The Higgs Boson: Searching for the God Particle

by Scientific American Editors

  Because the ILC’s linacs can be shorter (and hence less expensive) if the cavities can generate a stronger electric field, the design team has set an aggressive goal of improving the performance of the SCRF system until it can give the particles an energy boost of 35 million electron volts (MeV) for every meter they travel. Several prototype cavities have already exceeded this goal, but it remains a challenge to mass-produce such devices. The key to high performance is ensuring that the inner surface of the cavity is ultraclean and defect-free. The preparation of the cavities and their installation in the cryomodules must be done in clean-room environments.

  The ILC in a Nutshell

  The ILC design team has already established the basic parameters for the collider. The machine will be about 31 kilometers long, with most of that length taken up by the two superconducting linacs that will set up electron- positron collisions with 500 GeV energies. (A 250-GeV electron striking a 250-GeV positron moving in the opposite direction will result in a collision with a center-of-mass energy of 500 GeV.) At a rate of five times per second, the ILC will generate, accelerate and collide nearly 3,000 electron and positron bunches in a onemillisecond- long pulse, corresponding to an average total power of about 10 megawatts for each beam. The overall efficiency of the machine—that is, the fraction of electrical power converted to beam power—will be about 20 percent, so the two linacs will require a total of about 100 megawatts of electricity to accelerate the particles.

  To produce the electron beam, a laser will fire at a target made of gallium arsenide, knocking off billions of electrons with each pulse. These particles will be spin-polarized—all their spin axes will point in the same direction— which is important for many particle physics investigations. The electrons will be rapidly accelerated in a short SCRF linac to an energy of 5 GeV, then injected into a 6.7-kilometer storage ring at the center of the complex. As the electrons circulate and emit synchrotron radiation, the bunches of particles will be damped—that is, their volume will decrease, and their charge density will increase, maximizing the intensity of the beam.

  When the electron bunches exit the damping ring 200 milliseconds later, each will be about nine millimeters long and thinner than a human hair. The ILC will then compress each electron bunch to a length of 0.3 millimeter to optimize its acceleration and the dynamics of its subsequent collisions with the corresponding positron bunch inside the detector. During the compression, the bunches will be boosted to an energy of 15 GeV, after which they will be injected into one of the main 11.3-kilometer-long SCRF linacs and accelerated to 250 GeV.

  Midway through the linac, when the particles are at an energy of 150 GeV, the electron bunches will take a small detour to produce the positron bunches. The electrons will be deflected into a special magnet known as an undulator, where they will radiate some of their energy into gamma rays. The gamma photons will be focused onto a thin titanium alloy target that rotates about 1,000 times per minute, and the impacts will produce copious numbers of electronpositron pairs. The positrons will be captured, accelerated to an energy of 5 GeV, transferred to another damping ring and finally sent to the other main SCRF linac at the opposite end of the ILC. Once the electrons and positrons are fully accelerated to 250 GeV and rapidly converging toward the collision point, a series of magnetic lenses will focus the high-energy bunches to flat ribbon beams about 640 nanometers (billionths of a meter) wide and six nanometers high. After the collisions, the bunches will be extracted from the interaction region and removed to a so-called beam dump, a target that can safely absorb the particles and dissipate their energy.

  Every subsystem of the ILC will push the technological envelope and present major engineering challenges. The collider’s damping rings must achieve beam qualities several times better than those of existing electron storage rings. What is more, the high beam quality must be preserved throughout the compression, acceleration and focusing stages. The collider will require sophisticated diagnostics, state-of-the-art beam-tuning procedures and a very precise alignment of its components. Building the positron production system and aiming the nanometer-size beams at the collision point will be demanding tasks.

  Developing detectors that can analyze the collisions in the ILC will also be challenging. To determine the strengths of the interactions between the Higgs boson and other particles, for example, the detectors will need to measure the momentum and creation points of charged particles with resolutions that are an order of magnitude better than those of previous devices. Scientists are now working on new tracking and calorimeter systems that will allow researchers to harvest the rich physics of the ILC.

