Web edition: July 4, 2008
Print edition: July 19, 2008; Vol.174 #2 (p. 16)
The hammering has stopped, the whining of power tools has abated. Only the hum of electronic detectors reverberates through the cavernous, eight-story space below the Swiss-Franco border that is stuffed with 9,300 magnets and enough niobium-titanium wire to stretch to the sun and back five times.
But by September, if all goes according to plan, two narrow
beams of protons moving in opposite directions will begin making laps around
the underground laboratory’s 27-kilometer–long subatomic racetrack. The protons will pass from
The most violent of those collisions will generate the heat, energy and densities that existed just a trillionth of a second after the Big Bang. And like a movie in perpetual rewind, these primordial re-creations will repeat about once a second.
This is the Large Hadron Collider, or LHC, the mammoth atom smasher operated by the European Organization for Nuclear Research, better known by its French acronym, CERN. More than 15 years in the making, everything about the LHC is enormous, from the energies it generates — 14 trillion electronvolts — to the nearly 60 metric tons of liquid helium required to cool its magnets, to the 20,000 tons of metal it houses and to the staff of thousands of scientists involved. All this just to study the tiniest particles in the universe.
But more than the $8 billion price tag is riding on the LHC. Depending on what’s detected, physicists may find out if they understand the fundamental building blocks of nature, or if “everything that physicists have been talking about for 45 years is wrong,” says CERN theoretical physicist John Ellis.
More parochially, the success of the LHC may vault Europe over the
As revealed in Einstein’s famous equation, E=mc2, the enormous energies generated at the LHC will translate into large masses — and, particle physicists hope, a cornucopia of subatomic particles never before seen. Many are betting that they will see evidence of the elusive Higgs boson, a hypothetical particle first proposed in the 1960s that could help explain why some elementary particles have mass. Finding the Higgs would be the crowning achievement for what physicists call the standard model, a highly successful theory that unifies three of the four known forces in nature and groups the fundamental constituents of matter into two broad categories.
But researchers are hoping for more than a confirmation of the standard model. “I’m certainly not going to yawn if they find a Higgs boson; we’ll break out the champagne and won’t answer the phones for a week,” Ellis says. “But that’s somehow an expected discovery.”
Physicists hope the LHC will lead them beyond the standard model — to signs of extra dimensions, curled up into volumes of space so tiny they’re barely detectable; and to rapidly evaporating, microscopic black holes that the accelerator might forge.
And if scientists are really lucky, Ellis says, the experiments at LHC might double the scientists’ pleasure, revealing a whole new set of elementary particles. According to a theory known as supersymmetry, every particle in nature has a heavier partner whose spin differs by a half integer. Supersymmetry would unite two seemingly disparate groups of particles.
On one side of the divide are the bosons, the particles that act as messengers for the four fundamental forces in nature. These include the photon, which communicates the electromagnetic force; the W and Z, which mediate the weak nuclear interaction; and the gluon, which transmits the strong nuclear force. On the other side stand the fermions, the particles that react to those forces — electrons, quarks and the like. Supersymmetry says that the two groups belong under the same umbrella, residing in one big happy family.
Moreover, the lightest supersymmetric particle, called the neutralino, could be a candidate for dark matter, the long-sought, invisible material that astronomers say must exist to keep galaxies intact and galaxy clusters from flying apart. There simply isn’t enough ordinary visible matter to provide the requisite gravitational glue. The dark matter particles would provide the extra tug and account for more than 80 percent of the mass of the universe.
“That would be the fantastic breakthrough— that we would
finally know what most of the matter in the universe is,” says physics Nobel
laureate Steven Weinberg of the
Meeting the experiments
Inside the LHC tunnel, the twin, hair-thin proton beams rev up to speeds approaching that of light. Steering them around the racetrack are 1,232 dipole magnets. Each magnet weighs 35 metric tons and is supercooled to 1.9 degrees Celsius above absolute zero.
