A T&M View of the Higgs Boson Discovery: Pt 2, What they detect
Ransom Stephens- July 25, 2012
The two Higgs hunting experiments at CERN's LHC are performed in separate, complementary detectors: ATLAS and CMS. Both are apartment-complex sized devices through which experimentalists observe what happens when two protons collide.
Think of the detectors as cylinders whose axes coincide with the beam pipe through which bunches of protons circulate in opposite directions. Magnets direct the protons into collisions at the centers of each detector. It's better to think of the stuff that protons are made of as colliding than the protons themselves. Like bags of marbles with one marble hitting another, as we discussed in Part 1, the point-like objects within the protons interact. They can interact in many different ways, including, perhaps forming a Higgs Boson.
A candidate Higgs event in the CMS detector, notice the cylindrical symmetry with the beam pipe entering and emerging through the end caps (Photo courtesy of CERN).
The products of the interactions decay almost immediately. A Higgs boson won't get a micron before decaying. Only relatively stable objects make it through the beam pipe and into the detectors. The experiments have to reconstruct what happened during the collision by measuring the decay products of whatever formed.
As it works out, only a handful of objects actually pass it into the detector:
1. Electrons and positrons: e±
Positrons are the antimatter equivalent of electrons. They have exactly the same properties, except for their electric charge which is positive; the equal opposite of the charge of an electron.
2. Photons: γ
Gamma rays are more accurately thought of as electrically neutral particles than as electromagnetic waves. If you want to think of them as electromagnetic nuggets to get some scale, these guys have frequencies ranging from about 1025 to 1028 Hz, more than ten billion times the frequency of visible light.
3. "Hadrons" = charged pions and kaons: π±, K±
Protons are made of stuff called quarks and gluons which, as far as we know, are point-like particles. Physicists call them "fundamental" because they haven't been able to break them into smaller things the way they can tear apart atoms, protons, and neutrons. Since the "strong nuclear" force affects the behavior of quarks and gluons far more than the electromagnetic force, they have properties that seem quite strange. For instance, quarks can only be observed at length scales longer than about 1 fm (10-15 m), in pairs or triplets. These pairs and triplets are called "hadrons."
You probably recall seeing old black and white pictures from cloud chambers in science classes perhaps accompanied by a long list of "particles;" that's what these hadrons turned out to be. The ones that last long enough to get into the detector are mostly charged pions and kaons along with very few protons and neutrons.
None of these guys are stable either, their typical lifetime is a few dozen nanoseconds, but since they're traveling at a healthy fraction of the speed of light, time passes slower for them and they get well into the detector and can be observed directly before they are absorbed and decay.
At CERN's LHC, as well as at Fermilab's TeVatron before it, hadrons stream into the detector by the hundreds and are called jets because their reconstructed images look like jet contrails.
4. Muons: μ±
Muons are like electrons except that they're much heavier and harder to stop. Since they're also formed by cosmic rays colliding with the atmosphere, you're exposed to muons about once every second. These guys are the keys to big discoveries; one of the experiments even included muon in its name: Compact Muon Spectrometer = CMS.
5. Neutrinos: ν
Neutrinos hardly interact with matter at all. Neither TLAS nor CMS can actually see them, instead, they are "observed" as something that's missing. We'll get to this next time.
I've listed these five categories of stuff that the experiments can detect (add a "sort of" to neutrinos). You might be wondering how reasonable this assumption is, how can they be sure that nothing else makes it into the detector? The answer is that all other known particles would decay into these stable particles before penetrating the beam pipe.
So, you ask, what about unknown particles? And you're right. If something had a lifetime long enough to get from the collision into the detector that I didn't list, it would be a discovery so huge that it would make the Higgs look like discovering a pebble among sand.
(Ransom, the author, would like it noted that in Geneva, the city closest to CERN, bottles of wine cost less per milliliter than bottles of water, and that this should be adopted by the UN as the defining property of civilization.)