Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Journey to the Heart of the Large Hadron Collider
The ATLAS complex, home to the largest scientific measuring device in the world dedicated to particle physics, offers no hint of its grandeur from street level. From Route de Meyrin, the heavily trafficked road that separates its ground-level structure from CERN’s main campus, it looks just like a warehouse near an ordinary filling station. Until I walked through the doors of its main entrance, I wasn’t quite sure what to expect.
CERN, the European Organization for Nuclear Research (from the French acronym for Conseil Européen pour la Recherche Nucléaire), located on the Swiss-French border near Geneva, prides itself on its openness. Unlike a military facility, it allows anyone with permission to visit to snap pictures anywhere. Nevertheless, the dangerous and delicate nature of modern particle detectors warrants extremely tight protocols for entering the “caverns” where they are housed.
To tour a site that wasn’t yet completely finished, I put on a helmet like that a construction worker would use. My hosts, researchers Larry Price and Charlie Young, were wearing radiation badges—practice for the precautions needed when the beam line would be running. After getting final clearance, they entered the code for the gateway to the inner sanctum, and it electronically opened.
Before journeying underground, we viewed the two enormous shafts used to lower the detector’s components more than 350 feet down. I stood on the brink of one of these wells and gazed into the abyss. Awe and vertigo rivaled for my attention as I craned my neck to try to see the bottom.
Above the other well a giant crane was poised to lower parts down below. Transporting the original components of the detector from the various places they were manufactured and then putting them all together within such deep recesses in such a way that their delicate electronics weren’t destroyed surely was an incredible undertaking. The meticulous planning for such a complex project was truly phenomenal—and it continues.
We took a speedy elevator ride down to what is called “beam level.” Now we were at the same depth as the beam pipes for the Large Hadron Collider (LHC)—the vast ring that will be used to collect protons and other particles, accelerate them in opposite directions, and smash them together at record energies. The ATLAS detector is located at one of the intersection points where protons will collide head on. Half of the detector is above the beam level and half below to accommodate the floods of particles gushing out in all directions from these crashes.
The passages from the elevator to the control rooms and viewing platform are twisty, to provide barriers in case of a radiation leak. Most forms of radiation cannot travel through thick walls. Gauges sample the radiation levels to try to minimize human exposure.
The air in the hallways below seemed a bit stale. Pumped in by means of the ventilation system, it is monitored extremely closely. One of the components of the detector is liquid argon, cooled to only a few degrees above absolute zero—the baseline of temperature. If for some reason the argon heated up suddenly, became gaseous, and leaked, it could rapidly displace all of the breathable air. Warning systems are everywhere; so if such a danger were imminent, workers would be urged to escape via elevator before it was too late.
I finally reached the viewing platform and was astounded by the panorama in front of me. Never before had I seen such a vast array of sparkling metal and electronics, arranged in a long, horizontal cylinder capped with a giant shiny wheel of myriad spokes—nicknamed the “Big Wheel.” It was like encountering the largest alien spaceship imaginable—docked in an equally vast spaceport.
If massive new particles are discovered in coming years—such as the Higgs boson, theorized to supply mass to other natural constituents—this could well be the spawning ground. It would come not with the push of a button and a sudden flash but rather through the meticulous statistical analysis of gargantuan quantities of data collected over a long period of time. Not quite as romantic as seeing a new particle suddenly materialize from out of the blue, but statistics, not fireworks, is the way particle discoveries transpire these days.
The inner part of the detector had been hermetically sealed long before my visit, so it was impossible to see inside. Of the outer part, I could barely make out the vast toroidal (doughnut-shaped) magnets that serve to steer the charged elementary particles that escape the inner core. Researchers call these penetrating particles, similar to electrons but more massive, muons. Indeed the main reason for the detector’s huge size is to serve as the world’s finest muon-catcher. Each “spoke” of the Big Wheel is a muon chamber.
Modern particle detectors such as ATLAS are a bit like successive traps—each designed to catch something different. Particles that slip through one type might be caught in another. The net effect of having an assortment is to capture almost everything that moves.
Suppose a house is full of various vermin and insects. Placing a mousetrap on the kitchen floor might snag some of the rodents but would allow ants to roam free. An ant trap might lure some of those insects but would be inadequate for flies. Similarly, ATLAS consists of multiple layers—each designed to pin down the properties of certain categories of particles.
