Primed: The smashing science behind particle accelerators

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Long before the Large Hadron Collider (LHC) could smash its first atoms, researchers manning the Tevatron collider at Fermilab, in a quiet suburb 40 miles west of Chicago, raced to find evidence that the Higgs boson exists. After roughly three decades of service, the Tevatron shut down for good in late 2011, dealing the city of Batavia's largest employer a significant blow. Less than 18 months later, the LHC (the Tevatron's technological successor) also went offline - albeit temporarily. Only four years after recording its first proton collisions, the team at CERN is already scrambling to upgrade the staggering LHC, which lies under parts of no less than five cities in both France and Switzerland. With the world's largest particle colliders smashing a whole lot of nothing together for the next two years at least, the field of high-energy physics research is starting to look resource-starved. Of course, many might ask why exactly we need giant atom smashers like this, or even how they work. It turns out that first part is quite a bit easier to answer than the second.

During the last several decades, particle accelerators have revealed the existence of elementary particles such as quarks, led to the discovery of antimatter and generally helped us unlock the mysteries of the universe. And once they were done splitting atoms and probing the darkest corners of theoretical physics, accelerators often led to breakthroughs in medical imaging and cancer research. So, as massive colliders seem ready to land on the endangered species list, it seems as good a time as any to explain what a particle collider is, how it works and what we as a society have to gain from the research.

What is a particle collider?

A computer-generated schematic of the LHC's 27km subterranean tunnel [blue] with vertical shafts at each of the four main experiments. (CERN)

Well, let's start a little broader, since a collider is actually just a particular subcategory of devices called particle accelerators. So, what exactly is a particle accelerator, you ask? It's more or less exactly what it sounds like: a device that propels charged particles at high speeds. Accelerators come in a variety of sizes, from the cathode ray tube in your grandparents' TV to the 17-mile-long Large Hadron Collider, but they all operate in basically the same way: an electromagnetic field fires particles (anything from hydrogen atoms to electrons or protons) in a concentrated beam. To build that momentum, the device can use either a static field (like the CRT) or an oscillating field, though the former is severely limited in the amount of energy it can generate without producing an electrical discharge. By using multiple, oscillating, lower-voltage sources, an accelerator is able to put much more oomph on a particle beam and approach the speed of light. Modern colliders use a special type of vacuum tube called a klystron to generate these waves of energy that push the particles along. These are actually souped-up versions of the same tubes that powered radar equipment for the Axis during World War II.

Diagramming ALICE (A Large Ion Collider Experiment), one of the four main experiments of CERN's LHC. (CERN)

Electrostatic accelerators are also limited to firing particles in a straight line, while ones powered by oscillating fields can curve the path of the beam with the help of magnets. In the case of facilities commonly called "atom smashers," those beams are set on a collision course with a target, be it stationary or a second stream of accelerated particles. When the particles hit their target, they release a massive amount of energy and throw off smaller component parts of themselves, such as quarks, the sub-subatomic particles that make up protons and neutrons (more on that later). A collider, specifically, is actually comprised of two accelerators built on top of one another that intersect at various points along ring-shaped tracks that guide the particles. Where the beams cross and the particles collide (hence the name), there are large detectors, several stories high in some cases, that record the subatomic wreckage and offer us a glimpse into physics at the smallest of scales.

These high-energy physics laboratories make up only the tiniest portion of active accelerators in the world, however. Of the tens of thousands of particle accelerators around the globe, most are built for ion implantation (often used in manufacturing semiconductors) or radiotherapy (used in the treatment of cancer). In fact, according to CERN, almost half of the particle accelerators in the world are used for medical purposes.

Gaining speed

Above: Vibrant nighttime view of Fermilab's 4-mile-circumference Tevatron accelerator, which began service in 1983 and operated for 28 years until it was shut down in 2011. (Fermilab); Below: An early cyclotron designed by American physicist Ernest Lawrence, circa 1932. (Photograph courtesy of the science museum / science & society picture library)

Accelerators basically come in two different shapes: linear and circular. In a linac (short for linear accelerator), there is only one chance to accelerate a particle to the desired speed. Since the beams travel in a straight line, the only way to make a particle travel faster is to crank up the energy or build a longer accelerator. Single-pass devices of this kind have their limitations, most obviously their need to have long stretches of continuous space available for construction. That's why the modest (and now shuttered) 4.26-mile-long, ring-shaped Tevatron dwarfs even the largest linear accelerator in the world, the 2-mile-long SLAC National Accelerator Laboratory.

There are actually many different varieties of circular accelerators. These use magnetic fields to control the trajectory of the beam, passing them through an acceleration chamber over and over again, gradually increasing the speed of the particles inside. The earliest was the cyclotron. Developed in 1929 by Ernest Lawrence at UC Berkeley, a cyclotron houses a pair of D-shaped magnets that accelerate a charged particle in an expanding orbit. A rapidly alternating current is applied to the magnets, which guides the particle in a growing spiral towards whatever the target happens to be. Although there are still some cyclotrons in service for research purposes, most are used for radiation therapy or PET scans.

