A T&M View of the Higgs Boson Discovery: Pt 3, What they actually measure

-August 01, 2012

Only a few characteristics of the particles that emerge from the proton-proton collisions at CERN’s LHC (Large Hadron Collider) are directly observed by the detectors. Everything else is derived. The observable properties of the particles that enter the detector are:

•    electric charge
•    energy
•    momentum

You remember the pictures of particles going in circles from cloud chambers in the 1950s? They usually had a geeky looking dude in a lab coat with a pocket protector waving his hands around, too. The idea was that when a charged particle goes zipping through a thick cloud, it ionizes the gas, leaving behind a visible track that they could photograph. The idea is the same, but instead of using gas and photographs they use silicon pixels or grids of wires immersed in liquid argon to measure the particle trajectories.

Both ATLAS and CMS have central detectors that are encompassed by large, superconducting solenoid magnets. The center of the solenoid is aligned, like the detectors themselves, with the beam pipe so that the magnetic field is parallel to the proton beams. As charged particles propagate through the magnetic field, they experience a force in a direction that is perpendicular to both their direction and the direction of the magnetic field. The result is that, viewed end-on, the charged particles follow arced paths (you remember the Lorentz Force Law: F = qv×B, right?).

The direction of the force indicates the particle’s charge and the radius of the arc gives the component of the particle’s momentum perpendicular to the magnetic field.

Notice that I said momentum rather than speed. They don’t measure a particle’s speed because near the speed of light, a particle’s speed isn’t very useful. Nature has a speed limit, the speed of light in vacuum, but it doesn’t have a momentum limit. The momentum you recall from college, mv, is the product of the particle’s mass and its velocity. Near the speed of light, it’s a little different, but the idea of momentum is still a combination of inertia and motion, think of it as the amount and direction of motion.

I don’t want to bother you with a bunch of jargon that’s irrelevant to both your understanding and personal pursuits, so I try to limit it to 1-2 words/phrases in each blog entry. The jargon for today is “calorimeter” – the devices that measure energy. The basic idea is to measure how far charged particles can penetrate dense absorbers like lead. The farther they go, the higher their energy. By separating layers of absorber with active layers of electronics, their penetration distance can be measured and their energies derived.

Another, far cooler type of calorimeter uses dense crystals. They also stop incident particles, but in the process liberate light that is easy to measure. The amount of light liberated is proportional to the particles’ energies. We’ll see later how CMS uses this technology and why it makes for a “Compact” Muon Spectrometer.

Since different types of matter, electrons, hadrons, and muons, go farther than others in different types of media, calorimeters double as particle identifiers which we’ll cover in Part 4..

(Ransom, the author, recalls spending his first few weeks in particle physics research with a group of exalted (in his eyes) research physicists at the Stanford Linear Accelerator Center spending hours arguing about which epoxy was best for holding tiny wires in place. Since then, whenever he hears the word “epoxy,” he falls asleep.)

Read Part 1 and Part 2 of this blog series

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