The Justice

The view from the side C of the ATLAS cavern in November 2007 with the end-cap magnet toroid and muon big wheels installed. Photo courtesy of the European Center for Nuclear Research

A Toroidal LHC Apparatus is designed to help scientistsresearch dark matter and particle mass. Shown here, the ATLAS muon wheel. Photo courtesy of the European Center for Nuclear Research

Muons are particles whose mass is slightly larger than electrons'. Above, a computer-generated image of the ATLAS Muons Subsystem. Photo courtesy of the European Center for Nuclear Research

Prof. James Bensinger participated in the design and construction of the LHC's optical alignment system. Photo courtesy of the Brandeis website

Within a few months, subatomic particles will begin colliding at 99.99 percent the speed of light in one of the most important physics experiments ever performed.

The experiments, conducted in the massive particle accelerator known as the Large Hadron Collider, will provide a window to some of the least understood and most fundamental phenomena in the universe.

Prof. James Bensinger (PHYS) represented Brandeis' involvement in the unprecedented development. Bensinger, who recently returned to Brandeis after a week at the European Center for Nuclear Research (CERN) in Switzerland, designed and constructed the accelerator's optical alignment system.

The accelerator is a 16.6-mile circular tunnel straddling the French-Swiss border. It uses magnets super-cooled to a temperature of -456°F to blast twin beams of subatomic particles around the ring 11,245 times per second in opposite directions before synchronizing a collision. Data collected from the collisions will tell physicists about the properties of known particles and possibly lead to the discovery of new particles.

Though orchestrated by CERN, an organization of 20 European member nations, the LHC represents an international collaboration involving 80 countries, hundreds of institutions and thousands of people.

For the thousands involved, the LHC is the chance of a lifetime. As Prof. Masahiro Morii of Harvard University's physics department explained, particle physics is bound by the ability to build accelerators. For decades, physicists have known that existing facilities would be inadequate to complete the group of theories outlining what we know of particle physics known as the Standard Model. Now, the LHC is on the brink of breaking into a whole new realm of understanding.

Says Morii, who only recently became involved in the project: "I was aware of the collider, and I was waiting for it. As the experiment approaches I ask, 'What can I do to help?'"

Physicists hope that data from the accelerator will explain why particles have mass, a property that is not intrinsic to the Standard Model. There are a number of theories, including the famed Higgs particle, which is often cited as the reason for building the collider. The LHC may also give insight into how gravity relates to the other three fundamental forces-the electromagnetic, strong and weak forces-hopefully leading to a unified theory, the Holy Grail of physics.

The LHC scientists will try to answer fundamental questions about the universe such as: What is the nature of dark matter, the mysterious particles that compose most of the mass in the universe? Why do particles have mass? What was the universe like right after the Big Bang? Are there more dimensions than the four (space and time) in which we exist?

A Toroidal LHC Apparatus, one of four experiments at the LHC, is designed to address these questions, particularly the search for dark matter and the reason for mass. Like the rest of the accelerator, ATLAS is a massive piece of machinery, a cylinder about 65 feet high. Proton beams collide in the center, jetting newly formed particles out through concentric layers of detectors. ATLAS has three basic layers: an inner tracker, a calorimeter and a muon spectrometer. Particles created in proton collisions travel first through the inner tracker, which maps the path of charged particles through a magnetic field. The calorimeter then measures the energy of these same particles. Most particles end their trajectories in the calorimeter, but muons, particles almost identical to electrons but with greater mass, continue through to the muon spectrometer. The spectrometer then records the energy and path of the muons.

Keeping track of muons is especially important since, at high energies, they could be indicators that a Higgs boson or some other theorized phenomenon formed during the experiment.

The United States' main contribution to the international collaboration is the so-called end-cap muon system, a section of the muon spectrometer that was built by a collection of institutions headed by the Boston Muon Consortium.

BMC consists of professors, and often their students, from Brandeis, Boston, Tufts and Harvard universities and the Massachusetts Institute of Technology.

Bensinger, one of the key members of BMC since its start in 1994, designed and built the optical alignment system that keeps track of the exact location of the 11,000 chambers in the muon detector. This was one of the major challenges of the project, since ATLAS must be both very big and very precise to be effective. Indeed, certain parts of the detector must be located to a precision of 40 microns, or 0.00016 inches.

The need for such precision is apparent in the work of Prof. Lawrence Kirsch (PHYS), also of Brandeis. Kirsch worked with Harvard to build a series of aluminum tubes that track the actual path of muons in the end-cap system.

"The system [of tubes] depends on the constancy of properties-quality control was Brandeis' responsibility," explained Kirsch.

When a muon travels through one of the tubes, it ionizes the gas in the tubes. This means that there are free electrons in the tube that are electromagnetically drawn to a wire running down the center of the tube. Based on the time the electrons take to reach the center wire, analysts can calculate the path of the muon to about 100 microns. The accuracy of the calculation, however, depends on knowing exactly where the center wire is at all times.

In order to collect the data from the detector, BMC had to develop a comprehensive electronics and software system. Physics Prof. John Huth of Harvard coordinated the design and construction of electronic chips, one per tube, to record data on the electrons hitting the wire.

At Tufts University's physics department, Prof. Krysztof Sliwa helped to develop simulations of how the detector would work and create a system to handle the collected data.

"The problem is that those detectors are collecting an enormous amount of data, orders of magnitude more than anything before, so you have a problem even how to read this amount of data or how to store the results," said Sliwa.

The MONARC project, in which Sliwa was involved, simulated a data collection system called "the grid," which Kirsch described as an extension of the Internet, that will be used for analyzing the LHC information.

Other physics professors in the BMC include Hermann Wellenstein and Craig Blocker of Brandeis, Anthony Mann of Tufts, George Brandenburg of Harvard, Steve Ahlen of BU, Frank Taylor of MIT and a number of students and alumni from all five universities. Along with Brookhaven National Laboratory, University of Washington and University of Michigan, they represent the American contribution to one of the most significant physics experiments in history.

"The U.S. tends to be arrogant and insular," said Taylor, who returned to MIT this summer from three years at CERN. "These big projects will keep us from being insular in the future because the frontier is at the international scale."

According to Kirsch, "The sociology is unbelievable, with so many people trying to somehow get it to work, but we have faith that people will learn more about the world."

Physics students at Brandeis recognize the LHC's tremendous potential to increase scientific knowledge. "It's a really cool device," says Josh Eisenberg '12, who works with Prof. Bensinger and Prof. Blocker. "It's not going to end the world, but it will for sure change physics, be it the death of the standard model or a deeper proof it."

The construction of the LHC will be delayed slightly due to technical problems that emerged after a transformer malfunction on Sep. 11. According to Bensinger, however, some delays are to be expected with an experiment of this size. "It is not the only project that was ever delayed," Bensinger explained. "ATLAS was actually ready before [the] LHC as a whole."

As LHC powers up and the first low-energy tests begin, the immense effort of thousands of people over two decades is finally paying off.

Says Huth, "Pinch me; I'm dreaming."

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