Knocking on Heaven's Door (22 page)

The collisions we’ll study at the LHC are akin to those that took place in the first trillionth of a millisecond after the Big Bang. They will teach us about small distances and about the nature of matter and forces at this very early time. You might think of the Large Hadron Collider as a super-microscope that allows us to study particles and forces at incredibly small sizes—on the order of a tenth of a thousandth of a trillionth of a millimeter.

The LHC achieves these tiny probes by creating higher energy particle collisions than ever before achieved on Earth—up to seven times the energy of the highest existing collider, the Tevatron in Batavia, Illinois. As explained in Chapter 6, quantum mechanics and its use of waves tells us these energies are essential for investigating such small distances. And—along with the increase in energy—the intensity will be 50 times higher than at the Tevatron, making discovering the rare events that could reveal nature’s inner workings that much more likely.

Despite my resistance to hyperbole, the LHC belongs to a world that can only be described with superlatives. It is not merely large: the LHC is the biggest machine ever built. It is not merely cold: the 1.9 kelvin (1.9 degrees Celsius above absolute zero) temperature necessary for the LHC’s superconducting magnets to operate is the coldest extended region that we know of in the universe—even colder than outer space. The magnetic field is not merely big: the superconducting dipole magnets generating a magnetic field more than 100,000 times stronger than the Earth’s are the strongest magnets in industrial production ever made.

And the extremes don’t end there. The vacuum inside the proton-containing tubes, a 10 trillionth of an atmosphere, is the most complete vacuum over the largest region ever produced. The energy of the collisions are the highest ever generated on Earth, allowing us to study the interactions that occurred in the early universe the furthest back in time.

The LHC also stores huge amounts of energy. The magnetic field itself stores an amount equivalent to a couple of tons of TNT, while the beams store about a tenth of that. That energy is stored in one-billionth of a gram of matter, a mere submicroscopic speck of material under ordinary circumstances. When the machine is done with the beam, this enormously concentrated energy is dumped into a cylinder of graphite composite eight meters long and one meter in diameter, which is encased in 1,000 tons of concrete.

The extremes achieved at the LHC push technology to its limits. They don’t come cheaply and the superlatives extend to cost. The LHC’s $9 billion price tag also makes it the most expensive machine ever built. CERN paid about two-thirds of the cost of the machine, with CERN’s 20 member countries contributing to the CERN budget according to their means, ranging from 20 percent from Germany to 0.2 percent from Bulgaria. The remainder was paid for by nonmember states, including the United States, Japan, and Canada. CERN contributes 20 percent to the experiments themselves, which are funded by international collaborations. As of 2008, when the machine was essentially built, the United States had more than 1,000 scientists working on CMS and ATLAS and had contributed $531 million toward the LHC enterprise.

THE BEGINNING OF THE LHC

CERN, which houses the LHC, is a research facility, with many programs operating simultaneously. However, CERN’s resources are generally concentrated in a single flagship program. In the 1980s, that program was the
SpbarpS
collider,
³⁸
which found the force carriers essential to the Standard Model of particle physics. The stellar experiments that took place there in 1983 discovered the weak gauge bosons—the two charged
W
bosons and the neutral
Z
boson, which communicate the weak force. Those were the key missing Standard Model ingredients at the time, and the discovery earned the accelerator project leaders a Nobel Prize.

Even so, while the SpbarpS was operating, scientists and engineers were already planning a collider known as LEP, which would collide together electrons and their antiparticles known as positrons to study the weak interactions and the Standard Model in exquisite detail. This dream came to fruition in the 1990s, when through its very accurate measurements, LEP studied millions of weak gauge bosons that taught physicists a great deal about Standard Model physics interactions.

LEP was a circular collider with a 27 kilometer circumference. Electrons and positrons were repeatedly boosted in this ring as they orbited around. As we saw in Chapter 6, circular colliders can be inefficient when accelerating light particles such as electrons, since such particles radiate when accelerated on a circular path. The electron beams at the LEP energy of about 100 GeV lost about three percent of their energy each time they went around. This wasn’t too great a loss, but if anyone had wanted to accelerate electrons around this tunnel at any higher energy, the loss during each rotation would have been a deal breaker. Increasing the energy by a factor of 10 would have increased energy loss by a factor of 10,000, which would have made the accelerator far too inefficient to be acceptable.

For this reason, while LEP was being envisioned, people were already thinking about CERN’s next flagship project—which would presumably run at even higher energy. Because of the electron’s unacceptable energy losses, if CERN was to ever build a higher-energy machine, it would require proton beams, which are much heavier and therefore radiate much less. The physicists and engineers who developed LEP were aware of this more desirable possibility so they built the LEP tunnel sufficiently wide to accommodate a possible proton collider in the future, after the electron-positron machine would be dismantled.

Finally, some 25 years later, proton beams now race through the tunnel originally excavated for LEP. (See Figure 24.) The Large Hadron Collider is a couple of years behind schedule and about 20 percent over budget. That’s a pity, but perhaps not so unreasonable given that the LHC is the biggest, most international, most expensive, most energetic, most ambitious experiment ever built. As the screenwriter and director James L. Brooks jokingly said when hearing about the LHC’s setbacks and recovery, “I know people who take approximately the same amount of time to get their wallpaper just so. Understanding the universe just might have a better kick to it. Then again there’s some pretty great wallpaper out there.”

