Beyond the God Particle (4 page)

It was thought that LEP might actually discover the Higgs boson, an ingredient of the Standard Model that had been hypothesized by theorist Steven Weinberg to provide the origin of the masses of all the particles. The optimism of a LEP discovery had sprung from certain popular theories that had argued the Higgs mass was actually less than that of the Z
⁰
boson. To achieve the required energies to make a Z
⁰
boson with the precision afforded by using electrons and positrons (see chapters
7
and
8
), LEP had to be an enormous circular ring, housed underground in a deep tunnel. CERN therefore built a 27-kilometer (almost 17 miles) circumference circular tunnel, the construction of which ultimately proved decisive for a pathway to the LHC.

The excavation of the LEP tunnel was Europe's largest civil-engineering project prior to the Channel Tunnel. The tunnel is actually tilted, with its high side under the Jura Mountains, and this presented enormous and somewhat unforeseen engineering challenges, particularly in controlling high-pressure water leaks from springs within the mountains that threatened nonstop and major flooding. Three tunnel-boring machines started excavating the tunnel in February 1985, and the ring was completed three years later.

LEP did not find the quarry it had sought—the elusive Higgs boson. In a machine such as LEP the “signal-to-background” ratio for the Higgs boson would have been optimal for a discovery. Though many scientists expected the Higgs boson to be within LEP's reach and were disappointed at the Higgs boson's failure to emerge at LEP, the machine nevertheless made remarkably detailed precision measurements of the properties of the Z
⁰
boson that have had major impact on our detailed understanding of the Standard Model. LEP was even upgraded for a second operation phase, to
produce and study W particles, but the energy of the machine still limited the reach for a Higgs boson to a mass scale of less than about 115 times that of the proton. It still did not, even with the higher energies, find the Higgs boson. In the meantime, the SSC was canceled in the US, and CERN saw a golden opportunity to convert the LEP collider to a much higher-energy proton–proton collider. The LHC project was born.

The LEP collider was closed down in November 2000 to make way for the construction of the Large Hadron Collider (LHC) within the same tunnel. The LHC was built and commissioned, and it is, today, the world's most powerful particle accelerator, the most deeply penetrating tool we have into the inner workings of matter and energy.

COMMISSIONING THE WORLD'S LARGEST PARTICLE COLLIDER ISN'T EASY

Einstein said you should always drill through the thick part of the wood. Progress can only be made by extraordinary effort. Extraordinary effort often implies overcoming extraordinary setbacks. Particle physics drills deeper into the wood than any other human endeavor. It must necessarily have major setbacks from time to time.

The Large Hadron Collider at CERN spans the border between Switzerland and France, sitting about 300 feet underground in the LEP tunnel, a large circle with a circumference of about 17 miles. The LHC is the world's most powerful microscope and is used by physicists to study the smallest known particles and processes—the fundamental building blocks of all things. Two beams of protons travel in opposite directions inside the circular accelerator, gaining energy with every lap. Particles from the two beams collide head-on at very high energy within the centers of the two enormous detectors (“eyepieces”) called ATLAS and CMS. Teams of physicists from around the world then analyze the collisions. Two additional medium-size experiments, ALICE and LHCb, have specialized detectors that analyze the LHC collisions for a wide assortment of other phenomena.
⁸

Fermilab, with many US universities, collaborates mainly with CMS, and many US university physicists work with ATLAS. Fermilab built the LHC Remote Operations Center for the CMS experiment within its main
building, Wilson Hall. Here US scientists can be part of the action and play a vital role in actually managing the operation of the experiment without having to hop on a transatlantic flight.

On September 10, 2008, 1:30 a.m. in Fermilab's Remote Operations Center, the first circulating proton beam in the LHC in Geneva was celebrated by a partying audience of dozens of scientists, who were pulling an all-nighter, many clad in robes and pajamas. The mood was confident and exuberant. A new age for particle physics was dawning.

