How to Make an Apple Pie from Scratch

by Harry Cliff

  Things are different for the gluons. Each gluon carries a combination of color and anti-color, and since gluons are attracted to colored particles, they will actually interact with each other. This means that the strong force between two quarks is completely different from the electromagnetic force between, say, a proton and an electron.

  We’re almost at the point of understanding why no one has ever seen a naked quark, as it were, so bear with me. Imagine an electron and a proton sitting a little distance apart, like they might in a hydrogen atom. One way to conceptualize the electromagnetic force between them is to imagine the proton and electron both firing off photons in every direction,*6 a bit like one of those spherical lights you sometimes still see at 1980s-style discos. Since the proton and electron are close together, a large number of the photons emitted by the electron will be attracted toward the proton and absorbed, and vice versa. It’s this exchange of photons that creates the attractive force between the two charged particles.

  Now imagine that we grab hold of the electron and proton and start to pull them apart. As the distance between them increases, fewer and fewer of the emitted photons will be absorbed by the opposing partner, and the attractive force between the electron and proton gets weaker and weaker. At first you have to work hard against the attractive force, but as you separate the two particles, it gets easier and easier until eventually you’re left with a free electron and a free proton.

  Now let’s consider the equivalent situation for two quarks. Instead of photons, the two quarks now fire off gluons in every direction. Gluons that get shot out in the direction of the other quark get attracted to it and absorbed, creating an attractive force just like with the proton and electron. However, this is where the fact that gluons carry color starts to change things. The exchange of gluons creates an excess of color in the region between the two quarks. You can picture the gluons flowing back and forth between the two quarks as a tube of red, green, and blue, with a quark on either end. This colorful tube attracts other nearby gluons, drawing them into the gap and making the tube even denser and more colorful. Eventually there is so much color in the tube that all of the gluons emitted by both quarks get sucked in—forming a mighty multicolored bond between the two quarks.

  Now let’s say we decide to try to separate the two quarks. We grab hold and start to pull. It’s bloody hard work, but gradually the quarks start to creep apart. However, since all the gluons are still concentrated in a tube between the two quarks, the force we’re fighting against doesn’t get any weaker. Instead, the gluon tube stretches like an elastic band, and just like an elastic band, as we stretch it, more and more energy gets stored in the tension of the tube. Now—and this is the fun part—once the amount of energy stored in the tension of the gluon tube is equal to the mass of a new quark-antiquark pair, the tube catastrophically snaps, but instead of ending up with two free quarks, a new quark and antiquark are created from the energy stored in the stretched gluon tube, each attaching themselves to one of the broken ends. What we’ve got are two pairs of quarks, each of them still locked firmly together.

  This is the reason we have never seen a bare quark. Try to pull a quark out of a hadron and like a magician pulling handkerchiefs out of their sleeve, you instead end up with an ever-growing chain of hadrons, which gets longer and longer the harder you pull. When we smash protons together at the LHC, instead of knocking quarks out, we end up with great jets containing dozens of hadrons, all created from the energy of the initial kick that sent the original quarks flying apart.

  From this kind of argument, it seemed that quarks were doomed to remain trapped inside hadrons forever. But in 1973 the theorists David Gross, Frank Wilczek, and David Politzer made a stunning discovery about the nature of the strong force. They calculated that as you collide hadrons at ever higher energies, the viselike grip of the strong force should start to weaken. This implied that at sufficiently high energies, the strong force becomes so weak that hadrons effectively melt, turning into a superheated gas of free quarks and gluons.

  This superheated stuff is known as a “quark-gluon plasma,” a stupendously hot and dense state of matter where quarks and gluons are finally free to zip about outside the confines of individual hadrons. To create one requires temperatures and densities far beyond anything that had ever been achieved in the lab in the mid-1970s. In fact, there was only one time in the universe’s history when conditions had been extreme enough to create a quark-gluon plasma, that crucial first millionth of a second after the big bang.

  Back then, the universe was so hot and dense that no hadrons could form; the entirety of space would have been filled with this seething mass of quarks and gluons. However, as the universe expanded it cooled, and after about a microsecond the temperature dropped low enough for quarks and gluons to fuse together to form the first protons and neutrons. That means that if physicists wanted to understand the ultimate origins of matter, they would need to find a way to study a quark-gluon plasma in the lab.

  Enter RHIC, a 4-kilometer-circumference collider buried in a shallow tunnel cut through the soft sandy soil of Long Island. The principle of RHIC is similar to any other collider: two beams of particles are fired around the roughly hexagonal ring, one going clockwise, the other counterclockwise, kept on course by powerful electromagnets. On each orbit of the ring, high voltage electric fields give a kick to the particles as they pass by, gradually increasing their energy. Once the particles have reached the desired energy, the paths of the two beams are adjusted using magnets until they collide head-on inside large detectors, whose job is to record the subatomic debris that comes flying out from the collisions.

  What makes RHIC different from other colliders is the projectiles it uses. As the name—Relativistic Heavy Ion Collider—suggests, RHIC’s primary goal is to collide ions*7 of heavy elements, including aluminum, copper, uranium, and, sexiest of all, gold. The nuclei of these elements contain hundreds of protons and neutrons, and so when they collide enormous densities are created, potentially high enough to produce a quark-gluon plasma.