[
FIGURE 24
]
The setting for the Large Hadron Collider, with the underground tunnel illustrated in white, and Lake Geneva and mountains in the background. (Photo courtesy of CERN)

THE FELLOWSHIP OF THE RINGS

Protons are everywhere around and within us. However, they are generally bound into nuclei surrounded by electrons inside atoms. They aren’t isolated from those electrons and they aren’t collimated (aligned into columns) inside beams. The LHC first separates and accelerates protons and then steers them to their ultimate destiny. In doing so, they utilize the LHC’s many extremes.

The first step in preparing proton beams is to heat hydrogen atoms, which strips off their electrons and leaves the isolated protons that are their nuclei. Magnetic fields divert these protons so that they are channeled into beams. The LHC then accelerates the beams in several stages in distinct regions, with the protons traveling from one accelerator to another, each time increasing their energy before they are diverted from one of the two parallel beams so that they can collide.

The initial acceleration phase takes place in CERN’s
linac,
which is a linear stretch of tunnel along which radio waves accelerate protons. When the radio wave is peaked, the associated electric field accelerates the protons. The protons are then made to drift away from the field so they don’t decelerate when the field goes down. They subsequently return to the field when it peaks again so that they repeatedly accelerate from one peak to the next. Essentially the radio waves pulse the protons in the way you push a child on a swing. The waves thereby boost the protons, increasing their energy, but only a tiny amount in this first acceleration stage.

In the next stage, the protons are kicked via magnets into a series of rings where they are further accelerated. Each of these accelerators functions similarly to the linear accelerator described above. However, because these next accelerators are ring shaped, they can repeatedly boost the protons’ energies as they circle around thousands of times. These circular accelerators thereby transfer quite a bit of energy.

This “fellowship of the rings” that accelerates protons before they enter the large LHC ring consists of the proton synchrotron booster (PSB) that accelerates protons to 1.4 GeV, the proton synchrotron (PS) that brings them up to 26 GeV in energy, and then the super proton synchrotron (SPS) that raises their energy to the so-called injection energy of 450 GeV. (See Figure 25 to see a proton’s journey.) This is the energy the protons carry when they enter the last acceleration stage in the large 27 kilometer tunnel.

A couple of these accelerating rings are relics of previous CERN projects. The proton synchrotron, which is the oldest, celebrated its golden anniversary in November 2009, and the proton synchrotron booster was critical to the operation of CERN’s last major project—namely, LEP—in the 1980s.

After protons leave the SPS, their 20 minute long
injection phase
begins. At this point the 450 GeV protons that emerged from the SPS are boosted to their full energy inside the large LHC tunnel. The protons in the tunnel travel along two separate beams going in opposite directions through narrow three-inch pipes that extend on the 27 kilometers of the underground LHC ring.

[
FIGURE 25
]
The path a proton travels on when accelerated by the LHC.

The 3.8 meter (12 ft.) wide tunnel that was built in the 1980s but that now houses the proton beams in their final acceleration stage is well lit and air conditioned and large enough to comfortably walk around in, as I had the opportunity to do while the LHC was still in the construction phase. I took only a short stroll inside the tunnel on my LHC tour, but it still took me far longer to traverse my few steps than the 89 millionths of a second it takes for the accelerated highly energetic protons traveling at 99.9999991 percent of the speed of light to make it around.

The tunnel sits about 100 meters underground, with the precise depth varying from 50 to 175 meters. This shields the surface from radiation and also means CERN didn’t have to buy up (and destroy) all the farmland lying over the tunnel’s location during the construction phase. Property rights did, however, delay tunnel excavation back in the 1980s when it was originally constructed for LEP. The problem was that in France, landowners are entitled to the entire region to the Earth’s center—not just the farmland they plow. The tunnel could be dug only after the French authorities blessed the operation by signing a “Déclaration d’Utilité Publique,” thereby making the underlying rock—and in principle the magma underneath too—public property.

Physicists debate whether the reason for the tilt in the tunnel’s depth was geology or if it was done to further defl ect radiation, but the fact is the tilt helps with both. The uneven terrain was in fact an interesting constraint on the tunnel’s depth and location. The region lying under the CERN site is mostly a type of compact rock known as molasses, but underneath the fluvial and marine deposits lie gravel, sand, and loam containing groundwater, and this would not be a good place for a tunnel. The slope keeps the tunnel in the good rock. It also meant that one section of the tunnel at the foot of the beautiful Jura Mountains lying at the edge of CERN could be a little less deep so that getting stuff in and out of vertical shafts in this location was a bit easier (and cheaper).

The final accelerating electric fields in this tunnel are not arranged in a precisely circular fashion. The LHC has eight large arcs alternating with eight 700-meter-long straight sections. Each of these eight sectors can be independently heated up and cooled down, which is important for repairs and instrumentation. After entering the tunnel, protons are accelerated in each of the short straight sections by radio waves, much as they were in the previous acceleration stages that brought them up to injection energy. The acceleration occurs in radio-frequency (RF) cavities that contain a 400 MHz radio signal, which is the same frequency you use when you remotely unlock your car door. When this field accelerates a proton bunch that enters such a cavity, it increases the energy of the protons by a mere 485 billionths of a TeV. This doesn’t sound like much, but the protons orbit the LHC ring 11,000 times a second. Therefore, it takes only 20 minutes to accelerate the proton beam from its injection energy of 450 GeV to its target energy of 7 TeV, about 15 times higher. Some protons are lost during collisions or stray loose, but most of those protons will continue to circulate for about half a day before the beam is depleted and needs to be dumped into the ground and replaced by fresh newly injected protons.