Some particles, such as electrons and photons (particles of light), are captured by one of the inner layers, called the electromagnetic calorimeter. That’s where the light-sensitive liquid argon takes part in measuring their energy released. Other particles, such as protons and neutrons, are stopped by a denser layer just beyond, called the hadronic calorimeter. Those were the inner layers I couldn’t see.
For the most part, muons evade the inner detector completely. They are the only charged particles that manage to make it through. That’s why the outer envelope of the detector is called the muon system. It consists of what looks like a sideways barrel—centered on the beam line—framed on both sides by two massive “end-caps.” These serve to track as many muons as possible—making the detector ideal for any experiments that produce such particles.
As good as ATLAS is at trapping particles, some do manage to make it through all of the hurdles. Extremely lightweight neutral particles, called neutrinos, escape unhindered. There’s not much the researchers can do about that, except to calculate the missing energy and momentum. Neutrinos are notoriously hard to detect. Also, because the beam line obviously can’t be plugged up, collision products are missed if they head off at low enough angles. Given that most interesting results involve particles whirling off at high angles from the beam line, the lack of information about those traveling close to the beam line is not critical.
A Medusa’s head of cables connects the detector’s delicate electronics with the outside world, allowing for remote data collection. These connections make it so that relatively few scientists working on the project will need to venture down to the actual detector, once it is operational. Using a system called the Grid, scientists will be able to access and interpret the torrent of information produced via computers located in designated centers around the globe. Then they will look for special correlations, called signatures, corresponding to the Higgs boson and other sought-after particles.
It was humbling to think that the huge artificial cave housing ATLAS comprises but a portion of the LHC’s scope. Through one of the cavern walls, beam pipes extend from ATLAS into the formidable tunnel beyond. Miles from where I stood lay other grottos housing various other experiments: CMS, a multipurpose detector with a strong central magnet; ALICE, a specialized detector designed to examine lead-ion collisions; LHCb, another specialized detector focusing on the interactions of what are called bottom quarks; and several other devices.
Back on the surface, I took some time to explore the French countryside above the LHC ring. Most of the seventeen-mile-long circular tunnel lies under a bucolic border region known as Pays de Gex, or “Gex Country.” Passport in hand, I caught the bus from Geneva’s central station that heads toward the French village of Ferney-Voltaire. According to the map I had with me, the village is roughly situated above one part of the LHC tunnel.
In that quaint locale, where Voltaire once philosophized, mail is still delivered by bicycle. Boulangeries bake fresh baguettes according to the ancient tradition, and fromageries serve regional cheeses such as tangy Bleu de Gex. Stucco houses, faded yellow or green and capped with burgundy-tiled roofs, line the roads out of town. On the face of it, the community seemed little touched by modernity. The illusion was suddenly broken when I saw a white van turn the corner. Its prominently displayed CERN logo reminded me that this cozy village and its pastoral surroundings play a role in one of the leading scientific endeavors of the twenty-first century.
Back on the Meyrin campus of CERN, I noted a similar juxtaposition of old and new. CERN is a laboratory keenly aware of its history. Its streets are named after a wide range of people who have spent their careers trying to discover the fundamental components of nature—from Democritus to Marie Curie and from James Clerk Maxwell to Albert Einstein. Scattered around its museum area are an assortment of accelerators and detectors of various shapes, sizes, and vintage. Comparing the small early detectors to ATLAS served to highlight the unbelievable progress made in particle physics during the last seventy-five years.
CERN makes good use of many of its historical devices. Particles entering the LHC will first be boosted by several different older accelerators—the earliest built in the 1950s. It is as if the spirits of the past must offer their blessings before the futuristic adventures begin.
With this lesson in mind, before propelling ourselves into modern issues and techniques, we must first boost ourselves up to speed with a look at the history of elementary particles and the methods used to unravel their secrets. I intend this book not just as a guide to the Large Hadron Collider and the extraordinary discoveries likely to be made there, but also as a scientific exploration of humankind’s age-old quest to identify nature’s fundamental ingredients. Like a ride through a high-energy accelerator, it is a fantastic journey indeed.