences between the two models exist, too. For example, in a ring-accelerator, particles left over from the previous collision cycle can be made to go round and round in the ring, preserved for study. In a linac, that's not possible. Ring-accelerators, because of their design, can accelerate particles over longer distances and excite them to higher energies. Therefore, while the LHC can achieve peak collision energies of 14,000 GeV, the ILC is looking at 500 GeV.
All these factors combine to position the ILC as a complementary device to the LHC, its purpose. Because it smashes protons, which are not fundamental particles but are made up of them, detectors on the hadron collider have to map a stupendous variety of particles. The ILC smashes electrons and positrons, both of which are fundamental particles. During collisions, they may gain energy or lose energy to undergo some transformative processes, but physicists won't have to put up with quarks, antiquarks and gluons, whose physics is hardly understood.
In short, the ILC will make precision measurements where the LHC leaves off. For instance, the linac will measure the mass, spin and strength of the Higgs boson's interaction with other particles. It will also go after the elusive dark matter.
The construction of the ILC is slated to begin in 2015 and is estimated to take at least 10 years. The site of the machine is yet to be finalised, too, but Japan is touted to be a likely candidate because its government is willing to fund half of the estimated $25-billion budget.