For most of their journey around the ring, the beams travel in separate vacuum pipes, but at four points they collide. These are the hearts of the main experiments, known by their acronyms: ALICE (A Large Ion Collider Experiment), ATLAS (A Toroidal LHC Apparatus), CMS (Compact Muon Solenoid) and LHCb (Large Hadron Collider beauty).
ATLAS and CMS, the largest, are the major players. Both ATLAS and CMS fully enclose the portals where collisions happen, leaving no gaps for particles to escape without detection. Although they can’t flee, many of the particles rapidly decay into a spray of other, more stable members of the subatomic zoo. Like CSI detectives, scientists will measure the energy, mass and paths of those final particles to find out what happened in the collisions.
The cores of ATLAS and CMS — the parts closest to the collision sites — contain particle trackers made of silicon wafers. Charged particles traveling through the wafers create electrical signals that reveal their passage. The CMS experiment alone has enough wafers to tile an 8-meter–deep Olympic-sized swimming pool.
Just outside the wafers lie calorimeters, devices that slow down and absorb particles in order to measure their energies. Muons, the heavy cousins of electrons, are an especially precious commodity because they are both easy to detect and would be produced as end products in any reactions involving the Higgs boson.
Both CMS and ATLAS have powerful magnets that curve the paths of charged particles. The amount of curvature reveals the particles’ momentum and charge, allowing researchers to identify which charged particles are created directly or as a by-product of the collisions. CMS’ magnet is in the shape of a solenoid, a cylindrical coil of superconducting wire that generates a field 100,000 times stronger than that of Earth (measured at the surface). The field is confined by a steel clamp or yoke, which accounts for the bulk of the detector’s 12,500 tons.
In contrast, ATLAS uses a doughnut-shaped magnetic system, consisting of a ring of eight supercooled coils. The design requires no yoke and allows ATLAS to be eight times the size of CMS — 46 meters long and 25 meters high — yet only half the weight, notes Fabiola Gianotti, an ATLAS researcher at CERN.
Though searching for the same particles, the two mammoth
experiments are independent. CMS, located beneath
Together, quarks and gluons, the particles that bind quarks together, are the elementary constituents of particles such as the proton. And in some sense, it’s not the protons that really collide in the LHC, but the quarks and gluons they contain. “What we measure is not the individual quarks or gluons but the results of their [breakup], as they decay into other particles,” Gianotti says.
Evidence for extra dimensions
Both ATLAS and CMS will also explore the ghostly realm of hidden dimensions. Particle physicists tend to think of subatomic particles as point masses, but string theory attempts to unify all forces and particles by viewing them as different vibrations of strands or loops called superstrings.
Although the superstrings are probably too tiny to observe directly, the theory makes several predictions, including the existence of seven hidden dimensions of space. These dimensions would be tightly compacted or curled up. But through the production of new particles that might move or wind around these extra dimensions, ATLAS and CMS experiments will have the sensitivity to detect extra dimensions one-ten-billionth the size of an atom.
If this theory of extra dimensions is correct, the LHC could become a factory for making microscopic black holes.
In Einstein’s theory of gravity, which assumes a universe confined to three space dimensions and one of time, black holes could be generated by an accelerator much bigger than Earth. But in an alternative theory, gravity leaks out into other, unseen dimensions — a possible explanation of why gravity appears to be so much weaker than the other forces in nature.
In this scenario, gravity is weak only if observed at long distances because the extra, hidden dimensions dilute its strength. Conversely, at the high energies and small scales probed by the LHC, gravity would become much stronger than it is in ordinary three-dimensional space, cramming enough matter together to form microscopic black holes as often as once a second.
Such black holes, according to research by Stephen Hawking in the 1970s, ought to rapidly radiate away their energy and evaporate in an instant, and would not be dangerous. As they nearly instantaneously evaporate, they would radiate distinctive sprays of elementary particles, which stand out in the LHC detectors.
