The Higgs Boson: Searching for the God Particle

by Scientific American Editors

  More than 20 physicists have won Nobel Prizes for work that has contributed to the Standard Model, from the theory of quantum electrodynamics (the 1965 prize) to the discovery of the neutrino and the tau particle (1995) and the theoretical work of Gerardus ’t Hooft and Martinus J. G. Veltman while at the University of Utrecht (1999). Nevertheless, although it is a great scientific achievement, confirmed by a plethora of detailed experiments, the Standard Model has a number of serious flaws.

  First, it does not consistently include Albert Einstein’s theory of the properties of spacetime and its interaction with matter. This theory, general relativity, provides a beautiful, experimentally very well verified description of the fourth force, gravity. The difficulty is that unlike general relativity, the Standard Model is a fully quantum-mechanical theory, and its predictions must therefore break down at very small scales (very far from the domain in which it has been tested). The absence of a quantum-mechanical description of gravity renders the Standard Model logically incomplete.

  Second, although it successfully describes a huge range of data with simple underlying equations, the Standard Model contains many apparently arbitrary features. It is too byzantine to be the full story. For example, it does not indicate why there are six quarks and six leptons instead of, say, four. Nor does it explain why there are equal numbers of leptons and quarks—is this just a coincidence? On paper, we can construct theories that explain why there are deep connections between quarks and leptons, but we do not know if any of these theories is correct.

  Third, the Standard Model has an unfinished, untested element. This is not some minor detail but a central component: a mechanism to generate the observed masses of the particles. Particle masses are profoundly important—altering the mass of the electron, for example, would change all of chemistry, and the masses of neutrinos affect the expansion of the universe. (A neutrino’s mass is at most a few millionths of an electron’s mass, but recent experiments show that it is not zero. The scientists who led two pioneering experiments that made this discovery were awarded a share of the 2002 Nobel Prize for Physics.)

  Higgs Mechanism

  Physicists believe that particle masses are generated by interactions with a field that permeates the entire universe; the stronger a particle interacts with the field, the more massive it is. The nature of this field, however, remains unknown. It could be a new elementary field, called the Higgs field after British physicist Peter Higgs. Alternatively, it may be a composite object, made of new particles (“techniquarks”) tightly bound together by a new force (“technicolor”). Even if it is an elementary field, there are many variations on the Higgs theme: How many Higgs fields are there, and what are their detailed properties?

  Nevertheless, we know with virtually mathematical certainty that whatever mechanism is responsible, it must produce new phenomena in the LHC’s energy range, such as observable Higgs particles (which would be a manifestation of ripples in the underlying field) or techniparticles. The principal design goal of the LHC is therefore to discover these phenomena and pin down the nature of the mass-generating mechanism.

  These new phenomena may be discovered before the LHC comes into operation by experiments at Fermilab’s Tevatron, which started colliding beams of protons and antiprotons again in 2001 after a major upgrade. These experiments could find new phenomena beyond the range already explored by LEP. But even if they do “scoop” the LHC, they will reveal only the tip of a new iceberg, and the LHC will be where physicists make comprehensive studies of the new processes.

  If the Tevatron does not observe these new phenomena, then the LHC will pick up the chase. The exploratory power of the LHC overlaps that of LEP and the Tevatron, leaving no gaps in which new physics could hide. Moreover, high-precision measurements made in the past decade at LEP, the Stanford Linear Accelerator Center and Fermilab have essentially eliminated worries that the Higgs boson might be out of reach of the LHC’s energy range. It is now clear that either the Higgs boson or other new physics associated with the generation of mass will be found at the LHC.

  Emulating the Big Bang

  To address this kind of physics requires re-creating conditions that existed just a trillionth of a second after the big bang, a task that will push modern technologies to their limits and beyond. To hold the 7-TeV proton beams on course, magnets must sustain a magnetic field of 8.3 tesla, almost 100,000 times the earth’s magnetic field and the highest ever used in an accelerator. They will rely on superconductivity: large currents flowing without resistance through thin superconducting wires, resulting in compact magnets that can generate magnetic-field strengths unobtainable with conventional magnets made with copper wires. To maintain the superconductivity under operating conditions—with 12,000 amps of current—the magnets’ cores must be held at –271 degrees C around 22.4 kilometers of the tunnel. Cryogenics on this scale has never before been attempted.

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  ACCELERATOR MAGNET is shown in cross section. The superconducting coils carry 12,000 amps of current and must be kept cooled to below two kelvins. Each beam pipe carries one of the two countermoving proton beams. Other magnets focus the beams and bend them to cross at collision points within the detectors.

  Illustration by Slim Fims

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  In December 1994 a full prototype section of the LHC was operated for 24 hours, demonstrating that the key technical choices for the magnets are correct. Since then, tests on prototypes have simulated about 10 years of running the LHC. Magnets that surpass the design criteria are now being produced in industry and delivered to CERN for final testing and subsequent installation.