As the particle energy in the LHC is increased, the magnetic field strength in the accelerator beam pipe must also increase to hold the protons in fixed circular orbits within the machine. This is accomplished with 1,232 special “dipole magnets” (and thousands of other magnets that serve effectively as “correcting and focusing lenses” in the system; the system has about 5,000 magnets in total
⁹
). These magnets use electrical current to generate controlled magnetic fields. This can be varied to sweep over the required field strengths needed to maintain the delicate particle orbits, as their energies increase during acceleration (this process is called the “ramp”). Each LHC dipole magnet is a massive steel structure, about 50 feet in length. Their magnetic field ranges from zero to 100,000 times that of the earth's magnetic field. When the maximum field strength is reached, each magnet contains the potential energy of about one-quarter of a ton of TNT. This energy would be accidentally and explosively released if the current flow in the magnet were instantaneously disrupted.

To operate efficiently, the magnets use the principle of superconductivity and must be cooled down to near absolute zero temperature (0° Kelvin, which is 459° Fahrenheit). The magnets are housed inside enormous cryogenic containers, called “cryostats,” which contain the ultra-cold liquid helium. At temperatures approaching absolute zero the special magnet coils, made of copper-clad niobium-titanium cables, become superconducting, and then offer no resistance whatsoever to the flow of electrical current. Without this, the power bill to operate a collider would be unaffordable.

For the days after the initial beam was circulated, and the Fermilab pajama party on September 10, the accelerator physicists at CERN began to gradually step the machine up to its higher design energy. The precision-engineered coil windings of these magnets must be secure against any tiny movements as the magnetic field becomes stronger. This ultra-strong magnetic
field exerts enormous stress on the magnet structure, and the structure must literally contain the force of the field against an explosion. Slight changes in the magnet can create “normal,” or non-superconducting “hot-spots.” These hot-spots could “quench” the magnet, such that it loses its superconductivity, by being driven out of its cold, superconducting state.
¹⁰

In a quench, to prevent an explosion, the enormous electrical current flowing through the magnet coils must be quickly drained out of the system. A “quench protection system” is in place to minimize the effects of any unwanted quench incident. Astonishingly, copper, which is a good conductor of electricity at normal temperatures and is used in the wires of your home, is actually used as an
electrical insulator
on the superconducting cables, since the current will always flow through the superconductor and not the copper at the low temperatures! If a hot spot quench arises, the current can then be safely carried away through the surrounding copper, minimizing any damage to the system. A quench in any one of the totality of about 5,000 LHC superconducting magnets could disrupt the machine operation for several days with the quench protection system, but would be catastrophic without it.

Superconducting magnets have to be “trained” to reach higher and higher magnetic fields, as smaller and smaller glitches are relaxed from the coils. The engineers use advanced computer monitoring systems to watch for any possible quenches and induced stresses before they develop into larger problems. The enormous currents that flow within any particular magnet must also pass onto the next magnet. This is done outside of the superconducting environment of the cryostats and requires enormous copper junctions, joined together at face-to-face soldered copper plates. Even these low-tech solder joints are monitored by computers for any changes in temperature during the operation of the system.

OH, $%&#!

By September 19, 2008, things had settled back to “business as usual” at CERN. The LHC was being ramped up to full magnetic field strength as the magnets were being trained. Gradually, carefully, and systematically the LHC operators in their Swiss control room, with the precision of watchmakers, pumped more and more electrical current into the massive
superconducting magnets that steer the beam in its 8-kilometer (5-mile) diameter (that's the same as a 27-kilometer, or 17-mile,
circumference
) circle.

Suddenly…
a massive cataclysmic explosion ripped through the tunnel!

The electrical circuit feeding one magnet to an adjacent one had inexplicably “opened.” Later it would be discovered that the solder joint between external copper plate connectors conducting the electrical current from magnet to magnet had melted. While the monitors had detected some heating in the copper joints, evidently the meltdown of the solder happened too quickly to be detected. The soldered joints had failed and the electrical circuit opened up.
¹¹

When the current flow through an electromagnet is suddenly interrupted, i.e., when the circuit is “opened,” there develops a virtually infinite voltage. This happens because nature opposes the interruption and strives to restore the current flow that is required to maintain the magnetic field. This is the basis of spark coils in automobile engines, Tesla coils, and any devices that step up voltage from one value to another, such as a transformer. For the largest magnets in the world, opening of the circuit produced a monstrous spark. At the LHC this human-made bolt of lightning, like a super-bolt from Odin's scepter, blasted through the neighboring magnets and pierced the cryostat, the large vessel that holds all of the liquid helium that keeps the magnet cold and superconducting.