  The Next Steps

  Although the ILC team has chosen a design for the collider, much more planning needs to be done. Over the next few years, while the LHC starts collecting and analyzing data from its proton-proton collisions, we will strive to optimize the ILC design to ensure that the electronpositron collider achieves the best possible performance at a reasonable cost. We do not yet know where the ILC will be located; that decision will most likely hinge on the amount of financial support that governments are willing to invest in the project. In the meantime, we will continue to analyze the sample ILC sites in Europe, the U.S. and Japan. Differences in geology, topography, and local standards and regulations may lead to different construction approaches and cost estimates. Ultimately, many details of the ILC design will depend on exactly where the collider is built.

  In any event, our planning will allow us to move forward at full speed as soon as the scientific discoveries at the LHC reveal the best targets for follow-up research. In parallel with the technical design work, we are creating models for dividing the governance of the ILC project so that each constituency of physicists will have a say. This ambitious undertaking has been truly global in its conception, development and design, and we expect it to be thoroughly international in its construction and operation as well.

  -Originally published: Scientific American 298(2), 54-59 (February 2008)

  Higgs Won’t Fly

  by Graham P. Collins

  In a move that surprised and dismayed many physicists, one of the world’s leading laboratories has chosen not to continue an experiment that showed every sign of being on the verge of discovering an elusive particle that would have placed the capstone on a century of particle physics. The experiment was the last gasp of the venerable Large Electron-Positron collider (LEP), located near Geneva, Switzerland, and part of the European laboratory for particle physics (CERN). The particle was the long-sought Higgs, which is profoundly unlike any other particle discovered in human history and is the final jigsaw piece needed to complete the Standard Model of particle physics. The decision came down to the judgment of one man, Luciano Maiani, CERN’s director general, who chose to shut down LEP on schedule to avoid delaying construction of CERN’s next big experiment, the Large Hadron Collider (LHC), which is slated to be turned on in 2005.

  Postulated independently by British physicist Peter Higgs and others in 1964, the Higgs plays a unique role in particle physics. In one guise, the Higgs is a field permeating the universe and giving the other particles their mass. If the field were turned off, the particles making up your body would presumably fly apart at the speed of light like so many photons. We have no way of directly detecting the allpervasive Higgs field, but its other guise—individual Higgs particles, like tiny concentrated knots in the field—should be producible in violent collisions at accelerators. By studying the particle, physicists can verify the theory and pin down the Higgs’s many unknown properties.

  In 2000 researchers optimized the 11-year-old LEP to conduct one last search for the Higgs, pushing it to achieve collision energies of 206.5 billion electron volts (GeV)—about 14 GeV beyond its original design parameters. Most likely the Higgs would be too massive to fall within LEP’s extended reach, but in the summer, physicists saw signs of Higgs particles. Out of millions of collisions, nine produced Higgs candidates. A one-month extension to LEP yielded
additional results, sufficient to conclude that the odds that the results were noise were one in 250—a tantalizing result but much too uncertain to proclaim “discovery.” The data indicated that the Higgs has a mass of about 115 GeV (the remaining collision energy goes into creating a so-called Z particle at 91 GeV). By comparison, a proton is 1 GeV. A 115-GeV Higgs would agree nicely with predictions of supersymmetry models— the idea that particles in the Standard Model have “supersymmetric” partners.

  Hoping to gain enough data to reduce the odds of error below the one in a million needed for a discovery, experimenters pleaded for a year’s reprieve to LEP’s scheduled dismantling, but after vigorous debate they were turned down. It was time to make way for the $4-billion LHC, which is to occupy the same 27-kilometer-circumference tunnel as LEP. Running LEP in 2001 would have cost CERN $65 million, including $40 million in civilengineering contract penalties for delaying the LHC.

