Naming the animals in the particle zoo

[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. Yesterday’s post was an earlier excerpt from the same chapter. I apologize for the length of the excerpt, but it was hard to trim down without losing something of the flavor of the chapter.]

Unlike most sites where the business of cosmology is done, Fermilab isn’t in a remote spot. The facility is in Batavia, Illinois, part of the sprawling metroplex of Chicago, and it’s just a short drive from two major tollways. The Standard Model describes a plethora of particles, but it has nothing on the number of fast-food joints and auto shops within ten minutes’ drive of the Fermilab gates. My friend hosting me during my stay in Illinois wasn’t even aware of the lab’s location, despite having friends living close by—the area around it is that densely packed.

Wilson Hall, the headquarters building of Fermilab. [Credit: FNAL, ’cause my camera died before I could take a photo myself]
Nevertheless, portions of the extensive Fermilab grounds are as close to wilderness as you can achieve in suburbia. Though surrounded by all the trappings of modern urban sprawl, Fermilab is home to a small herd of bison, along with coyotes, herons, and other wildlife — a true nature preserve, worth visiting for that reason alone. Additionally, many of the buildings are architecturally interesting: spirals, scalloped roofs, and other geometrical features appear throughout the facility. (I described the SiDet building with its icosohedral roof in Chapter 1, which you’ll probably have to wait for the actual book to read.) Even the power lines running into the complex were designed to look like the Greek letter pi (π). Mixed in with the architectural monuments to the heady heyday of 1970s particle physics are the hideous utilitarian sheds and trailers housing many of the research scientists, and the unimpressive boxy metal structures containing the DZero  and CDF detectors. Overall, Fermilab is a glorious heterogeny of past, future, beautiful, ugly, natural, and artificial — a metaphor for modern science.

Wilson Hall, the 15-story Fermilab headquarters, reportedly was inspired by the Gothic cathedral in Beauvais, France, though to this highly professional physicist’s eyes, it looks more like the Atari logo. The building has an inner courtyard extending nearly the height of the building, with glass from base to ceiling on either end creating a space filled with light. The courtyard level, which visitors enter via doors at the top of a broad set of stairs, contains a garden of ficus plants. Balconies overhung with ivy look over the open cafeteria space at the far end. Overall, Wilson Hall’s atrium is one of the most attractive spaces in science, like an art museum dedicated to particle physics.

I attended a cosmology conference at Fermilab during my graduate student days several years ago, but due to time constraints and active experiments going on during the visit, I hadn’t been able to see much. This trip would be very different, for unfortunate reasons. In September 2011, the Tevatron — Fermilab’s largest collider — shut down after budget cuts made it too expensive to continue operations. What this meant is that I could actually tour parts of the facility that would be inaccessible while the collider was running, but it also meant many conversations were weighted with the past tense, and the future tension of uncertainty. What will happen at Fermilab in the short term, ten years later, twenty years later? That existential doubt affected nearly every question I asked, and every answer I received.

(As a side note, my borrowed camera failed me right before DZero, so I don’t have any photos from this part of the tour.)


When the Tevatron was operating, the protons and antiprotons entered DZero via a narrow pipe, nearly unnoticeable when compared with the detector apparatus. The name “Tevatron” refers to a tera-electronvolt[1] (abbreviated as TeV), a large measure representative of the energy scale with which the particles slam into each other.  One TeV is greater than what you’d get converting the mass of 1000 protons into pure energy. In terms of everyday objects, that’s not very much—you possess more energy than that simply by walking at a normal pace across the room—but it’s packed into two beams, consisting of individual protons and antiprotons.

That feat is accomplished by accelerating the particles nearly to light-speed, and involves overcoming several challenges. First, protons and antiprotons are tiny by the standard of daily life, but they’re relatively massive, so they take a bit of pushing to reach high velocities and to steer. Second, while protons are really common—you can get them just by taking hydrogen and stripping the electron off—antiprotons aren’t. Third, if researchers are to have any hope of understanding the products of collisions, the protons and antiprotons should all have the same speed (and therefore the same energy), at least within a tiny margin of error. Finally, protons and antiprotons are really tiny compared to the equipment needed to study them, but enough of them need to collide inside DZero for any results.

