Higgs Boson Unmasked by World's Biggest Test Instruments

Two papers in the 17 September issue of Physics Letters B formally report independent discoveries of a particle that looks like a Higgs boson, walks like a Higgs boson, and quacks like a Higgs boson. The papers cautiously venture only that the particle is “consistent, within uncertainties, with expectations for the standard model Higgs boson”: It has a rest mass of about 125 GeV (gigaelectronvolts), no electrical charge, and a spin different from one. (For the record, the Intrade Prediction Market declared the Higgs boson found on September 12 and paid off its Higgs positions.)

It’s been a long road. Planning for the two experiments—the papers are from the ATLAS and CMS consortia— started in the 1990s, long before CERN’s Large Hadron Collider was built. Over the years, the project rosters have included more than 7000 scientists, engineers, technicians, and other collaborators. Each paper opened with a dedication to contributors who “did not live to see the full impact and significance of their experiment.”

Each paper is the result of a separate experiment, depending on a different detector—each of which is among the biggest, most complex, most expensive instruments ever built.

So the papers’ publication throws a spotlight (or, more accurately, a 7 TeV volt proton beam) on the two general-purpose detectors used at the Large Hadron Collider (LHC), itself the largest machine ever built, a 27-km-circumference ring tunneling 100 meters under the French-Swiss border near Geneva.

The detectors are big. ATLAS (A Toroidal LHC Apparatus, illustration above; note the two small figures standing on the barrel) is the larger of the two: 46 meters long and 25 meters in diameter. Think of something about as long as the Statue of Liberty is tall from the top of the pedestal to the tip of the torch, but a lot bulkier. ATLAS could fit inside the Lincoln Memorial, but it wouldn’t have a lot of room to spare.

CMS (Compact Muon Solenoid), the smaller detector, is 21 meters long and 15 meters in diameter. That’s roughly the size of a 2x3x3 stack of school buses.

Though different in detail, both instruments follow the same general plan. They are built as nested sensor cylinders, like a series of tin-cans-within-tin-cans, ranging from the size of a shoe-box near the core to basketball-court size at the outer limits. The detectors are symphonic variations on a theme by Rutherford and Einstein: incident particles strike a target material, throwing off a shower of particles; the shower particles strike a secondary material, stimulating emission of photons or electrons; the photons or electrons energize a sensor (a diode, a fiber optic cable, or a copper wire) to produce a signal, and mark the time, position, and nature of the original strike.

Both machines must be designed for ridiculously demanding environments—the sine qua non is ultrafast and ultra-accurate logging and triage of billions of events per second in a typhoon of radiation under magnetic fields otherwise found only in MRI machines.

ATLAS (AToroidal LHC Apparatus) Experiment

As noted, ATLAS is the bigger of the two instruments, half an American football field long and eight stories tall.

ATLAS has three main sections—an inner detector, a pair of calorimeters to measure particle energies, and a muon spectrometer—assembled on two of the world’s largest electromagnets and tied together by a data-processing system that catches and interprets a many-petabyte-per-second flood and throttles it down to a manageable one petabyte per year.(Click here for a video tour.)

Enmeshed in the outer parts of the detector are two massive electromagnets, a five-ton, 2.4-meter-diameter, 2-Tesla solenoid and an 830-ton, 20-meter-diameter, 4-Tesla toroidal barrel magnet. These intense fields curve the paths of charged particles. Interactions with the various sensor layers yield information on position, time, and the energy of individual interactions (like tracking a cannonball through a cornfield by watching the tassels wave). Tying the individual data points together over time gives the particle’s trajectory, and the curve reveals charge, mass, and velocity.

Inner detector. ATLAS’s inner detector (image at right) is an array of sensors that measure the momentum of each charged particle.

First comes the pixel detector, (see Spectrum's description)  about the size of a jeroboam of champagne (or, for those of us of more modest means, a couple of five-gallon paint cans), three nested cylinders containing 80 million bipolar diodes designed to weather intense ionizing radiation. 

Next comes the 7-meter-long semiconductor tracker. This medium-range system wraps the pixel detector in eight layers of silicon microstrip detectors that log momentum, impact parameters, and position, streaming 6 million channels of data.