In modern high-energy physics research, synchrotrons like the LHC are much more common. Instead of accelerating outwards from a central source, particles in a synchrotron are fired around a ring in a consistent track. This allows the particle to be accelerated indefinitely... at least in theory. As the particles approach the speed of light, the forces of relativity begin to act on them and they lose energy through radiation. The faster they go, the more radiation is generated. This synchrotron radiation can pose a problem for physicists simply looking to create the most powerful beam of particles possible, but it turns out that this is actually a useful source of X-rays, which can be manipulated to act like a microscope.

Inside the LHC

Professor Peter Higgs, after whom the Higgs boson particle was named, visits the CMS experiment section of the LHC. (CERN)

Perhaps the best way to understand how these devices work is to take a detailed look at one. And what better accelerator to use as an example than the world's largest and most powerful, the Large Hadron Collider? Protons race around the 17-mile track in opposite directions at 99.9999991 percent the speed of light, performing more than 11,000 complete laps every second. These protons don't just materialize inside the main ring. First, scientists must generate them by filling a cylinder with hydrogen and stripping away the electrons. Then the newly freed protons begin a long journey through several smaller accelerators, starting with the Linac 2. From there, they then pass on to the Proton Synchrotron Booster, the Proton Synchrotron and, finally, the Super Proton Synchrotron (SPS), a process that gradually pushes their energy levels towards 450 GeV (gigaelectronvolts) before entering the collider. The protons aren't simply spat out in a constant stream either. The SPS releases the particles in bunches, equally distributed between each track, traveling in opposite directions. This is to ensure that collisions happen at regular, predictable intervals.

Now, the other parts are essential and impressive feats of engineering in their own rights, but it's the collider itself that deservedly garners the most attention. The primary beam tube straddles the borders of France and Switzerland at a minimum depth of 160 feet, crossing back and forth between the countries four times. As the accelerator cranks up the speed, pushing the protons towards 7 TeV (teraelectronvolts), more than 1,600 magnets (most topping 27 tons) steer the subatomic particles. Those powerful magnets need to be chilled with liquid helium to a rather frosty 1.9 degrees Kelvin (about 456 degrees below zero Fahrenheit). The "track," as the researchers call it, which contains the proton beam is an extreme vacuum on the inside. In fact, there are fewer stray particles inside the track than there are in a similar volume of outer space. That vacuum is essential to preventing contaminants from colliding with the protons.

All of this hard work would be for naught without a way to actually study the subatomic flotsam cast off by the colliding particles. In total, the LHC ring houses seven experiments, spread out among four intersections, that seek to answer questions about the moments immediately following the Big Bang, why we live in a world composed of matter (as opposed to antimatter) and myriad other physics mysteries. At each of these points, the two separate tracks meet and magnets squeeze the beams together. Then, ignoring all advice from Dr. Spengler, they cross the proton streams, generating as many as 600 million collisions per second. The smashing of particles throws off gluons and quarks and all manner of exotic material while generating temperatures not reached since the moments after the Big Bang. The massive detectors at each site (ATLAS, CMS, ALICE and LHCb) use roughly 150 million sensors to record these fleeting events. For example, strange quarks (more on those later) decay into stable up or down quarks in the tiniest fraction of a second. These detectors obviously put out an absolutely massive amount of data - roughly 700MB per second, which is over a petabyte a month.

The Matter with Matter

An event recorded in 2012 within the CMS experiment, showing characteristics expected from the decay of the SM Higgs boson into a pair of photons (shown as yellow dashes and solid green lines). (CERN)

While fabricating semiconductors and fighting cancer are arguably more important and productive uses for particle accelerators, it's the atom smashers like the LHC that have captured the imagination of the public. This is partially thanks to a rather effective media blitz run by CERN, but also because of their scale and the potential to unlock the mysteries of the universe. After all, it's hard not to sit in awe of a marvel of engineering like the LHC, which, if uncurled, would be longer than Manhattan, or of the Tevatron, which used more than 1,000 magnets to steer beams of protons (clockwise) and antiprotons (counter-clockwise) in bunches many times smaller than a human hair. Those particles traveled at close to the speed of light into a head-on collision.

The exciting part is what happens when these particles do, in fact, smash together. In high school, you were probably told that matter was composed of atoms, and those atoms were composed of protons, electrons and neutrons, and that was it. In truth, protons and neutrons are hadrons, or composite particles, that are actually made up of even smaller constituents called quarks. When these subatomic particles collide at speeds approaching that of light, they generate intense amounts of heat and throw off quarks, including rare ones that don't often occur naturally in the universe and are given names like "strange" and "charm." Up and down quarks are more common and make up the vast majority of observable matter in the universe. Their more oddly named siblings decompose pretty quickly, stabilizing as either up or down.

