The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (9 page)

The Bevatron contributed to two Nobel Prizes: in 1959 to Emilio Segrè and Owen Chamberlain, for the discovery of the antiproton, and in 1968 to Luis Alvarez, for the discovery of too many particles to count—all those pesky hadrons. Sometime later, Alvarez and his son Walter were the ones who first demonstrated that an asteroid impact was the likely cause of the extinction of dinosaurs, by discovering an anomalously high concentration of iridium in geological strata that formed around that time.

The idea behind particle accelerators is simple: Take some particles, accelerate them to very high velocities, and slam them into some other particles, watching carefully to see what comes out. The procedure has been compared to smashing together two fine Swiss watches and trying to figure out what they are made of by watching the pieces fly apart. Unfortunately, this analogy has it backward. When we smash particles together, we’re not looking for what they are made of; we’re trying to create brand-new particles that weren’t there before we did the smashing. It’s like smashing together two Timex watches and hoping that the pieces assemble themselves into a Rolex.

To attain these velocities, accelerators use a basic principle: Charged particles (such as electrons and protons) can be pushed around by electric and magnetic fields. In practice, we use electric fields to accelerate particles to ever-higher speeds, and magnetic fields to keep them moving in the right direction, such as around the circular tubes of the Bevatron or the LHC. By delicately tuning these fields to push and nudge particles in just the right way, physicists can reproduce conditions that would otherwise never be seen here on earth. (Cosmic rays from outer space can be even more energetic, but they are also rare and hard to observe.)

The influence of a magnetic field on moving particles. If the magnetic field is pointing upward, it pushes positively charged particles in a counterclockwise direction, negatively charged particles in a clockwise direction, and neutral particles not at all. Likewise, stationary particles just remain at rest.

The technological challenge is clear: Accelerate particles to as high an energy as we can, smash them together, and look to see what new particles are created. None of these steps is easy. The LHC represents the culmination of decades of work learning how to build bigger and better accelerators.

E = mc
²

When the Bevatron created antiprotons, it wasn’t because there were antiprotons hidden in the protons and atomic nuclei they were working with. Rather, the collisions brought new particles into existence. In the language of quantum field theory, the waves representing the original particles set up new vibrations in the antiproton field, which we detect as particles.

In order for that to happen, the crucial ingredient is that we have enough energy. The insight that makes particle physics possible is Einstein’s famous equation,
E = mc
²
, which tells us that mass is actually a form of energy. In particular, the mass of an object is the minimum energy that object can have; when something is just sitting perfectly still, minding its own business, the amount of energy it possesses is equal to its mass times the speed of light squared. The speed of light is a big number, 186,000 miles per second, but its role here is just to convert units of measurement from mass to energy. Particle physicists like to use units where speed is measured in light-years per year; in that case
c
is equal to one, and mass and energy become truly interchangeable,
E = m
.

What about when an object is moving? Sometimes discussions of relativity like to talk as if the mass increases when a particle approaches the speed of light, but that’s a little misleading. It’s better to think of the mass of an object as fixed once and for all, while the energy increases as it goes faster and faster. The mass is the energy that the thing would have if it was not moving, which by definition doesn’t change even if it happens to be moving. Indeed, energy grows without limit as you get closer and closer to the speed of light. That’s one way of understanding why the speed of light is an absolute limit to how fast things can go—it would take an infinite amount of energy for a massive body to move that fast. (Massless particles, in contrast, always move at exactly the speed of light.) When a particle accelerator pushes protons to higher and higher energies, they are coming closer and closer to the speed of light, never quite getting there.

Through the magic of this simple equation, particle physicists can make heavy particles out of lighter ones. In a collision, the total energy is conserved but not the total mass. Mass is just one form of energy, and energy can be converted from one form to another, as long as its total amount remains constant. When two protons come together at a large velocity, they can convert into heavier particles if their total energy is large enough. We can even collide perfectly massless particles to create massive ones; two photons can smack together to make an electron-positron pair, or two massless gluons can come together to make a Higgs boson, if their combined energy is larger than the Higgs mass. The Higgs boson is more than a hundred times heavier than a proton, which is one of the reasons it’s so hard to create.

Particle physicists enjoy using units of measurement that make no sense to the outside world, as it lends an aura of exclusivity to the endeavor. Also, it would be a pain to use one set of units for mass and another for other forms of energy, since they are constantly being converted back and forth to one or the other. Instead, whenever we’re faced with an amount of mass, we simply multiply it by the speed of light squared to instantly convert it into an energy. That way we can measure everything in terms of energy, which is much more convenient.

Scale of energies. Particle physicists measure temperature, mass, and energy on the same scale, using electron volts as a basic unit. Common expressions include milli-eV (1/1000 eV), keV (1000 eV), MeV (1 million eV), GeV (1 billion eV), and TeV (1 trillion eV). Some values are approximate.

The energy unit favored by particle physicists is the electron volt, or “eV” for short. One eV is the amount of energy it would take to move an electron across one volt of electrical potential. In other words, it takes nine electron volts’ worth of energy to move an electron from the positive to the negative terminals of a nine-volt battery. It’s not that physicists spend a lot of time pushing electrons through batteries, but it’s a convenient unit that has become standard in the field.

