Beyond the God Particle (29 page)

Let's examine the most recent highest-energy colliders that have ever been constructed to see how this symphony of components perform.

THE WORLD'S GREAT COLLIDERS

TEVATRON

The Tevatron was a synchrotron particle accelerator at Fermilab.
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It was the highest-energy particle collider in the world until it was superseded by the LHC at CERN. The Tevatron accelerated protons in one direction in the machine and antiprotons in the opposite direction, and brought them into collision at two points within the machine. At these locations were the detectors, the CDF and D-Zero, which played the role of the “eyepieces of the microscope.” The Tevatron ring was 3.9 miles in circumference, and it accelerated the protons and antiprotons up to energies of 1 TeV, producing collisions up to 2 TeV in energy.

The Tevatron was completed in 1983, and significant upgrades were made continually during in 1983 through 2011. The Tevatron discovered of the top quark, made the most precise measurement of the W-boson mass (and initially a much improved measurement of the Z-boson mass, which
was soon superseded by CERN's LEP accelerator), saw the first hints of CP-violation in “b”-quark physics, and made numerous other measurements concerning the strong interactions of quarks.

The first large accelerator at Fermilab was called the Main Ring, and construction on it began on October 3, 1969, with the groundbreaking led by the lab's first director, Robert R. Wilson (Fermilab was then known as the National Accelerator Laboratory). This would become the 3.8-mile-circumference tunnel that eventually housed the Tevatron. The Main Ring used conventional copper wire magnets and achieved a typical beam energy of 300 GeV by 1973, and a record beam energy of 500 GeV in 1976. To go to higher energies required more powerful magnets, and this required the technology of superconductivity. The Main Ring accelerator was shut down on August 15, 1977, and newly developed superconducting magnets and a new beam pipe were mounted on top of the old Main Ring magnets. The superconducting “Tevatron” produced its useful beam energy of 900 GeV in November 1986.

On September 27, 1993 the cryogenic cooling system of the Tevatron was proclaimed an International Historic Landmark by the American Society of Mechanical Engineers. The system, which provides liquid helium to the Tevatron's superconducting magnets, was the largest industrial scale cryogenic system in existence upon its completion in 1978. The cryogenics maintains the coils of the magnetism a superconducting quantum state, and the magnets consume only 1/3 of the power they would be required with copper magnets at normal non-cryogenic temperatures.
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The Fermilab accelerator complex was much like the transmission in your car, having different “gears” that sequentially deliver more energy as you increase the car's speed. We began in first gear with a high-voltage Cockcroft–Walton pre-accelerator. This device was a large-voltage source that gave one big initial energy kick, very similar to that used in old TV picture tubes but on a much larger scale (TV picture tubes used a voltage of about 20,000 volts, while the Cockcroft–Walton at Fermilab produced 750,000 volts). The accelerated beam then passed into a 150-meter linac, accelerating up to 400 MeV, then into the “Booster,” a small synchrotron about 100 meters in diameter. Here the protons circulate in their orbit about
20,000 times and attain an energy of around 8 GeV. From the Booster the particles pass into another synchrotron called the Main Injector, which accelerates the protons up to 120 GeV. Some of the protons at this stage were used to create antiprotons, which were collected into a sophisticated device called the antiproton source.
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This made the antiprotons available to be injected back into the Main Injector, and the antiproton energy was increased back to 120 GeV. Finally the protons and antiprotons were both injected into the Tevatron.

The Tevatron could accelerate the protons and anti-protons from the Main Injector in opposite directions up to 980 GeV each. To maintain the particles in their synchrotron orbits the Tevatron used 774 niobium-titanium superconducting dipole magnets, cooled to superconducting temperatures in liquid helium. 240 quadrupole magnets were used as magnetic lenses to focus the beam.