Up until the autumn of 2012, a wonderfully mad-scientist-looking apparatus called a Cockcroft-Walton generator[2] provided the initial proton acceleration. However, this type of generator is finicky, so present and future Fermilab experiments use more reliable radio-frequency quadrupole generators. These have the added advantage of consuming less power than their predecessors, even at the detriment of looking rather plain. Whichever device starts the process, it feeds the protons into a linear accelerator: a long straight tunnel lined with electromagnets. Moving protons experience a force in a magnetic field, so careful accelerator design can bring them up to a substantial fraction of light-speed.

Fermilab researchers solved the problem of antiproton production by using more protons. They direct the beam of protons onto a “fixed target”: a piece of nickel in an underground facility located near Wilson Hall. The collision of protons with nickel atoms produces a number of particles, including the desired antiprotons. Further instruments separate the antiprotons from the other debris and channel them into a focused beam.[3]

Despite the challenges of creating them, antiprotons have a few major advantages. Since they have opposite electric charge as protons, they experience the same electromagnetic forces, but in the opposite direction. That means researchers can use the same tunnels to steer them into collisions, whereas proton-proton colliders (like the Large Hadron Collider (LHC) in Europe) require two sets of tunnels — and magnets — to get the protons traveling in opposite directions.

The Tevatron itself is a ring, a rough circle 6.9 kilometers in circumference. Well, at least it looks circular from the air, but it’s more a complicated polygon with rounded corners. From basic physics, the protons and antiprotons will tend to fly in straight lines if left alone (hence the linear passages in the tunnel). However, it’s almost impossible to start all the particles in perfect paths, so the Tevatron is lined with magnets to focus the beams and steer them around the track. The pipe through which the protons and antiprotons travel is buried about 8 meters deep, far enough belowground to help control temperature and block a lot of cosmic rays: particles from deep space that could mimic the ones we’re really interested in.

I’m following particle physicists by using the term “beam” for the particles traveling around the Tevatron, but it’s not like the steady beam of a flashlight. Instead, the protons and antiprotons are injected into the collider in bunches (the actual term!) of a specific size; the beam consists of many of these bunches. The design of the bunches and beams ensures enough protons and antiprotons will actually run into each other, despite being very small. The result: about 2 million collisions inside DZero every second, each producing cascades of other particles for the detectors to collect.

Anatomy of a proton

If a proton meets its antimatter partner at sufficiently high energies, they will interact as their component pieces rather than as whole particles. That’s where things get really interesting: in the early Universe, all particles possessed very high energy, so colliders recreate in a very small way some of those conditions present when the cosmos was young. More prosaically, collisions provide the only way we have to “see” inside protons and get to the fundamental structure of matter itself. Since light isn’t available for this work, we use particles to “see” inside each other.

It’s best to think of a proton as a soup of quarks and other particles, which are sometimes collectively known as partons.[4] A proton is made of two up quarks and a down quark, the big chunks in the soup, which are called “valence” quarks: the components that are most obvious and define the overall flavor. I use that word deliberately: flavor is the general term for the type of particle. The proton’s mass is a lot greater than the mass of the three quarks put together. As with the binding energy of a nucleus, that’s E = mc2 again, but in reverse: this time the energy holding the proton together contributes to the mass. The binding particles are known as gluons (a word derived from “glue”). Even though gluons are massless and quarks are relatively light,[5] the whole proton mass is the result of the detailed interactions via the nuclear force.