The possibility of creating tiny black holes at the LHC is “quite a long shot,” admits Steve Giddings of the University of California, Santa Barbara. But he’s hoping that long shot comes through. “Not only would we learn things about gravity and the fabric of spacetime,” he says, “but we would apparently have direct evidence for extra dimensions of space.”
Two medium-sized experiments get to the heart of other fundamental questions about the universe. Physicists analyzing data from the LHCb will try to gain insight about why the universe has so much more matter than antimatter. Theory suggests that the cosmos began with equal amounts of both. But if matter and antimatter were mirror copies of each other, they would have annihilated, leaving behind only pure energy. Fortunately, an imbalance arose, allowing the tapestry of galaxies, the solar system and our own planet to coalesce.
LHCb examines the decay of B mesons, particles composed of a specific pairing of a quark and antiquark — the bottom antiquark and either an up or a down quark.
To catch the quarks, LHCb uses a 20-meter–long stack of detectors on a movable track that can intercept the spray of particles created during collisions. Studying how these quarks decay may reveal subtle differences between matter and antimatter. Rather than behaving as exact replicas of each other, the particles and antiparticles may exhibit slight distortions, akin to staring at a reflection in a wavy mirror.
Within the fireballs, the neutrons and protons that make up the lead nucleus melt away, freeing quarks from their bonds with gluons. For a few precious moments, they form a new state of matter called the quark-gluon plasma.
In previous experiments at both CERN and Brookhaven National Laboratory
All about the Higgs
Finding the Higgs is the driving force behind much of the research at the LHC. According to the simplest version of the standard model, every particle in the universe ought to be massless. Electrons, for instance, ought to weigh exactly the same as photons, the particles of light — absolutely nothing. That, of course, is not the case.
In the early 1960s, physicist Peter Higgs proposed that a hypothetical field, now called the Higgs field, permeates the universe like a cosmic vat of molasses. Many particles — but not all — would slow down as they propagated through the goo.
Slowing down is a critical step. That’s because Einstein’s theory of relativity draws a sharp distinction between particles that have mass and those that don’t. Massless particles move at the speed of light, while massive particles never reach that ultimate of all speeds. So slowing down, according to relativity theory, is tantamount to acquiring mass. Electrons, protons, neutrons and the like bulked up because the Higgs field slowed them down, while photons somehow remained immune to the molasses and stayed weightless.
The Higgs field cannot be directly measured, but high-energy collisions like the ones possible at the LHC could excite the field. The decay of the Higgs boson would be the measurable signpost of that excitation. Theory suggests that the Higgs ought to lie in the mass-energy range that can be achieved at the LHC, Weinberg says.
“The Higgs boson is not a slam dunk, but it is something that is expected, and it’s really important for the future of fundamental physics to see whether in fact it’s found,” he says.
But, Weinberg adds, “the greatest fear, I think, that the particle physics community has is that the LHC will find the Higgs boson and not find anything else. Because if that happens, we will simply have verified what is now the popular version of the standard model, and we won’t have any experimental clues as to how to go beyond the model.”
If the Higgs particle has a mass at the high end of its predicted range, about 115 billion to 200 billion electronvolts, it should be easily detected, its decay products standing out dramatically against the background of particle debris from other processes. But if the particle tips the scales at the lighter end, the signal will be trickier to discern, Weinberg says.
If the detection of the Higgs involves finding an extra, unaccounted-for signal in the collision debris, finding a supersymmetric particle requires finding a deficit.
The lightest supersymmetric particle is thought to have no charge and to interact only weakly, which is why it is a candidate for the invisible dark matter. “As such, these particles would not be seen in a detector, hence the energy it carries would be missed,” notes Ellis. It’s like the curious incident of the dog that did nothing in the nighttime, from the Sherlock Holmes story, he notes. It’s what’s missing that counts.
Whatever happens, says Weinberg, particle physics is about to awaken from some 30 years of slumber. Come September, the fun begins.
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