The protons and antiproton beams were rather diffuse and generally didn't interact, passing freely through one another throughout most of the length of the circumference of the Tevatron. However, at certain special points around the Tevatron the beams were “squeezed” together and collisions occurred. This is the basic principle of a collider—the beams are not hitting a fixed target like a glass slide with protozoans in a drop of water. Rather, the beams are colliding head-on with one another! Surrounding these special “squeeze points” were the detectors, which collect and measure the products of the collisions. At the Tevatron there were two such detectors, CDF and D-Zero, electronically collecting and charting the debris of trillions of proton–antiproton collisions at 1.96 TeV.

The Main Injector, which replaced the old Main Ring of the Tevatron, was the last addition to the Fermilab complex, built at a cost of $290 million. The Tevatron collider Run II began on March 1, 2001, after the completion of the Main Injector, with the beam energy of 980 GeV. The Main Injector was the last particle accelerator for high-energy physics built in the US—and it was begun over 20 years ago in 1993. The Main Injector remains operational as an important part of the ongoing Fermilab program investigating neutrinos.

The Tevatron ceased operations on September 30, 2011, and some of its components have now been cannibalized for other accelerators and experiments.

LARGE ELECTRON–POSITRON COLLIDER

The Large Electron–Positron Collider was built at CERN and began operation in 1989. It was a circular synchrotron collider with a circumference of 27 kilometers and was constructed in the tunnel that now houses the LHC.

The concrete-lined “LEP tunnel” was a major construction project, undertaken between 1983 and 1988. The tunnel crosses the border, underground, between Switzerland and France, with most of it lying under France. The tunnel is tilted, and the high part of the ring is under the Jura Mountains to the west of Geneva. The tunnel therefore has a variable depth ranging from about 160 to 570 feet. Construction had to overcome serious challenges posed by underground water at high pressures in the mountains.
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Hydrostatic cement held the day!

LEP accelerated electrons (in bunches) in one direction and positrons (antielectrons) in the opposite within a common beam pipe. Each particle bunch initially reached a total energy of about 45.5 GeV, yielding a combined energy of 91 GeV. This allowed the direct production of the Z
⁰
boson, which has a mass of 91 GeV. LEP was later upgraded to go to higher energies, which enabled the production of a pair of W bosons, each having a mass of 80 GeV. The LEP collider energy eventually achieved a beam energy of 104 GeV, for a total collision energy of 209 GeV.

Like the Tevatron, the particle acceleration at LEP was done in stages. The older CERN Super Proton Synchrotron was used initially to accelerate and inject bunches of electrons and positrons into the LEP ring. Once the particle bunches were accelerated to the desired beam energy, an electron bunch heading one direction and a positron bunch heading the other direction were squeezed to cause head-on collisions within the particle detectors. When an electron and a positron collide, they can annihilate to make a Z boson. The produced Z boson decays instantly into other elementary particles, which are then detected by the particle detectors.

The LEP collider had four detectors, situated symmetrically around the synchrotron, where the bunches of particles were “squeezed” to produce collisions. The four detectors of LEP were called Aleph, Delphi, Opal, and L3. These detectors, slightly differing in their designs, yielded complementary information about the physics at LEP. The detectors were quite large, each about the size of a small house. They could measure the decay particles
from the Z boson produced in the collision. The detectors allowed a reconstruction of the process that produced them. By performing complex statistical analyses of this data, physicists could infer the properties of the Z boson in great detail.

The beam energy of the LEP collider was so precisely monitored it could detect the motion of the French
Train de Grande Vitesse
(
TGV
) as it rolled though the French-Swiss countryside en route to Paris or Lyon from downtown Geneva. Physics-wise, by scanning the Z
⁰
boson, one could measure and “count” the number of decays of the Z
⁰
boson into undetectable (or “invisible”) particles called neutrinos. This confirmed the result that there were only 3 kinds (or “flavors”) of very light-mass neutrinos in nature. The precision measurements of the Z
⁰
boson mass and decay have provided a major constraint on the indirect quantum effects in the Standard Model. Together with the discovery of the top quark at the Tevatron, this analysis gave strong clues as to where the Higgs boson would be found.