Gluons are elementary particles that mediate the strong nuclear force between quarks. If that sentence makes your head spin a bit, don't worry, all you really need to know is that they're exchange particles, almost like neurotransmitters in the human body, controlling the strong nuclear interaction simply by travelling back and forth. One of the major purposes of the LHC is to study quark-gluon plasma, a thick soup of these matter-building blocks that existed in the extreme heat and density following the Big Bang.

Hunting the Higgs

A transverse view of the same 2012 event shown above. While possibly indicating the Higgs boson, these results "could also be due to known Standard Model background processes." (CERN)

The most exciting (and vaguely anticlimactic) purpose of the LHC, however, was to find evidence of the Higgs boson. This theoretical elementary particle has no spin, electric charge or color charge, making it unique among elementary particles. It's also believed to be extremely unstable, decaying into another particle almost immediately after bursting into existence. Proof of the Higgs boson would go a long way towards confirming the existence of the Higgs field, an essential part of the Standard Model of particle physics, which has informed scientific research for much of the last 50 years. The Higgs field would explain why some particles have mass, even when other factors suggest they should be massless, and why the weak nuclear force has a much shorter range than the electromagnetic force. It would also mark an end to a 40-year search for a solution to one of the greatest unanswered questions in all of physics. The Higgs boson wouldn't lead to any immediate technological breakthroughs, but it would validate decades of scientific research and guide the search for a unified theory of everything that reconciles gravity with the other forces in nature: electromagnetic, weak and strong interactions.

In March 2013, researchers announced that the LHC detected a spinless particle that fit the Higgs boson's profile in many ways. Though the scientists were unable to say for sure that it was, in fact, a Higgs particle, they are quite confident that further study will prove that it is. The one major concern was its rather sizable mass. The problem is that, although there's still plenty of number crunching to do, we won't be any closer to confirming the existence of the Higgs boson without more experimentation. But, with the LHC offline until 2015 and the Tevatron shuttered for good, there's little hope that we'll be adding more fuel to the Standard Model fire anytime soon. The biggest shame is that there are a small handful of accelerators left performing high-energy physics research. And none can even approach the levels of energy generated at the two aforementioned colliders.

The Sky is Falling; the Sky is Falling!

No discussion of the LHC would be complete without addressing the fear that it would destroy the Earth. A small, but vocal set of alarmists worried that by colliding particles at such high speeds, the accelerator would create microscopic black holes that would devour the Earth and all life on it. Obviously, this never happened. While the basic logic seems sound (smashing particles into an extremely small space could cause the resulting mass to collapse in on itself creating an inescapable well of infinitely dense matter), the science simply doesn't back up the apocalyptic paranoia. If, and that is a big if, the LHC could create such tiny black holes, it turns out there would be very little reason to be afraid. For one, collisions of much higher energies happen naturally in the universe all the time. As of yet, none of these collisions have led to the creation of a world-eating black hole. Additionally, black holes actually evaporate slowly through Hawking radiation. With such a low amount of mass, these micro black holes would likely evaporate very quickly.

Now what?

The ALICE absorbers and surrounding structures -- totaling three main sections and weighing an aggregate 400 tons -- were precisely aligned at a tolerance of 1-2 mm. (CERN)

Well, accelerator research isn't about to end anytime soon. While high-energy physics labs are going through lean times, there will always be room for research into medical imaging and synchrotron light sources, which put all that pesky X-ray radiation to use. But let's not forget, the LHC is not dead yet. The collider has only been shut down temporarily, as researchers believe they've received the best results they can out of this particular hardware iteration. Now many of the LHC's components are undergoing a significant retrofit that should lead to higher energies and more accurate measurements. CERN tentatively scheduled its return to service for 2015 and, when it fires back up, it could possibly push 14 TeV - roughly twice its current energy levels. And a second round of proposed upgrades in 2018 will help the LHC reach even greater heights, bumping up its luminosity.

The next great leap is expected to come from a new generation of colliders, such as the International Linear Collider or the Compact Linear Collider, which are backed by competing conglomerates of scientific institutions from around the globe. These proposed linear accelerators, neither of which is far enough along to have a geographical home yet, would smash electrons into their antimatter counterpart, positrons. Since they're linear and use elementary particles, the ILC and CLIC would be capable of generating much more accurate results than the comparatively clumsy hadron colliders that have been common over the last several decades. The ILC, if approved, would stretch a minimum of 19 miles, making it not just the largest linear accelerator ever built, but also the largest particle accelerator ever. Though the total energy generated by collisions at the ILC would be lower than at the LHC, the accuracy of the data collected would be much greater. The ILC could be the key to nailing down the Higgs boson as well as potentially unlocking the existence of extra dimensions, and discovering candidates for dark matter. But, we're sure it will raise just as many questions as answers.

Have we armed you with the knowledge necessary to build your own personal proton collider? No, we probably couldn't teach you how to build a salad spinner. But hopefully the next time you spot an article about the LHC, you'll have a better appreciation for the work that goes on there. And, more importantly, you'll have to look up a few less things on Wikipedia to make sense of it.