  With the 1993 demise of the planned 40-TeV SSC, the 14-TeV LHC became the only accelerator project in the world that can support a diverse research program at the high-energy frontier. The LHC’s intense beams present those designing the experiments with remarkable challenges of data acquisition. The beams will consist of proton bunches strung like beads on a chain, 25 billionths of a second apart. At each collision point, pairs of these bunches will sweep through each other 40 million times a second, each time producing about 20 proton-proton collisions. Collisions will happen so often that particles from one collision will still be flying through the detectors when the next one occurs!

  Of these 800 million collisions a second, only about one in a billion will involve a head-on quark collision. To keep up with this furious pace, information from the detector will go into electronic pipelines that are long enough to hold the data from a few thousand collisions. This will give “downstream” electronics enough time to decide whether a collision is interesting and should be recorded before the data reach the end of the pipeline and are lost. LHC detectors will have tens of millions of readout channels. Matching up all the pipelined signals that originate from the same proton-proton collision will be a mind-boggling task.

  When Quarks Collide

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  A toroidal LHC apparatus (ATLAS) detector uses a novel toroidal magnet system. Protons collide in the center, producing a spray of particles. The concentric layers of ATLAS detect different species of particles, some precisely tracking the particle trajectories, others (“calorimeters”) measuring the energy carried

  Illustration by Slim Films

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  Particle detectors are the physicists’ electronic eyes, diligently watching each collision for signs of interesting events. LHC will have four particle detectors. Two will be giants, each built like a Russian matryoshka doll, with modules fitting snugly inside modules and a beam collision point at the center. Each module, packed with state-of-the-art technology, is custom-built to perform specific observations before the particles fly out to the next layer. These general-purpose detectors, ATLAS and CMS, standing up to 22 meters high, will look for Higgs particles and supersymmetry and will be on the alert for the unexpected, recording as much as possible of the collision debris. Two smaller detectors, ALICE and LHCb, will concentrate on different specific areas of physics
.

  Both ATLAS and CMS are optimized to detect energetic muons, electrons and photons, whose presence could signal the production of new particles, including Higgs bosons. Yet they follow very different strategies. Years of computer simulations of their performance have shown that they are capable of detecting whatever new phenomena nature may exhibit. ATLAS (a toroidal LHC apparatus) is based on an enormous toroidal magnet equipped with detectors designed to identify muons in air. CMS (compact muon solenoid) follows the more traditional approach of using chambers inside the return yoke of a very powerful solenoidal magnet to detect muons.

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  COMPACT MUON SOLENOID (CMS) detector uses a more traditional magnet design than ATLAS does and is optimized for detecting muons. CMS has muon detectors (yellow) interleaved with iron layers (orange) that channel the magnetic field produced by the superconducting solenoid coil. The electromagnetic calorimeter (blue) contains 80,000 lead-tungstate crystals for detecting electrons and photons. Above, a computer simulation shows a collision in which a Higgs particle decays into two muons (the tracks at about “4 o’clock”) and two jets of hadrons (at about “11 o’clock”).

  Illustration by Slim Films

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  Part of the CMS detector will consist of crystals that glow, or scintillate, when electrons and photons enter them. Such crystals are extremely difficult to make, and CMS benefits from the experience gained from a recent CERN experiment, L3, which also used crystals. (The L3 detector was one of four that operated from 1989 to 2000 at the LEP collider, performing precision studies of the weak force that told us that exactly three types of zero- or low-mass neutrino exist.) Before L3, such crystals had been made only in small quantities, but L3 needed 11,000 of them. Crystals of the type developed for L3 have been widely used in medical imaging devices. CMS needs more than seven times as many crystals made of a more robust material. In due course the superior CMS crystals are likely to have an even bigger effect on the medical field.

  ALICE (a large ion collider experiment) is a more specialized experiment that will come into its own when the LHC collides nuclei of lead with the colossal energy of 1,150 TeV. That energy is expected to “melt” the more than 400 protons and neutrons in the colliding nuclei, releasing their quarks and gluons to form a globule of quark-gluon plasma (QGP), which dominated the universe about 10 microseconds after the big bang. ALICE is based around the magnet of the L3 experiment, with new detectors optimized for QGP studies.

  There is good evidence that experiments at CERN have already created a quark-gluon plasma. Over the coming years, Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) has a good chance of studying QGP in detail by packing 10 times more energy per nucleon into its collisions than CERN does. The LHC will extend this by a further factor of 30. The higher energy at the LHC will complement the more varied range of experiments at RHIC, guaranteeing a thorough study of an important phase in the universe’s early evolution.

  B mesons, the subject of LHCb’s investigations, could help tell us why the universe is made of matter instead of equal amounts of matter and antimatter. Such an imbalance can arise only if heavy quarks and antiquarks decay into their lighter cousins at different rates. The Standard Model can accommodate this phenomenon, called CP violation, but probably not enough of it to account completely for the dominance of matter in the universe. Physicists observed CP violation in the decay of strange quarks in the 1960s, but data on heavy “bottom” quarks and antiquarks, the constituents of B mesons, are also needed to establish whether the Standard Model description is correct.