Though the original goal of the LEP collider was to discover the Higgs boson, we now know that it was slightly too massive to be produced at LEP energies and that it would have to await the LHC. LEP nonetheless made definitive and precise measurements of the properties of the Z boson. By carefully calibrating the beam energy and scanning the Z boson by varying the energy of the LEP collider, it was possible to infer many details about how the Z boson decays. A slightly higher-energy “super-LEP” machine might be built one day to produce the Higgs (in electron-positron colliders the Higgs boson is produced together with the Z
⁰
boson and requires about 240 GeV and higher luminosity than LEP). CERN terminated LEP to make way for the LHC in 2000.

LHC

The Large Hadron Collider is the world's largest and highest-energy particle accelerator.
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The collider is contained in the circular tunnel and has a circumference of 27 kilometers (17 miles); it was originally constructed for LEP. It fully recovered from its “helium incident” (aka “major magnet explosion” on September 19, 2008; see
chapter 1
) and began doing physics in November of 2009.

Recall that the Tevatron accelerated protons (of charge +) in one direction
and antiprotons (of charge –), in the opposite direction. This could be accomplished in one beam pipe with a common set of magnets, since both bunches would be held in a common orbit circulating in opposite directions. However, the LHC collides protons head-on with protons. This spares the necessity of making antiprotons, but it requires two adjacent parallel beam pipes (and more complex magnets) since protons cannot be circulated in the same pipe in the same circles in opposite directions. The two beam pipes must also intersect to create collisions.

1,232 dipole magnets keep the beams on their circular paths, while an additional 392 quadrupole magnets keep the beams focused. In total, there are over 1,600 superconducting magnets made of copper-clad niobium-titanium that are kept at their operating temperature of 1.9 K (−271.25° C) by 96 tons of liquid helium. The LHC has now eclipsed the Tevatron in another aspect: it is the largest cryogenic facility in the world operating at liquid helium temperature. Most recently the LHC operated at a beam energy of 4 TeV, or 8 TeV in the total collision energy. This was sufficient to discover the Higgs boson but was short of the planned design energy.

As of this writing (March 2013), the CERN LHC is down for upgrades. It will come back online around January 2015 at the full design energy, whence the protons will each have 7 TeV, giving a total collision energy of 14 TeV. The increase in total collision energy from 8 TeV to 14 TeV causes almost twenty times as many gluon-gluon collisions that produce Higgs bosons and new particles, and will yield much more data. It will also nearly double the discovery reach for new particles that are unanticipated in the Standard Model.

The run of the LHC starting in January 2015 and extending through to around 2018 will be one of the most important voyages the human species has ever taken into an unknown wilderness. Only this run of the LHC with the large detectors—the experiments called ATLAS and CMS—has a chance of seeing if there is anything “beyond the Higgs boson.”

THE DETECTORS

Of course, as in the case of microscopy, the art of particle physics is not simply a matter of accelerating and colliding beam particles. It involves the
eyepiece of the microscope, i.e., how to see the debris of these collisions and how to detect any new and unexpected particles produced within the collisions. We have said very little about the behemoth detectors of particle physics in this book. Alas, the detectors of particle physics necessitate another whole book, and we must suffice to point you to the Internet for more. Just search on the keywords “ATLAS CERN” and “CMS CERN,” and you're on your way. You'll find the web addresses
http://atlas.ch/
and
http://cms.web.cern.ch/
; you can see the remarkably short URLs these two experiments enjoy, perhaps in part because CERN was the origin of the World Wide Web in the era of Tim Berners-Lee. (You can also search for other detectors such as “CDF” and “D-Zero,” formerly at Fermilab, and the LEP detectors mentioned above.)

But we do want to take a moment to salute an old late friend and colleague, and a hero of the resistance movement in France during World War II: Georges Charpak.
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In the 1960s through 1970s, particle detection involved mainly taking photographs of collisions within large and cumbersome devices called “bubble chambers,” followed by the examination of these photographs by a human eyeball. This is a slow, non-automated, and labor-intensive method. It could not possibly deal with the very high statistics demanded of particle physics experiments that seek to find something like a Higgs boson, a needle in a haystack of trillions of collisions.