Beyond the God Particle

by Leon M. Lederman

  FIGURE 8.31. Bar Magnet. The magnetic field of a bar magnet.

  The poles of a magnet are called north (N) and south (S). If a small iron bar magnet is hung from its middle by a string, it becomes a compass needle, and its N end will point northward, thus its S end points southward. The N end of a magnet will repel the N end of another magnet, S will repel S, but N and S attract each other. Hence, N and S are like positive and negative charges (we call N and S magnetic monopoles). However, we can never have an isolated N without, somewhere, having a compensating S; all magnets are therefore “dipoles,” i.e., having two opposite poles, with equal but opposite N and S. This is a consequence of the magnetic field being set up by electrical currents, rather than having magnetic charges, or “magnetic monopoles,” as their sources.15 Either pole of a magnet will induce magnetization in a nearby magnetic material. Therefore, either pole can attract iron-containing objects, such as paper clips, because the magnet will induce magnetization in the paper clip. The paper clip becomes itself a temporary magnet, with its N pole facing an S pole, or vice versa.

  If we arrange a flat white sheet of paper over a bar and sprinkle over the paper little iron filings, the filings will align with the magnetic field and allow us to visualize the magnetic field itself!16

  CYCLOTRONS

  We'll only mention cyclotrons in passing, since they are rather passé in modern particle physics, and will instead refer the interested reader to the extant literature, e.g., search online for “cyclotrons” or see the Wikipedia entry.17 The idea of a cyclotron is to accelerate charged particles but to hold them in circular spiral motion with a constant magnetic field. For example, we can inject particles into the center of a circular machine with a perpendicular magnetic field. We give the particles a little kick in energy, and they will move in a circle. Each time the particles complete one full turn, they are given another “kick” of energy from the same electric field, and then the cycle repeats. As the particle receives each kick in energy, it will tend to spiral outward into a circular orbit with a larger radius.

  The cyclotron was invented in 1932 by Ernest Lawrence of the University of California, Berkeley, with much of the development in collaboration with his student, M. Stanley Livingston. The cyclotron was an improvement over the linac of the 1920s, when it was invented, being more compact and cost-effective due to the circular repetitive acceleration process.

  For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments; several cyclotrons are still in use for this type of research. Cyclotrons are still actively used in medical applications to treat cancer and to produce radioactive isotopes for medical imaging. Ion beams from cyclotrons can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path. Cyclotron beams can be used to bombard other atoms to produce short-lived positron-emitting isotopes suitable for PET imaging.

  SYNCHROTRONS

  The most advanced circular accelerator, and the one used most commonly in particle physics, is the synchrotron. The first electron synchrotron was constructed by Edwin McMillan in 1945, although the principle had already been published in a Soviet journal by Vladimir Veksler.18 The first proton synchrotron was designed by Sir Marcus Oliphant and built in 1952. Protons, and even muons, can be accelerated to very high energies in large rings without appreciable synchrotron radiation loss. The Large Hadron Collider and former Tevatron were both proton synchrotrons (the Tevatron also circulated and accelerated antiprotons in the opposite direction in the machine).

  In a synchrotron, particles are held in a fixed circular orbit. Each time they complete a cycle in the machine they receive a kick of energy from RF cavities. As the energy of the particles increases, the magnetic field is also slightly increased to maintain the same orbit. This allows the vacuum beam pipe that contains the motion of the particles to be a very large circular shape, rather than a large disk, as in the cyclotron. The smaller cross-section beam pipe allows for the magnetic fields to be localized within the pipe. These factors allow us to build very high-energy machines that are far less expensive than cyclotrons or linacs.

  Synchrotrons “hit a wall” when they are used to accelerate electrons. Owing to its small mass, an electron tends to radiate photons copiously when placed in a circular orbit at high energies. This is called “synchrotron radiation.” To achieve high energies an electron synchrotron has to be quite large, or else most of the acceleration energy will be radiated away. The Large Electron–Positron Collider (LEP) at CERN that collided electrons with positrons was a synchrotron that occupied the large tunnel that today houses the LHC, with a diameter of about 5.3 miles. LEP pushed the limit of achievable synchrotron energy with a beam of electrons circulating in one direction and positrons in the other, each beam having about 100 GeV of energy per particle (hence 200 GeV total; LEP went a bit above this energy scale in its last days). Energies of about 45 GeV per particle create a total energy in the collision of 90 GeV, allowing direct production of the Z0 boson. The synchrotron energy loss per orbit at LEP was about 0.2 percent. At the highest energies, about 100 GeV per beam, the electrons lost about 2 percent of their energy every time they orbited in the machine due to the synchrotron radiation. Synchrotron radiation itself is useful, however, in many applications, particularly in the study of chemical and biological reactions. Large circular electron machines are often designed to be sources of the high-energy photons from synchrotron radiation, or “synchrotron light sources.”19

  MAGNETIC LENSING

  Recall that a lens can, ideally, focus all the photons that are moving parallel to the axis of the lens to a point. It turns out that we cannot make a magnetic field that focuses charged particles exactly like a lens. With a “quadrupole magnet,” however, we can focus the electrons that are moving in one plane—let's say the “horizontal plane,” but then we end up defocusing by the same amount in the perpendicular, or vertical plane. If we rotate the quadrupole about its axis by 90o we will get exactly the opposite: particles in the vertical plane are now focused, while those in the horizontal plane are defocused.20

  However, here we exploit the fabulous trick that was used by microscope and telescope lens makers to correct for chromatic aberration. We can have one quadrupole that focuses (defocuses) in the horizontal (vertical) plane, followed by a second quadrupole, rotated by 90 degrees, that that focuses (defocuses) in the vertical (horizontal) plane. However, recall that when a focus lens (F) is followed by some space (O), which is then followed by defocus (D) and more space (O), the net effect is to focus. That is, a compound lens that is focus-space-defocus-space, or “FODO,” is net focusing (see figure 7.27 caption)! We can therefore keep a tightly focused beam orbiting within our synchrotron by repeating this arrangement: FODOFODOFODO…. This technique is called alternate gradient focusing. This was the breakthrough that led to large synchrotrons, such as the first Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory, then eventually to the Tevatron and LHC.21

  Synchrotrons are unable to accelerate particles from rest, and they require a sequence of pre-acceleration stages. This can be done by a chain of other accelerators, like linacs or other smaller synchrotrons, and an initial kick from something simpler, like a high voltage.

  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.22 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.23

  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.24 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.25 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 Z0 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 Z0 boson, one could measure and “count” the number of decays of the Z0 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 Z0 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 Z0 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.26 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.