  Chris Tully, the Higgs coordinator for one of the four LEP detectors and the person responsible for combining the data from all four, complains that what is most frustrating is the perceived failure of CERN’s scientific decision-making process. Two different review boards discussed the Higgs evidence and the extension request, and both failed to recommend whether to proceed or not. Each board had roughly equal numbers of LEP and LHC scientists. Tully feels that part of the problem was the boards’ not keeping to their proper terms of reference. For example, the LEP Scientific Committee, instead of limiting itself to the scientific issues, also considered the potential effect on LHC finances.

  Maiani’s decision could have been overturned at a special November 17 meeting of the CERN Council, representatives of CERN’s 20 member countries—but again the result was a deadlock, and so Maiani’s decision stood. “CERN is following a scientific program based on indecision,” Tully says. Yet he doesn’t fault Maiani, who, he considers, “made the wisest choice” from the perspective of a director general, who must give highest priority to the future of the laboratory, meaning the LHC.

  LHC advocates insist that the decision was based on the science. Ana Henriques Correia, who leads construction on part of the LHC’s ATLAS detector, says, “The scientific evidence [for Higgs] was not strong enough to postpone LHC.” She points out that a sizable chance remained of no discovery by LEP even after a 2001 run.

  Supporters argue that LEP was uniquely positioned to discover or rule out a 115-GeV Higgs promptly: after 11 years LEP’s experimenters had a very good understanding of the performance of the accelerator and its four detectors. By comparison, the LHC’s extremely complicated detectors are unknown quantities. Although the LHC is scheduled to collide its first protons in July 2005, collection of scientific data will not begin until 2007—after the lengthy process of commissioning, understanding and calibrating the accelerator and its detectors. Furthermore, CERN is discussing moving the start-up date back to the end of 2005.

  The opportunity to discover the Higgs now passes to the Tevatron proton collider at the Batavia, Ill., Fermi National Accelerator Laboratory. The Tevatron discovered the top quark in 1995 and starts up again in March after a major upgrade. But it will take until about 2006 to gather sufficient data to claim discovery of the Higgs, if it is near 115 GeV (the device could see Higgs evidence up to 180 GeV). Paul Grannis, a member of the D-Zero experiment at the Tevatron, cautions that he doesn’t know enough about the various factors in play to second-guess the CERN decision, but nonetheless he has “a hard time imagining why they did not” choose to continue. “We would be globally in so much better shape if we knew whether the Higgs were there or not, in trying to map out the future [accelerator] program.”

  These matters interest experimenters planning what to build after the LHC. The U.S., Japan and Germany are working on plans for the next electron-positron colliders, which will explore higher energies than LEP had. These devices would map out the detailed properties of the Higgs and other new particles, such as supersymmetric particles, expected to be discovered at the LHC. A Higgs under 130 GeV favors supersymmetry, and physicists understand very well what kind of program is needed to find and study supersymmetry. Above 130 GeV, “it is most likely not supersymmetry,” Grannis says, “and then we’re on a fishing expedition to figure out what the hell is going on.”

  -Originally published: Scientific American 284(2) 17-18 (February 2001)

  The Large Hadron Collider

  by Chris Llewellyn Smith

  * * *

  CERN

  * * *

  When two protons traveling at 99.999999 percent of the speed of light collide head-on, the ensuing subatomic explosion provides nature with 14 trillion electron volts (TeV) of energy to play with. This energy, equal to 14,000 times that stored in the mass of a proton at rest, is shared among the smaller particles that make up each proton: quarks and the gluons that bind them together. In most collisions the energy is squandered when the individual quarks and gluons strike only glancing blows, setting off a tangential spray of familiar particles that physicists have long since catalogued and analyzed. On occasion, however, two of the quarks will themselves collide head-on with an energy as high as 2 TeV or more. Physicists are sure that nature has new tricks up her sleeve that must be revealed in those collisions—perhaps an exotic particle known as the Higgs boson, perhaps evidence of a miraculous effect called supersymmetry, or perhaps something unexpected that will turn theoretical particle physics on its head.