Then comes a transition radiation tracker, a sheath of some 350 000 “soda straw” sensors— 4-mm-diameter tubes filled with a mixture of xenon, carbon dioxide, and oxygen, with a gold-plated wire running down the axis. A pion or electron hitting the gas mixture sends out a shower of photons, which in turn kick out electrons. The electrons produce a current in the axial wire—more for an electron, less for a pion—proportional to the particle’s momentum. The system tracks multiple strikes along the particle’s path, and the path’s trajectory betrays its momentum.

Calorimeters. A two-stage calorimeter encloses the inner detectors, measuring the particle energies.

The inner layer, the electromagnetic calorimeter, measures the energies of photons and electrons. It consists of pleated baffles of lead and stainless steel, with liquid argon filling the gaps. Again, a particle striking the metal throws off a characteristic shower of particles, which stimulate photons in the argon and produce currents in a copper grid. The researchers can log the interactions along a particle’s track and add up the energies of the emitted photons to calculate the total energy.

The outer hadronic calorimeter measures the tracks and energies of protons, neutrons, and mesons. These are steel sheets interleaved with scintillating plastic tiles. Once again, the system detects the light from the particle shower and adds it up to find the energy of the original hadron.

Muon spectrometer. Only muons and neutrinos make it out of the hadronic calorimeter alive, and the muon stops here. These particles (with the same charge and spin as an electron, but 200 times heavier) arc through the detectors' soda-straw sensors (similar in design to those used in the transition radiation tracker), freeing electrons along their routes. The electron clouds drift to the straws’ sides and axial wires. The researchers log the resulting currents and can map the drift times back to reconstruct the particle path and energy. (Neutrinos almost never interact with anything: researchers have to infer their masses and energies the way bank examiners find embezzlements—they add up what they find in the till and subtract it from what they had to start with.)

Data System. Overall, ATLAS produces about 23 petabytes of raw data per second. The ATLAS data processing system makes the analysis a little easier by doing a lot of filtering on the front end. It uses a multi-stage triage process to reduce the data-stream from a billion or so events-per-second  to a hundred or so candidates qualified for further analysis—which still adds up to a petabyte a year.

There is a Level 1 Trigger that cherry picks information from the energy and muon detectors, deciding in about two microseconds (and that includes transmission delays) what to trash and what to pass on. Events that make it through Level 1 are cached for a fraction of a second, then passed on to the Level 2 and Level 3 Triggers, which further winnow the crowd. Overall, these filters reduce the incoming data to about a 1 KHz flow, with a tolerated latency up to 10 milliseconds.

Compact Muon Solenoid (CMS) Experiment

The Compact Muon Solenoid (CMS) detector (image below) follows the same general plan, with pixel (image at left) and silicon tracker sensors near the beam axis (in fact, both detectors use the same pixel sensor design), followed by electromagnetic and hadronic calorimeters, all wrapped up in the iron cocoon of the muon detector. (Click here for an animation of CMS detection strategies.) Though smaller than the ATLAS detector, the CMS device is considerably heavier: at about 15,400 tons, it is almost exactly twice as heavy as ATLAS. (Each of CMS’s 36 muon detector segments alone weighs in at almost 29 tons.)

CMS’s construction strategy was novel. Instead of being assembled down in the cavern, it was built on the surface, in 15 slices that were then lowered into place. This let the team start fabricating the CMS while tunnel excavators were still at work.

The magnet strategy is different, too. The solenoid that gives the experiment its name--CERN confidently calls it the largest superconducting electromagnet—is a single tube, with no toroidal barrel magnet running through the muon detector. The 4-Tesla solenoid is 13 meters long and 7 meters in diameter (almost three times the diameter of the ATLAS solenoid). Think of it as an MRI big enough to give a sperm whale a full-body scan.

Instead of pleated metal and liquid argon, the CMS electromagnetic calorimeter uses a swaddling of transparent lead tungstate crystals—75,848 of them. Each 2.5 by 2.5 by 23 centimeter prism weighs about three pounds and took two days to grow in Russian and Chinese factories. The lead tungstate scintillates when struck by electrons and photons. Avalanche photodiodes (APDs) catch the light and amplify the signal. (On the top and bottom of the lead-tungstate “can,” the radiation is too high for the silicon-based photodiodes, so CMS uses vacuum photodiodes.)

Images: CERN, CMS/ATLAS