  In 1999 experiments began at two B factories in California and Japan that can produce tens of millions of B mesons a year. These experiments have observed the CP violation predicted by the Standard Model in one B meson decay mode. The high luminosity of the LHC beams can churn out a trillion B mesons a year for LHCb. This will allow much higher precision studies in a wider variety of circumstances and perhaps uncover crucial exotic decay modes too rare for the other factories to see clearly.

  A Laboratory for the World

  Scientific experiments as ambitious as the LHC project are too expensive to be palatable for any one country. Of course, international collaboration has always played a role in particle physics, scientists being attracted to the facilities best suited to their research interests, wherever situated. As detectors have become larger and costlier, the size and geographic spread of the collaborations that built them have grown correspondingly. (It was the need to facilitate communication between the LEP collaborations that stimulated the invention of the World Wide Web by Tim Berners-Lee at CERN.)

  The LHC accelerator originally had funding only from CERN’s (then) 19 European member states, with construction to occur in two phases on a painfully slow timetable—a poor plan scientifically and more expensive in toto than a faster, single-phase development. Fortunately, additional funds from other countries (which will provide some 40 percent of the LHC’s users) will speed up the project. Contributions of money or labor have been agreed to by Canada, India, Israel, Japan, Russia and the U.S. For example, Japan’s KEK laboratory will supply 16 special focusing magnets. The U.S., with more than 550 scientists already involved, will furnish the largest national group; accelerator components will be designed and fabricated by Brookhaven, Fermilab and Lawrence Berkeley National Laboratory.

  Furthermore, 5,000 scientists and engineers in more than 300 universities and research institutes in 50 countries on six continents are building the ATLAS and CMS detectors. When possible, components will be built in the participating institutions, close to students (who get great training by working on such projects) and in collaboration with local industries. The data analysis will also be dispersed. It will be a formidable challenge to manage these projects, with their stringent technical requirements and tight schedules, while maintaining the democracy and freedom for scientific initiatives that are essential for research to flourish.

  Until now, CERN has been primarily a European laboratory. With the LHC, it is set to become a laboratory for the world. Already its 7,000 scientific users amount to more than half the world’s experimental particle physicists. In 1994 John Peoples, Jr., then director of Fermilab, summed it up nicely: “For 40 years, CERN has given the world a living demonstration of the power of international cooperation for the advancement of human knowledge. May CERN’s next 40 years bring not only new understanding of our Universe, but new levels of understanding among nations.”

  -Originally published: Scientific American 13, 52-59 (March 2003)

  The Discovery Machine

  by Graham P. Collins

  You could think of it as the biggest, most powerful microscope in the history of science. The Large Hadron Collider (LHC), now being completed underneath a circle of countryside and villages a short drive from Geneva, will peer into the physics of the shortest distances (down to a nano-nanometer) and the highest energies ever probed. For a decade or more, particle physicists have been eagerly awaiting a chance to explore that domain, sometimes called the tera scale because of the energy range involved: a trillion electron volts, or 1 TeV. Signifi cant new physics is expected to occur at these energies, such as the elusive Higgs particle (believed to be responsible for imbuing other particles with mass) and the particle that constitutes the dark matter that makes up most of the material in the universe.

  The mammoth machine, after a nine-year construction period, is scheduled (touch wood) to begin producing its beams of particles later this year. The commissioning process is planned to proceed from one beam to two beams to colliding beams; from lower energies to the terascale; from weaker test intensities to stronger ones suitable for producing data at useful rates but more difficult to control. Each step along the way will produce challenges to be overcome by the more than 5,000 scientists, engineers and students collaborating on the gargantuan effort. When I visited the project last fall to get a firsthand look at the preparations to probe the highenergy fronti
er, I found that everyone I spoke to expressed quiet confidence about their ultimate success, despite the repeatedly delayed schedule. The particle physics community is eagerly awaiting the first results from the LHC. Frank Wilczek of the Massachusetts Institute of Technology echoes a common sentiment when he speaks of the prospects for the LHC to produce “a golden age of physics.”

  A Machine of Superlatives

  To break into the new territory that is the terascale, the LHC’s basic parameters outdo those of previous colliders in almost every respect. It starts by producing proton beams of far higher energies than ever before. Its nearly 7,000 magnets, chilled by liquid helium to less than two kelvins to make them superconducting, will steer and focus two beams of protons traveling within a millionth of a percent of the speed of light. Each proton will have about 7 TeV of energy—7,000 times as much energy as a proton at rest has embodied in its mass, courtesy of Einstein’s E=mc2. That is about seven times the energy of the reigning record holder, the Tevatron collider at Fermi National Accelerator Laboratory in Batavia, Ill. Equally important, the machine is designed to produce beams with 40 times the intensity, or luminosity, of the Tevatron’s beams. When it is fully loaded and at maximum energy, all the circulating particles will carry energy roughly equal to the kinetic energy of about 900 cars traveling at 100 kilometers per hour, or enough to heat the water for nearly 2,000 liters of coffee.