  The last time that such violent collisions of quarks occurred in large numbers was billions of years ago, during the first picosecond of the big bang. They will start occurring again in 2007, in a circular tunnel under the Franco-Swiss countryside near Geneva. That’s when thousands of scientists and engineers from dozens of countries expect to finish building the giant detectors for the Large Hadron Collider (LHC) and start experiments. This vast and technologically challenging project, coordinated by CERN (the European laboratory for particle physics), which is taking the major responsibility for constructing the accelerator, is already well under way.

  The LHC will have about seven times the energy of the Tevatron collider based at Fermi National Accelerator Laboratory in Batavia, Ill., which discovered the long-sought “top” quark in experiments spanning from 1992 to 1995. The LHC will achieve its unprecedented energies despite being built within the confines of an existing 27-kilometer tunnel. That tunnel housed CERN’s Large Electron-Positron Collider (LEP), which operated from 1989 to 2000 and was used to carry out precision tests of particle physics theory at about 1 percent of the LHC’s energy. By using LEP’s tunnel, the LHC avoids the problems and vast expense of siting and building a new, larger tunnel and constructing four smaller “injector” accelerators and supporting facilities. But bending the trajectories of the 7-TeV proton beams around the old tunnel’s curves will require magnetic fields stronger than those any accelerator has used before. Those fields will be produced by 1,232 15-meter-long magnets installed around 85 percent of the tunnel’s circumference. The magnets will be powered by superconducting cables carrying currents of 12,000 amps cooled by superfluid helium to –271 degrees Celsius, two degrees above the absolute zero of temperature.

  To carry out productive physics experiments, one needs more than just high-energy protons. What counts is the energy of collisions between the protons’ constituent quarks and gluons, which share a proton’s energy in a fluctuating manner. The LHC will collide beams of protons of unprecedented intensity to increase the number of rare collisions between quarks and gluons carrying unusually large fractions of their parent protons’ energy. The LHC’s intensity, or luminosity, will be 100 times as great as that of previous colliders and 10 times that of the canceled Superconducting Super Collider (SSC). The SSC would have been a direct competitor to the LHC, colliding 20-TeV proton beams in an 87- kilometer-circumference tunnel around Waxahachie, Tex. The LHC’s higher intensity will mostly compensate for the lower beam energy, but it will make the experiments much harder. Furthermore, such la
rge intensities can provoke problems, such as chaos in the beam orbits, that must be overcome to keep the beams stable and well focused.

  At four locations around the LHC’s ring, a billion collisions will occur each second, each one producing about 100 secondary particles. Enormous detectors— the largest roughly the height of a six-story building—packed with thousands of sophisticated components will track all this debris. Elaborate computer algorithms will have to sift through this avalanche of data in real time to decide which cases (perhaps 10 to 100 per second) appear worthy of being recorded for full analysis later, off-line.

  Unanswered Questions

  As we study nature with higher-energy probes, we are delving into the structure of matter at ever smaller scales. Experiments at existing accelerators have explored down to one billionth of one billionth of a meter (10-18 meter). The LHC’s projectiles will penetrate even deeper into the heart of matter, down to 10-19 meter. This alone would be enough to whet scientific appetites, but pulses are really set racing by compelling arguments that the answers to major questions must lie in this new domain.

  In the past 35 years, particle physicists have established a relatively compact picture—the Standard Model—that successfully describes the structure of matter down to 10-18 meter. The Standard Model succinctly characterizes all the known constituents of matter and three of the four forces that control their behavior. The constituents of matter are six particles called leptons and six called quarks. One of the forces, known as the strong force, acts on quarks, binding them together to form hundreds of particles known as hadrons. The proton and the neutron are hadrons, and a residual effect of the strong force binds them together to form atomic nuclei. The other two forces are electromagnetism and the weak force, which operates only at very short range but is responsible for radioactive beta decay and is essential for the sun’s fuel cycle. The Standard Model elegantly accounts for these two forces as a “unified” electroweak force, which relates their properties despite their appearing very different.