[The following is based on an excerpt from Back Roads, Dark Skies, my book-in-progress. My book has failed to find a publisher, so I’m going to run a few bits of it here from time to time. This piece is derived from portions of Chapter 2: Of Bosons and Bison.]
Particle physics is the purest manifestation of human curiosity about the world in which we live. Human beings have always asked questions, and since the ancient Greeks more than two millennia ago, the impulse to explore has grown into a systematic, worldwide effort to discover the basic rules governing how the universe works. Particle physics arises directly from our restless desire to understand our world; it’s not the particles that motivate us, it’s our human desire to figure out what we don’t understand.
—from The Particle at the End of the Universe, Sean Carroll
If you housed Willy Wonka’s chocolate factory in a gray cinderblock building, you wouldn’t have a much greater disconnect than you get between the interior and exterior of the DZero detector. On the inside: a premiere particle detector, one of the most intricate and sophisticated experiments in the world, managed by an international collaboration. On the outside: a huge ugly plain blue metal shed, surrounded by trailers that house the researchers, engineers, and technicians that keep the detector running. It doesn’t help either that DZero is in the boondocks of Fermi National Accelerator Laboratory — known colloquially as Fermilab — on the opposite side of the long loop of the particle collider track from the highly photogenic Wilson Hall.
The exteriors were nearly lacking in any decorative features, with one very striking exception: a large cartoon dog on the side of one metal trailer. The dog is kind of a generic bull terrier, biting an itch it can’t scratch. The dog caught my notice for another reason, though: the distinctive drawing style marked it as the work of George Booth, one of my all-time favorite New Yorker cartoonists.[*] I asked my host, Kurt Riesselmann, about it, but he didn’t know why a Booth dog would grace a Fermilab trailer. However, Kurt is deputy head of the Office of Public Information at Fermilab, so he didn’t take long to find the answer: one of the proposals that eventually became DZero was called LAPDOG, and one of its leaders was George Booth’s neighbor on Long Island in New York. (From what I can glean from the internet, there was also a proposal for DZero named TOPD0G, though it’s not mentioned in the official history.)
As the Boothian dog indicates, DZero was designed to scratch an itch: looking for particles predicted by theory, but not yet seen experimentally. These particles include the top quark—the heaviest and most elusive of the type of particles that make up —and the Higgs boson, about which I’ll have more to say soon. DZero is one of two huge detectors at Fermilab, part of the Tevatron particle collider. Its science-fictionish name simply comes from its location: if you look at the site from a satellite view, the Tevatron forms a nearly circular ring. The designers gave letter addresses to six equally spaced points around the ring, like 2-hour increments on a clockface: A, B, C, D, E, and F. For more precise locations, numbers were appended, starting from zero. Thus, with Wilson Hall standing at A (noon), DZero — or D0 — sits opposite at point D (6 o’clock). Similarly, researchers conducted older experiments at the CZero detector located at the 4 o’clock position, while the other major Tevatron detector—the Collider Detector at Fermilab (CDF)—sits at BZero (the 2 o’clock position).
Inside the metal shed, DZero becomes much more impressive. The interior, which goes deep underground, consists largely of a huge multistory room, laddered and catwalked, with a variety of labs, work areas, and support structures running around its perimeter. When I visited, workers were assembling two big detectors in preparation for shipping to other sites, a common repurposing of the space since the shutdown of the Tevatron in October 2011. In fact, many of the DZero scientists were tied up in a big group meeting held that morning, so my first sherpa into the world of particle physics, the enthusiastic Dmitri Denisov, ran late and had to rush off early for his real duties. Thankfully, he had enough time to spare to take me inside the detector itself.Modern particle physics is a massive international effort, and DZero is no exception. Even after the shutdown, the project involves over 500 scientists from the United States, China, Brazil, Canada, South Korea, and 13 other nations. The flags of these countries festoon a huge cement wall, several stories high, that divide the human-safe portion of the facility from the actual DZero detector. As I learned, the key to blocking stray particles often is just massive, thick barriers to slow them down. When the Tevatron was accelerating particles, the area behind the wall was off limits to ordinary mortals. (The wall also helps shield the detector from cosmic rays, so protection runs in both directions.) By the sad misfortune of the Tevatron shutdown, I got more than a glimpse behind that wall.
DZero on the inside
By training, I’m a theoretical physicist: partly out of inclination, but mostly out of lack of the kind of practical mentality necessary to be a good experimentalist. As a result, I take any chance I get to put on a hardhat and climb around inside a real experiment. Unfortunately, my borrowed camera decided to play dead right before entering the detector, so this is the one part of my adventure where I have no photographic evidence.
In a way, though, that’s just as well. To go into DZero itself, Dmitri and I had to don hardhats, but we also checked out special tagged keys we used to enter the detector. The latter is standard practice for work areas with elevated levels of radiation, but not bad enough to require radiation suits or badges. Dragging a camera along would have just been an extra headache for me, so let’s assume its absence made me a little more mindful of my personal safety than I might have been otherwise.
DZero is a big gray canister, 10 meters tall and 20 meters long. Its apparent simplicity is deceptive: that canister is full of instruments, packed in layers like a cylindrical onion, all for the purpose of tagging the energy and direction of travel for as many particles as possible. In a sense, particle physics is the inverse of astronomy. When using a telescope, astronomers usually know what kinds of particles will be entering the instrument: photons, which are particles of light. They use those photons to infer things about the source of the light, tracing back through any intervening material. Particle physicists at Fermilab, on the other hand, know what their source is very well: colliding beams of protons and their antimatter partners, antiprotons. The particles that come out can only be inferred by their behavior, a reverse-engineering of extremely complex physical processes.
The huge variety of particles found in nature explains in part the size of DZero. We now have to face an unpleasant truth: physicists have no detectors that can simply distinguish between all those particles. Some are easier to tell apart than others, admittedly, but as a general rule the experimenters must begin with a set of detections and work backward using computer models to figure out what particles the collisions and subsequent decays actually produced. The problem is complicated, not least because some particles are too short-lived to trigger the detectors, or may not interact with the matter in the detectors at all. Different types of detectors are needed for hadrons than for photons, for muons than for electrons.
Climbing hard-hatted into DZero’s interior, the scale of the structure was overwhelming. I’ve certainly lived in apartments smaller than DZero; thankfully no place I’ve lived has involved so many wires, though. Every individual detector requires its own set of wires, running into huge banks of electronics. Each particle detection triggers the process of turning individual events into a reconstruction of the collision between protons and antiprotons, and the all-important aftermath.
Ultimately, the bulk of particle identification must be done by computer. There are simply too many signals, too many possibilities, too many particle jets and tracks and chances for false signals. For that reason, the DZero control room itself looks like something from a science fiction film, the bridge of a starship running scans for alien worlds. That these “worlds” are smaller than atoms makes them no less alien to us in many ways. Part of their strangeness, of course, comes from their inaccessibility to our senses, requiring — as always — the need to find new ways to see.
[excerpt to be continued tomorrow…]