The result was an explosive release of 3,000 gallons of liquid helium into the confines of the LHC tunnel. The resulting shock wave traveled down the tunnel at the speed of sound. A mile away steel doors were blown off their hinges. At least five of the massive magnets were destroyed, and about fifty others were damaged. The accelerator's entire vacuum tube was corrupted, as debris was sucked back into the beam pipe around its entire 27-kilometer circumference. It is said that the oxygen and nitrogen within the vicinity of the explosion was condensed out of the air itself and lay on the floor of the tunnel in a pool for six hours.

As a testimonial to modern safety standards, no human being was even slightly injured in this monstrous explosion. The catastrophe occurred at the time of international economic turmoil, however, and threatened the viability of the LHC project. The entire accelerator had to be cleaned out, and major repairs were required along nearly a half mile of the tunnel, including the replacement of five magnets. The event revealed a design flaw with the non-superconducting copper joiners that connect one magnet to
another, maintaining the current flow through the system, and these had to be replaced around the entire 27-kilometer circumference of the machine.

Though the entire future of the LHC was in doubt for a brief time, CERN persevered and brought it back online within two years. This was a spectacular and heroic feat, the kind of challenge that any risky new and cutting-edge endeavor must overcome, reminiscent of NASA's near-disaster with the Apollo 13 lunar mission. The LHC came back online, and less than four years after the “helium incident” successfully discovered the Higgs boson. LHC's performance since then has been jaw-droppingly spectacular. CERN's achievement with the LHC would surely have put a smile on the face of Albert Einstein—it has certainly drilled through the thick part of the wood.

At this writing (March 2013), the LHC is down for upgrading and will come back online around January 2015 for what will be the most important survey of the highest energy scales, or shortest distance scales humans have ever probed, at energies well beyond the mass of the Higgs boson.

So, since the LHC is the world's most powerful microscope, what is it we are looking for?

“ACH! IF I'D KNOWN THERE WOULD BE SO MANY PARTICLES I WOULD HAVE BEEN A BOTANIST INSTEAD.”

So said Einstein, supposedly, tongue in cheek, to a colleague in the lunch line at Princeton, who was explaining to the great master something of some newly discovered particles in the early 1950s. Back then these were called the “strange” particles, and they are known today to be composed of things dubbed “strange quarks.” The understanding of the nuclear particles, those associated with the strongly interacting particles found in the atomic nucleus, spanned the 1950s well into the 1970s. This branch of physics is still going on today in a modern form as we contemplate the ferociously energetic collisions at the LHC. Today, in the dawn of the third millennium, what is the new form of matter we would like to tell Professor Einstein about? That is the multi-billion-dollar question—the subject of this book: it's the Higgs boson, or the God Particle. And, as to what lies beyond—we simply don't know.

So, we might tell Professor Einstein in the lunch line at Princeton about the Higgs boson, and it goes as follows:

In its simplest theoretical incarnation, the Higgs boson is a form of matter named after one of the physicists who first considered the possibility. It forms a field, something like a magnetic field that is composed of photons, that fills all of space. Particles of matter interact with this field and acquire their masses. Heavy particles, like the top quark, interact more strongly with the Higgs field than light ones, like the electron. This should more properly be called the Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism after all of the authors who should share credit for it.
¹²
The idea of the Higgs boson, and the way in which it gives mass to other particles in nature derives from many sources in other fields of physics as well. The central idea, in fact, lies at the heart of superconductors and was first considered by people like Fritz London in the 1930s.
¹³
Following the initially somewhat general ideas of the Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism, Steven Weinberg put it all together and showed precisely how such a particle would fit into the overall scheme of nature, which ties together the so-called “weak” interactions with the “electromagnetic” interactions to form what we now call the Standard Model.
¹⁴
The name “Higgs boson” stuck. The Higgs boson does the job of making all the particles have their masses, and it becomes an essential ingredient. Through the Standard Model, the Higgs boson properties become precise, though the Higgs boson mass itself is
ab initio
unknown. Given knowledge of the Higgs boson mass, the Standard Model tells us how to produce the Higgs boson and how it will show up in detectors through its telltale fingerprints (i.e., via its “decay modes”
¹⁵
).