  I had come to Brookhaven to meet Helen Caines and Zhangbu Xu, the two leaders—referred to as “spokespeople”—of the STAR experiment,*8 one of two large detectors used to study the collisions produced by RHIC. We met over coffee in a large reception building close to the entrance to the Brookhaven site, a collection of office buildings and experimental halls spread across 21 square kilometers of land surrounded by dense woodland.

  Sitting among the hubbub of Brookhaven staff getting their first vital caffeine injection of the day, Helen and Zhangbu talked me through the highs and lows of two decades studying the universe’s most extreme state of matter. Helen cut her teeth as a PhD student at the University of Birmingham before crossing the Atlantic for her first research job in 1996. For someone interested in quark-gluon plasma, there was no better place to be in the late nineties. RHIC was only a few years away from delivering its first collisions, and as a young researcher, Helen got in on the ground floor, joining the STAR Collaboration as soon as she arrived in the United States. At the time, her future co-spokesperson, Zhangbu, was working on his PhD at Yale, having originally studied physics in his native China. When data collecting began at RHIC, the two young physicists would be perfectly placed to lead the search for quark-gluon plasma.

  However, before the experiments kicked off, the physicists at RHIC found themselves having to deal with unexpected headlines in the press courtesy of Hawaii resident Walter L. Wagner. Wagner was worried that the high-energy collisions at RHIC might end up destroying the world and obligingly provided a menu of doomsday scenarios to choose from. The collider might produce a tiny black hole that would gobble up the Earth or perhaps synthesize a new form of “strange matter” that would convert the planet into a formless blob. Most exciting of all was the prospect of creating a bubble universe with different laws of physics, which would then expand at the speed of
light, destroying not just our planet, but the entire cosmos.

  The theoretical physicist Frank Wilczek jumped in to debunk Wagner’s concerns, but that only seemed to fuel media interest, and eventually Brookhaven was forced to produce a lengthy report detailing why their new collider was unlikely to lead to the end of days.*9 Things calmed down after that, but it didn’t stop Wagner filing twin lawsuits in New York and San Francisco in an attempt to halt the start of collisions. Fortunately, when the first gold nuclei smashed into one another at Brookhaven on June 12, 2000, the world carried on turning.

  In the early days following the start of data collecting, some theorists were keen to claim that a quark-gluon plasma had already been created by RHIC based on the measurements taken by STAR and the other three detectors operating at the time. However, Helen, Zhangbu, and their experimental colleagues were far more cautious.

  The great challenge of knowing whether you’ve made a quark-gluon plasma is that it’s impossible to measure its properties directly. When two gold nuclei collide at RHIC the superheated blob of matter they form only exists for an instant. After a mere ten-trillionths of a trillionth of a second, this tiny fireball expands and cools down, transforming into an explosion of thousands of hadrons that tear through the detector at close to the speed of light.

  These hadrons are all that STAR sees. It’s only by studying their properties that you can infer whether a quark-gluon plasma formed. However, as time passed the physicists at RHIC started to see telltale signs. First of all, they found that the thousands of hadrons seen by their detectors in each collision were flowing out from the impact point in a collective way, like the movement of a herd of wildebeest across a plain, strongly implying that they all originated from a single unified blob of matter. What’s more, the number of jets produced by each collision was far lower than expected, almost as if the quarks were getting slowed down as they waded through a thick quark-gluon soup preventing them from converting their kinetic energy into jets of hadrons.

  It took five years for the physicists at RHIC to be sure, but in 2005 they were ready to announce to the world that they had pulled it off—they had created a state of matter that hadn’t existed in the universe since the big bang. They estimated that the quark-gluon plasma they’d produced had a temperature of around 2 trillion degrees, 130,000 times hotter than the center of the Sun, and had a density of around a billion tons per cubic centimeter.

  Most extraordinary of all, its bulk properties were completely different from what they had been expecting. Rather than a gas of free quarks and gluons, it behaved like a liquid, and not just any liquid, but a near perfect fluid. This strange substance seemed to flow without any internal resistance or stickiness (or to put it more technically, it has almost zero viscosity). During the first microsecond the universe was filled, not by a fireball, but with a trillion-degree soup.

  Our coffees drunk, we bid goodbye to Zhangbu, and Helen took me off to see STAR in the flesh. On the way we picked up her colleague and technical coordinator of the experiment, Lijuan Ruan. Like Zhangbu, Lijuan hails from China and arrived at Brookhaven as a young graduate student in 2002. Since then she’s been deeply involved in all aspects of the experiment but particularly likes getting her hands dirty. Her delight and pride in the detector were evident: “It’s only getting hands on with the hardware that you really start to get a feel for how the whole thing works.”

  The huge hall housing the STAR detector was on the other side of the campus so we jumped in Helen’s car for a short drive, passing the aptly named Thomson Road and Rutherford Drive along the way. First stop was the control room, a dark bunker-like room with dozens of ancient-looking rear-projection computer screens used to monitor the performance of the experiment. Compared to the gleaming modernity of the LHC, the whole place had a distinctly well-worn feel. Unsurprising, perhaps, for an experiment about to enter its third decade.