One electron volt is a tiny bit of energy. The energy of a single photon of visible light is about a couple of electron volts, while the kinetic energy of a flying mosquito is a trillion eV. (It takes many atoms to make a mosquito, so that’s very little energy per particle.) The amount of energy you can release by burning a gallon of gasoline is more than 10
²⁷
eV, while the amount of nutritional energy in a Big Mac (700 calories) is about 10
²⁵
eV. So a single eV is a small amount of energy indeed.

Since mass is a form of energy, we also measure the masses of elementary particles in electron volts. The mass of a proton or neutron is almost a billion electron volts, while the mass of an electron is half a million eV. The Higgs boson that the LHC discovered is at 125 billion eV. Because one eV is so small, we often use the more convenient unit of GeV, for giga– (1 billion) electron volts. You’ll also see keV for kilo– (1,000) electron volts, MeV for mega– (1 million) electron volts, and TeV for tera– (1 trillion) electron volts. In 2012, the LHC collided protons with a total energy of 8 TeV, and the eventual goal is 14 TeV. That’s more than enough energy to make Higgs bosons and other exotic particles; the trick is to detect them once they’re produced.

We can even measure temperature using the same units, because temperature is just an average energy of the molecules in a substance. From this perspective, room temperature is only two-hundredths of an electron volt, while the temperature at the center of the sun is about 1 keV. When the temperature rises above the mass of a certain particle, that means that collisions have enough energy to create that particle. Even the center of the sun, which is pretty hot, isn’t nearly high enough to produce electrons (0.5 MeV), much less protons or neutrons (about 1 GeV each). Back near the Big Bang, however, the temperature was so high that it was no problem.

The easiest way for nature to hide a particle from us is to make it so heavy that we can’t easily produce it in the lab. That’s why the history of particle accelerators has been one of reaching for higher and higher energies, and why accelerators get names like Bevatron and Tevatron. Reaching unprecedented energies is literally like visiting a place nobody has ever seen.

Energizing Europe

The official name of CERN, the Geneva laboratory where the LHC is located, is the European Organization for Nuclear Research, or in French Organisation Européenne pour la Recherche Nucléaire. You’ll notice that the acronym doesn’t work in either language. That’s because the current “Organization” is a direct descendant of the European Council for Nuclear Research, Conseil Européen pour la Recherche Nucléaire, and everyone agreed that the old abbreviation could stick even after the name was officially changed. Nobody insisted on switching to “OERN.”

The council was established in 1954 by a group of twelve countries that sought to reenergize physics in postwar Europe. Since that time, CERN has been at the forefront of research in particle and nuclear physics, and has served as an intellectual center for European science, as well as an important component of Geneva’s identity. In the second-largest city in Switzerland, a world center of finance, diplomacy, and watchmaking, one out of sixteen passengers passing through Geneva airport is somehow associated with CERN. When you land there, chances are there’s a physicist or two on your airplane.

Like most major particle physics labs, the story of CERN has been one of bigger and better machines reaching ever-higher energies. In 1957, there was the Synchrocyclotron, which accelerated protons to an energy of 0.6 GeV, and in 1959, saw the inauguration of the Proton Synchrotron, which reached energies of 28 GeV. It still operates today, providing beams that are accelerated further by other machines, including the LHC.

A major step forward came in 1971 with the Intersecting Storage Rings (ISR), which attained 62 GeV in total energy. The ISR was a proton
collider
as well as an accelerator. Previous machines had accelerated protons and aimed them at stationary blocks of matter, which are relatively easy targets to hit; the ISR collided beams moving one way with beams moving in the opposite direction. This technique presents a much greater technological challenge but also makes much higher energies accessible; not only does each beam carry energy, but every bit of the energy is now available to make new particles. (In fixed-target experiments, much of the energy goes into providing a push to the target.) Prospects for building a particle collider were studied in the 1950s by Gerard K. O’Neill, an American physicist, who later became more well-known for proposing and advocating human habitats in outer space, and small electron-positron colliders were constructed in Frascati, Italy, in the 1960s by Austrian physicist Bruno Touschek.

The ISR was about one and three-quarters of a mile in circumference. Big, but bigger was yet to come. The Super Proton Synchrotron (SPS), more than four miles in circumference, opened in 1976, and reached energies of 300 GeV. Just a few years later, in a bold move, CERN reengineered the SPS from its original task of accelerating protons to a new configuration in which it collided protons with
antiprotons
. As you might expect, antiprotons are hard to collect and work with. They’re not lying around like protons are; you have to make them in lower-energy collisions to start, and then work hard to gather them without their bumping into an ambient proton and annihilating in a flash of light. But if you pull off this trick, you can take advantage of the fact that protons and antiprotons have opposite charges to curve them around in opposite directions but in the same magnetic field. (The LHC collides protons with protons, and therefore has to use two separate beam pipes for the two directions.) Italian physicist Carlo Rubbia used the upgraded SPS in 1983 to discover the W and Z bosons of the weak nuclear force, picking up the Nobel Prize in 1984.