Following up on the introductory essay (below) to my series on the Large Hadron Collider (LHC), today I provide the first substantive posting, on the basics of particle-physics colliders.
A good place to start is with an analogy from George Mason University professor Robert Oerter, in his book The Theory of Almost Everything (which I reviewed here). The analogy is presented primarily on pp. 135-137, but the remainder of the chapter also provides important historical information.
To paraphrase Oerter's analogy, let's say we have a sponge-rubber Nerf football that may have objects embedded within it, such as a baseball or even a ball made of steel.
We are not allowed to squeeze or cut open the Nerf ball, nor can we see inside of it. Instead, we must fire pellets into the ball and make inferences about the inner content of the ball by what happens to the fired pellets. Depending upon whether pellets go completely through the ball and come out, get stuck in the ball, or ricochet back out at some angle, estimates of the size, position, and hardness of the inner objects can be made.
Similarly, it is through inferences from records of particle collisions that physicists learn about the micro world of matter. I will have a later write-up specifically on detection of particle collisions.
In looking at the LHC and earlier facilities such as Fermilab's Tevatron, CERN's Large Electron Positron collider (whose tunnel has been converted to use with the LHC), and Stanford Linear Accelerator Center, three major distinctions arise. These distinctions, and the pros and cons of different alternatives, are discussed below.
Fixed vs. Particle-Beam Targets
One distinction is between experiments that aim a beam of particles at a fixed target, and those that aim two particle beams into each other. Quoting from Peter Woit's book Not Even Wrong, "Accelerators that collide two beams together are now called colliders..." (p. 15).
This CERN document discusses the arguments in favor of fixed-target and beam-on-beam (collider) designs. In short, colliders are said to be more economical to run and can produce higher energies. On the other hand, "Firing a particle beam into a solid metal target or large tank of liquid ensures that almost every particle will collide with a nucleus but getting two particle beams to interact is much harder."
Lepton (e.g., electron) vs. Hadron (e.g., proton) Colliders
The same CERN document as discussed in the preceding section also addresses lepton vs. hadron colliders. Leptons are said to be advantageous for precision energy measurement, whereas hadrons are better suited to discovery of new particles. Electrons (one type of lepton) also have a problem with something called "synchrotron radiation," as discussed below.
For additional background on leptons and hadrons, you can look over a couple of my previous postings. One on leptons is available here, whereas one on hadrons can be found here.
Linear vs. Circular
This Wikipedia entry on particle accelerators lists some advantages of circular designs over linear ones:
In the circular accelerator, particles move in a circle until they reach sufficient energy. The particle track is typically bent into a circle using electromagnets. The advantage of circular accelerators over linear accelerators (linacs) is that the ring topology allows continuous acceleration, as the particle can transit indefinitely. Another advantage is that a circular accelerator is relatively smaller than a linear accelerator of comparable power (i.e. a linac would have to be extremely long to have the equivalent power of a circular accelerator).
Circular accelerators have evolved from the original cyclotrons to the more elaborate synchrotrons. As Woit explains, "Higher-energy accelerators could not be built using a single magnet, but instead used a doughnut-like ring of smaller magnets. This design was called a 'synchrotron'..." (p. 13).
The technical mathematical name for a doughnut-like structure is a torus. In fact, one of the experiments to be carried out at the LHC is known as ATLAS, for "A Toroidal LHC ApparatuS." See the Wikipedia page on the LHC for further information on the individual experiments.
According to the Wikipedia page on synchrotrons:
While a cyclotron uses a constant magnetic field and a constant-frequency applied electric field, and one of these is varied in the synchrocyclotron, both of these are varied in the synchrotron...
In a cyclotron the maximum radius is quite limited as the particles start at the center and spiral outward, thus this entire path must be a self-supporting disc-shaped evacuated chamber... The arrangement of the single pair of magnets the full width of the device also limits the economic size of the device.
Synchrotrons overcome these limitations, using a narrow beam pipe which can be surrounded by much smaller and more tightly focused magnets.
Under certain conditions, synchotrons can lose energy through a process called "synchrotron radiation," as is described in this excellent educational graphic from the Nobel Foundation.
Synchrotron radiation appears to be a problem when two conditions coexist: (1) a circular accelerator is being used, and (2) electrons are being studied. A Fermilab document states the following:
Linear electron accelerators do not produce much synchrotron radiation. Circular electron accelerators and storage rings produce copious synchrotron radiation in all the bending magnets, especially when the magnetic fields are high or the beam energy is high.
Further, Woit notes that:
While the main constraint on the energy of an electron-positron ring is the problem of synchrotron radiation loss, for a proton ring this is not much of an issue, since protons are so much heavier than electrons (p. 23).
For further information, I would suggest this CERN LHC page of Frequently Asked Questions and this PowerPoint slide show by Florida State University professor Horst Wahl.