A large block of metal, which forever afterward becomes too radioactive for other uses. Even a very intense beam of protons can be stopped by a few feet of lead because protons are very willing to interact with matter through the nuclear Strong force. The same is not true of muons: they pass through shielding much more readily because they mostly interact via electromagnetism. However, this means that the proton beam deposits a lot more heat, since it’s giving up all of its energy: 0.7 GJ or half a million BTU for the LHC at full intensity (these first tests are at low intensity). Which would make a better death-ray: something that deposits more energy or something that can’t be shielded against? (A purely academic question.)
> I wondered what CMS was; I assume it’s this.
Yes, the Compact Muon Solenoid experiment. Most of my posts will be CMS-centric, and some of the earlier ones give more detail about the experiment, writing the acronyms out in full. However, some acronyms have become words on their own: many physicists working on CMS would have to stop and think if you said “Compact Muon Solenoid.”
We’re colliding protons (a subset of hadrons— I think it should be called the Large Proton Collider), but that’s just to get enough energy into one place for nature to take over and make whatever it wants. When CLEO was operating at Cornell, we collided electrons and positrons for the same purpose. The species of particles being collided matters to a small degree: it changes the production rate of certain processes, both for the interesting signal and the problematic background, but the energy of collision matters a lot: if a particle is more massive than the collision energy, its production rate will be zero. At CLEO, we couldn’t dream of creating Z bosons— our collision energy would have had to have been nine times higher. At the LHC, we’ll make so many that we can use them for calibration.
In general, proton collisions create more of everything, though that’s often a liability. In an electron-positron collision, the collided state is neutral in many senses: no electric charge, no Weak charge, and no color (Strong) charge. It will probably create a particle-antiparticle pair which balance each other, or it will make a particle which is its own antiparticle (which subsequently decays into a particle-antiparticle pair). Protons, on the other hand, are not fundamental particles, but three stable (“valence”) quarks and a soup of short-lived quark-antiquark pairs and gluons creating and annihilating with one another all the time. When you collide two protons, you see a snapshot of that process, and any of the internal particles (generically called “partons”) are available for collisions. This way, you can get quark-antiquark, gluon-gluon, or any other combination. As you can see, the by-products don’t have to be neutral or balance each other’s momentum or anything, since not all parts of the protons were involved in the collision. That makes for a lot more possibilities: particles with lower-than-collision energy can easily be made from collisions that don’t take full advantage of the protons’ energies, the final state does not need to be charge-neutral, etc. That’s great for discovery when you don’t know what you’re looking for, but it also creates a lot more background for us to sift through, which makes it more difficult to identify something new and interesting.
Among new and interesting signals, the easiest to identify will be ones that decay into muons. The vast majority of uninteresting collisions are when partons riccochet off each other, pull a quark-antiquark pair out of the vacuum to stay color-neutral (see Color Confinement) and form mesons, or sometimes a “jet” of mesons mostly going in the same direction. This is predicted by QCD, but hard to quantify because QCD is a hard-to-solve non-abelian gauge theory (and also because it depends on the precise internal structure of protons, which involves the even more insoluble non-perturbative QCD). If your favorite process decays into anything that looks like that, good luck trying to distinguish it from the many-orders-of-magnitude larger background!
Physics related to the electroweak force, or electro-weak symmetry breaking in particular, has a much higher propensity to decay into muons. In electroweak theory, leptons such as electrons, muons, and taus have pretty much the same status as quarks and antiquarks, while QCD doesn’t care about leptons at all. Moreover, muons have this nice property that they can pass through feet of iron and yet still make an ionization trail in a detector. The outermost part of the “Compact Muon Solenoid” experiment includes layers of iron between the tracking chambers, so that everything but muons, neutrinos, and gravitons will be stopped, but only muons will be observed in the tracking chambers. It’s the easiest kind of particle to identify.
That said, electrons are also important because they’re second-easiest to identify. They leave an ionization trail and are all-too-easy to stop in material: they decay into showers of particles in the Electromagnetic Calorimeters. Taus, unfortunately, look a little like meson jets, but not entirely and clever algorithms can distinguish them sometimes. Then, of course, there are other, more complicated things we can do to look for signals among the hadrons, but with less confidence, and therefore less statistical precision. Signals such as massive Higgs (> 130 GeV) decaying into four muons will be easy to see, one of the first results, while a lighter Higgs (114-130 GeV) decaying into two “b-jets” will be much harder, and will require more time and more data.
No one has ever built a muon collider. It would be really cool (cleaner, more easily interpreted signatures than proton colliders), but it’s hard to collimate them into a narrow beam. And, of course, they decay in milliseconds.
[…] three LHC experiments, LHCb, had the honor of observing the first beam particles! The diagram in my August 24 post shows the parts of the LHC that have seen beam so far, which includes LHCb (on the red line). All […]