I’ve been meaning to write a little about my tour of the experiments when I visited CERN a few weeks ago. Sadly (or perhaps happily) I realize now that everything I was hoping to say was already said, at a level of precision I wasn’t intending to reach, by Jacques in a report of his own visit to CERN. I’ll say it anyway, but read his post for more details.

CERN, nestled in the Alps on the Swiss/French border by Geneva, is going all-out to complete the Large Hadron Collider by the target date of 2007. This is a giant ring that will accelerate protons in opposite directions, ultimately to smash into each other and (hopefully) create a bunch of new particles for us to explore. The energy of the collisions will be 14 TeV, where one TeV is about one thousand times the rest energy of a single proton; so there is more than enough energy available to see all sorts of fun things.

The LHC will have two large general-purpose experiments, ATLAS and CMS, and I had a chance to see each of them. (There will also be a nuclear-physics experiment, ALICE, and a B-physics experiment, LHC-b.) Both are truly amazing feats of engineering and physics. For one thing, they are big; each is of order 10,000 tons, and fits into a room the size of a large cathedral.

Here is a picture of ATLAS at the early stages of construction; to see its current state, check out the mandatory webcam.

CMS (which also has a webcam) is actually more impressive to see up close, as it is being put together as a series of slices, while ATLAS is being constructed more as concentric layers.

Interestingly, the reason why the experiments have to be so big is simply because of quantum mechanics. What we are really interested in is what is happening at extremely tiny distances. But the uncertainty principle tells us that we can’t probe small distances without reaching very large momenta, or equivalently ultra-high energies. At the energies probed by the LHC, the particles produced in each collision come zooming out at tremendous speeds, and the extraordinary sizes of CMS and ATLAS are necessary to capture and analyze all of these many energetic particles. Each detector is a series of concentric layers that serve to measure the properties of different kinds of particles — electrons and photons are easy, strongly-interacting particles (quarks and gluons) create elaborate jets, and muons require special treatment so that they don’t just punch right through the detector. Other particles (W and Z bosons, tau leptons) decay rapidly and are diagnosed by what they decay into. Still others (neutrinos, not to mention various hypothetical new species) zip right out of the detector completely unseen, but their presence can be inferred from “missing energy,” if the total energy of the reconstructed event is less than that of the initial collision.

As a theorist (and one who grew up in astronomy departments), one of the most fascinating concepts in high-energy experiments is that of a trigger. Each detector will witness approximately one billion collisions per second, which is a lot. You might imagine that you’re faced with two problems: simply recording all the data from each event, and then sifting through them for the interesting bits. You’re right, but it’s much worse than you think. That’s because each event isn’t just a few bytes if data; it’s of order one megabyte per event. There’s simply no way you could record all of the data.

Instead, you try to figure out which events are “interesting,” and record those — perhaps 100 events per second. That’s where the trigger comes in. While the data from each event are still streaming through the hardware, they are rapidly analyzed to see if they are worth keeping. This happens in levels; you do an ultra-rapid scan at the hardware level to see if anything potentially interesting is going on, and are able to cut down a billion events to about ten thousand. That’s the level-1 trigger; the level-2 trigger is a sophisticated piece of software that looks at more precise characterizations of the events (much like an ER doctor making a preliminary rapid diagnosis, then homing in with more delicate tests) to get you down to the one hundred events that are actually recorded for later analysis. (This is all part of the great computing challenge that Mark discussed a while back.)

Why are some events more interesting than others? Quantum mechanics again. Particle physics doesn’t predict what will happen at each event, only the probability that certain things will happen. The interesting bits (new physics, or events that help improve our understanding of established physics) will be swamped by well-understood processes, known affectionately as the “background.” (A generation or two of physicists worked tirelessly to establish the gleaming edifice we know as the Standard Model, and now we think of it as simply “background.”) As you might guess, a lot of hard thought (and spirited, collegial disagreement) goes into deciding which events to keep and which to toss away!

During my visit, the LHC folks seemed cautiously optimistic that they would really turn on the accelerator in 2007, and start taking useful physics data in 2008. All that will be necessary is the superhuman efforts of an army of physicists and engineers working twenty hours a day. But it will be worth it, as the LHC should revolutionize our understanding of the subatomic world. For the last 25 years or so, particle physics has been in an extremely unusual position — the theory just worked so well that all the new experiments kept finding particles that had already been predicted. This is the opposite of the historically common state of affairs, in which experimenters keep coming up with unexpected new phenomena that the theorists have to scramble to understand (as we’ve seen in cosmology in the last decade). I fully expect the tables to turn once again when CMS and ATLAS start releasing new results. We have lots of ideas about what might be around the corner — one or more Higgs bosons, supersymmetry, extra dimensions, various forms of strong dynamics — but my suspicion is that what we see won’t fit perfectly well into any pre-existing framework, at least not at first. That’s when we theorists will really have to earn our salaries, and physics at the high-energy frontier will be as exciting as it ever was.

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