In recent months many popular news outlets have described events at the Large Hadron Collider, located at the European Organization for Nuclear Research, or CERN. As a result, many people have become familiar with the term "particle collider." Despite the self-explanatory term, however, it might not be entirely clear what such a machine does.
In her piece, Hesla likens the LHC (which collides a multitude of particles, including protons and lead ions) to a bird watcher scanning an entire forest for one particular species. She goes on to describe a muon collider as a set of binoculars that the bird watcher can use to get a detailed look at the bird once he's spotted it.
Scientists think of the LHC as a universal particle physics discovery machine. It scans a wide range of energies to look for the elusive Higgs boson and other particles, the
way a birdwatcher would walk through a forest and look for a particular bird. But once he’s located the bird in a treetop and wants to get a close look, the naked eye won’t cut it. He’ll want a pair of binoculars to better study the bird’s plumage and behavior.
A muon collider would be a particle physicist’s set of binoculars. It would zoom in on a narrow region of energy to uncover the physics phenomena that the LHC can’t reveal on its own. It would provide a clear, unobstructed view of the subatomic world.
Much as the binoculars would ignore the larger context of the forest to focus on the bird's appearance and behavior, the muon collider would home in on one particular energy range where certain phenomena exist.
Hesla then describes the benefits of using these "binoculars" over scanning the entire forest with the naked eye:
Clean events arise because muons are indivisible. Unlike protons, which contain quarks and gluons, muons have no component parts. Two colliding protons are like two high-velocity bags of trash meeting in mid-air: the pieces of garbage inside the bags, not
the two bags themselves, are doing the colliding. The difficulty then lies in sorting through the mess each collision creates and tracing which bottle cap
triggered the trajectory of which candy wrapper.
With muons, there is no garbage. Being a muon is uncomplicated. When two colliding muons have opposite charges—one positive, one negative—they annihilate, and all their energy goes into making new matter. In contrast, when the LHC accelerates protons, it’s really the quarks and gluons inside the protons that are colliding, and each component carries only a fraction of the total proton energy.
The simple imagery in the first paragraph, using objects familiar to any reader, evokes the sense of a seemingly impossible task, and even helps illustrate that protons are made up of still smaller parts.
But the analogies stop there. If proton collisions are like two bags of trash, what does that make muon collisions? Perhaps single items, such as glass bottles, that shatter upon impact and scatter the resulting shards. Given that there are only two items initially, it is easier to decipher which pieces came from where, as well as the nature of the collision.
And, if we were to stick with the trash analogy, it is unlikely that one would use birdwatcher's binoculars to sort out strewn glass on the sidewalk.
Taken together (and they do appear one directly after the other in the article), the two analogies are a bit inconsistent. I know this is splitting hairs: magazine pieces are often limited by length and such in-depth explanations would detract from the ultimate goal of the story.
Taken individually, these two analogies work fabulously to explain their respective concepts, and Hesla's piece is certainly worth reading.