  From the control room we walked into a large hangar with a heavyset shield wall made from monolithic concrete blocks at one end. Much to my surprise there was no iris scan or radiation procedures to follow—the levels of radioactivity are well within safe limits as long as RHIC isn’t running—and before I knew it, I found myself standing beneath the hulking mass of the STAR detector.

  Weighing in at 1,200 tons and the size of a three-story building, the detector makes quite an impression when you first come face-to-face with it. The bulk of the barrel-shaped detector is made up of a huge electromagnet used to bend the particles as they come flying out from the collision point, allowing physicists to measure their momenta. Nestling inside the magnet is the delicate STAR tracking system, which reconstructs the trajectories of the thousands of charged particles that are released as each tiny blob of quark-gluon plasma expands and cools. On the day I visited, STAR had been opened up, allowing me to see into the heart of the detector, complete with twinkling LED lights that made it look like something out of a science fiction movie.

  As we stood on a raised gantry looking into the glowing heart of the detector, Helen and Lijuan told me about their plans for the next run of RHIC and the STAR experiment. Now that they’re routinely able to create and study quark-gluon plasma, the team is closing in on a crucial moment in our universe’s history, and one that’s critical to our story. Around a microsecond after the big bang, the temperature of the universe dropped enough that the quark-gluon plasma transformed into the first protons and neutrons. This is what physicists refer to as a “phase transition,” much like a liquid freezing to form solid ice. The plan for the next experimental run is to use RHIC to continually adjust the energy of the collisions, which roughly corresponds to varying the temperature of the quark-gluon plasma. The higher the energy of the colliding ions, the higher the temperature.

  By slowly scanning through the collision energy, Helen and her colleagues hope to pinpoint the tipping point where a quark-gluon plasma “freezes” to form hadrons. Figuring out how this process happened—effectively how protons and neutrons were cooked in the big bang—could have a profound influence on our understanding of how the first elements formed.

  Having led the world in particle physics for the second half of the twentieth century, RHIC is now the United States’ only remaining particle collider. For some years there were serious doubts about whether the research program led by STAR and its friendly rival and neighbor on the ring, PHENIX,*10 would continue to be funded. During the 2000s, RHIC was the only show in town when it came to studying quark-gluon plasmas, but in 2010 CERN’s Large Hadron Collider got in on the action with its own dedicated heavy ion experiment, ALICE.*11 In 2012, the far higher energy of the LHC allowed ALICE to smash RHIC’s record for the highest ever recorded temperature, when lead ion collisions provided by the LHC produced a quark-gluon plasma with a temperature of more than 5.5 trillion degrees.

  But while the LHC may dwarf RHIC in size and energy, there are still a few tricks its European rival cannot match. In particular, RHIC’s ability to reduce its collision energy to lower values than the LHC means it’s the only collider that can search for the critical point when free quarks and gluons fuse to form hadrons. In the short term at least, the funding situation for America’s last collider looks pretty rosy. With luck, it won’t be long before Helen, Zhangbu, Lijuan, and their colleagues close in on the ultimate recipe for a proton.

  Skip Notes

  *1 The reason for this has to do with Heisenberg’s uncertainty principle in quantum mechanics, which is a bit of a diversion for the purposes of our story right now. We’ll come back to it.

  *2 A gigaelectron volt is a billion electron volts, the kinetic energy of an electron after it’s been accelerated through a billion volts.

  *3 At the time it was claimed to be the world’s straightest object.

  *4 Before you try to break into CERN, I should add that things have been tightened up a bit since.

 
*5 Today, we know of six quarks in total. The up, charm, and top quark with electric charges of +²⁄₃ and the down, strange, and bottom quarks with electric charges of -¹⁄₃.

  *6 There is a subtlety here—the photons fired out by the particles in this example aren’t real, observable photons like the ones produced by a lightbulb. Instead, they’re what we call “virtual” particles. Virtual particles are completely undetectable and are only really a crutch for thinking about how forces are transmitted between particles. To be honest, I don’t find the concept of virtual particles particularly helpful—a far better explanation involves physical entities called “quantum fields,” which we’ll come to soon enough—but they are useful for the purposes of this analogy.

  *7 In this case an ion is an atom that has been stripped of some electrons, giving it an overall positive charge.

  *8 If you’re into acronyms, STAR stands for Solenoidal Tracker at RHIC.

  *9 The main reason why RHIC was unlikely to destroy the world is that cosmic rays with far higher energies than the collisions at RHIC have been bombarding the Earth, Moon, and other celestial bodies for billions of years. If producing world-destroying black holes, strange matter, or bubble universes was possible it would already have happened and we wouldn’t be here.

  *10 Wondering what PHENIX stands for? Well it’s Pioneering High Energy Nuclear Interaction eXperiment, apparently.

  *11 ALICE stands for A Large Ion Collider Experiment—a rare example of an acronym for a particle physics experiment that actually works.

  CHAPTER 9

  What Is a Particle, Really?