The interactions between the partons also mean that there’s more going on inside a proton: particles that exist in a state of perpetual quantum fluctuation, and whose properties average out to make them unimportant under ordinary conditions. In the soup metaphor, the proton broth contains other quarks, antiquarks, and possibly other ingredients whose existence is only suspected. Because all those things that aren’t valence quarks pop in and out of existence quickly, they are usually called “virtual” particles. In my opinion, a more enlightening term might be potential particles: they only potentially exist due to interactions, but can be liberated under the right conditions. That’s the goal of particle colliders like the Tevatron—to probe the insides of protons to get at the potential particles, seeing what they reveal about the forces giving them life. Those hidden ingredients could even include dark matter, an intriguing proposition.

Showers of particles

The original core of the Collider Detector at Fermilab (CDF), which is bristling with silicon chips and other means of detecting and identifying particles. CDF and DZero are the two big detectors at the Tevatron. [Credit: moi]
The original core of the Collider Detector at Fermilab (CDF), which is bristling with silicon chips and other means of detecting and identifying particles. CDF and DZero are the two big detectors at the Tevatron. [Credit: moi]
The hidden potential ingredients in the proton and antiproton soup explain why we can use just those two types of particle to get a lot of interesting things we might want to study. In fact, one of the major goals of particle physics has always been to find particles whose existence we didn’t suspect previously. Even for particles predicted by theory — such as the top quark or the Higgs boson, 2012’s big particle physics discovery — experiments are often required to measure some of their properties, most notably their masses. Over the years, physicists have found a few strong hints that the physics we know isn’t enough to explain everything. I’ll discuss two of the biggest examples of this, neutrinos and dark matter, in the next chapter.

The immediate result of proton-antiproton collisions inside DZero is a set of cascades, including partons, leptons (particles including electrons), and photons. The partons don’t exist as individual particles for any noticeable length of time: they quickly turn into hadrons, producing the jets of particles that contain much of the interesting stuff physicists are looking for.

So how do we know which product is which? Here’s the idea: detectors themselves necessarily consist of atoms, subject to the same fundamental forces of nature as the particles they’re trying to detect. What a given particle “sees” when it comes close to an atom depends on its properties, including its electric charge, mass, and whether it’s governed by the strong force or not. Additionally, since the paths of charged particles curve under the influence of magnetic fields, detectors are surrounded by powerful magnets; that also aids in separating the types of particles.

For starters, electrons are produced in huge numbers in collisions, but also are part of any ordinary atom. An incoming electron will “see” the whole atom and interact with it via the electromagnetic force. Similarly, as photons are manifestations of the electromagnetic force, they also “see” and interact with the whole atom. In that way, photons and electrons can be detected using the same devices, despite being very different in most respects. However, the situation is complicated because high-energy photons can produce new electrons and positrons when they scatter off atoms. Researchers must use the shape of the particle trajectories through the detector to determine which particle is which.

Detectors like DZero use electrically sensitive materials like silicon and other semiconductors for electron- and photon-hunting, but these are somewhat expensive. As a result, the silicon detectors are placed close to the center of the detector, near where the initial collisions take place. They consist of small chips or strips of material in layers; a photon or electron could potentially interact with many atoms during passage through the detector. By seeing where electrical signals are triggered, the researchers can reconstruct the particles’ trajectories in three dimensions — then work backward to see which was an electron, which was a photon, and which was a positron (though a lot fewer of those are produced).

Hadrons, on the other hand, will not “see” the whole atom: they tend to only interact with the nucleus, making their probability of interaction much lower. For that reason, hadron detectors often use solid materials comprised of big nuclei — ones with a lot of protons and neutrons to increase the chances of something hitting them. DZero uses uranium and argon together; uranium is the heaviest of the elements easily found on Earth, and it has lots of nucleons. When a hadron hits a uranium nucleus, it loses energy, which is converted to light. That light is amplified by the argon atoms, allowing the signal to be detected.

Most hadrons produced in collisions are pions:[6] the lowest-mass mesons comprised of up and down quarks, and their antiquark equivalents. Neutral pions are their own worst enemy: they are made of a quantum mixture of quarks and antiquarks of the same type, so they almost immediately self-annihilate into gamma rays. Most other hadrons aren’t stable either, so detectors must be on the lookout for their decay products.

