Physics questions about the LHC.

[Elok]

JM is right, by the way. Real life doesn't come with a Civilopedia to tell you what new stuff your research will unlock.

So why don't we invent one, instead of dicking around with all these colliders? Sheesh.

[KrazyHorse]

Do you find cloud chambers interesting?

No cloud chambers for 40 years.

[KrazyHorse]

Not if disappointment is conditional on hope of beating expectations.

Or if you had a non trivial prior distribution imposed...

[KrazyHorse]

Okay, here's a perhaps more interesting question about the LHC:

How does the LHC detect particles resulting from high-energy collisions?

The following is based on 3 year old recollection of ~3 years of research level work in LHC phenomenology.

First, you need to know that there are (in the context of high energy detectors) 6 different classes of particles that are relevant: hadrons, light electromagnetic particles (photon, electrons, positrons), muons, b mesons, tau leptons and neutrinos

Next, the overall layout of general purpose detectors: innermost is the pixel detector, then the electromagnetic calorimeter, then the hadronic calorimeter, then the muon chambers

Finally the description of what each layer does, why you would want to do it and how it accomplishes it:

The pixel detector tracks very accurately the paths of particles through the first few centimeters away from the primary collision. It does this both to better reconstruct paths of particles in the next layers as well as to detect decays of particles in the first few millimeters away from the collision (these will show up as displaced vertices; reconstructed tracks will intersect away from the primary vertex at the collision point). Displaced vertices are signs of tau leptons and b mesons. Both of these particles are signs of interesting physics, and both decay to the other families of particles mentioned earlier. The pixel detector is essentially an array of millions of charge coupled devices similar to the pixels in digital cameras; the passage of an electromagnetically interacting particle (charged particle or photon) releases a burst of current from the affected pixel, and these hits are recorded and correlated to reproduce tracks through the detector.

The electromagnetic calorimeter measures the energy and path of light EM particles (and ties the tracks back to the pixel detector). The EM calorimeter is essentially composed of cells of scattering material (e.g. steel) followed by an absorbing material (e.g. argon). When a high energy photon/electron/positron hits atoms of the scattering material, the collision releases a shower of lower energy particles. The energy of these particles is measured by the ionization that secondary scatterings they produce in the absorbing material. This process is repeated through multiple layers until all energy is absorbed.

The hadronic calorimeter performs the same job, except for heavier, strongly interacting particles (bundles of quarks). The scattering happens off the nuclei of a different scattering material (IIRC copper is a reasonable choice). The hadronic calorimeter and EM calorimeter together absorb the majority of energy coming out of each collision (even for most interesting ones)

By the time you reach the muon chamber, all types of particles other than muons and neutrinos have been absorbed. (muons have lost a small fraction of their intial energy and have left tracks in the detector). A strong magnetic field is applied in this region, and the path of the muons is recorded (I believe generally through a relatively low resolution network of tubes of gas that is ionized when charged articles pass through). The magnetic field bends the muons' paths and their energy is measured via the curvature of the path. Muons are also generally signs of interesting physics and in most events carry only a very small fraction of the initial energy; when they have a higher fraction it is usually interesting, which is why so much effort goes into measuring them.

Neutrinos are essentially invisible. They are detected only by the missing energy they carry away. Precise measurements of all the other energy in the collision is important to detect high energy neutrinos (again, these are signs of interesting physics)

[Dauphin]

No cloud chambers for 40 years.

No **** Sherlock.