Baryons, including protons and neutrons, are much rarer, since they involve three quarks instead of a quark-antiquark pair. However, even baryons may be more common than mesons involving the more exotic quarks. If our goal is to find new particles, those are likely to be hiding among the “none-of-the-above” products, the ones that leave experimenters scratching their heads, tearing their hair out, and double-triple-quadruple-checking their results to make sure they aren’t fooling themselves.

The final layers of DZero are ranks of muon detectors. Even though muons are like heavier cousins of electrons (with equal electrical charge), they interact differently. Mostly this is because every electron is identical, so when an electron approaches an atom, there’s a special symmetry involved in the interactions: the math is the same whichever electron is in the atom or outside it. While you can swap a muon in for an electron in an atom, muons are so short-lived they don’t last in that environment.[7] That mean there isn’t the symmetry between the electron in the atom and the muon outside it — and muons will tend to pass right through atoms without giving up much of their energy. To phrase another way, electrons “see” a whole atom as a potential barrier, but to a muon an atom is translucent.

As a result, muons detectors wrap around the rest of DZero, and comprise a very thick layer. As my guide, Dmitri Denisov, pointed out, the muons end up traveling through the metal electromagnets built to steer the charged particles through the rest of the detector, which is a complicated calculation. (I speak as someone who has taught electromagnetism on many occasions!)

Most neutrinos pass right through the whole detector without hitting anything. So, even though they carry a measurable amount of energy and momentum from the original collisions and subsequent decays, that won’t show up in any of the detectors. Both energy and momentum are conserved quantities, meaning the total amounts before and after the collision must balance. As with other detections, it’s a matter of working backwards: the missing stuff is most likely carried off by the invisible neutrinos, with relevant double-checks to make sure everything properly adds up before and afterwards.

[1] Spelling matters. If you write instead Tara Electron-Volt, you’re referring to the heiress of the Boston Electron-Volt family.
[2] According to the official history of Fermilab, director Robert Wilson loved the appearance of the Cockcroft-Walton generator so much, he wanted it to place it in the atrium where anyone could look at it. The members of the accelerator design team pointed out that it would require making the protons travel around a 90-degree bend, the engineers rebelled, and Wilson backed down. However, now that the Cockcroft-Walton generator is decommissioned, Wilson’s wish may be fulfilled.
[3] I’m leaving a lot of steps out of the process: the actual way the generators and accelerators work, storage of antiprotons, etc. Suffice to say the Tevatron is a major work of engineering, surprisingly delicate and precise for something so large and powerful. I’m also neglecting Felicia the Fermilab Ferret.
[4] Even though partons are fundamental constituents of matter, they are indifferent to you. Dolly partons, however, will always love you.
[5] Quarks don’t exist as isolated particles under anything but the highest energies and concentrations. As a result, a quark’s “bare mass”—the mass of a free quark—is hard to measure, and isn’t known very precisely even after decades of experiments.
[6] These are named after the symbol used to represent them, the Greek letter π (pi). Usually the name is pronounced like “pylon” without the “l”, but privately I like to pronounce it like “peon” because I am a twisted soul.
[7] The half-life of muons is about 2 millionths of a second, meaning if you have 100 muons, after 2 millionths of a second, about 50 will have decayed into electrons and neutrinos. The reason they’re able to get as far as they do in particle detectors is that they’re moving close to light-speed. That puts them into the realm of relativity, so from their perspective, the distance they travel is much shorter than what we measure.

2 responses to “Naming the animals in the particle zoo”

  1. […] Naming the animals in the particle zoo: The hows and whys of particle detection, in the context of the Tevatron at Fermilab. This excerpt also includes what may be my best joke yet, if I can say that about my own writing. […]

  2. Well written, Matthew. Shame that no publisher is picking it up. With writing like this, I would’ve thunk someone would’ve offered a contract. Egads. Is the science trade book market that bad that no one is buying? I’m curious because I’m throwing my (non-bowler) hat into the ring.

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