Assembly of supercollider detector reaches halfway point

Brown physicists contribute multiple components.

by Marty Downs

A sigh of relief rose from physicists around the world as one of the largest components of the CMS detector, scheduled to start operating at the Large Hadron Collider (LHC) later this year, was eased into place 100 meters underground. Three physics professors from Brown and many Brown graduate students and postdoctoral researchers were among those celebrating.

On February 28, the Compact Muon Solenoid (CMS) particle detector safely completed its descent into the 27-kilometer-diameter underground ring that makes up the world's largest supercollider yet constructed. A huge gantry crane lowered the CMS detector's assembled central section into place in the LHC accelerator at CERN in Geneva, Switzerland. At 1,950 metric tons, the section, which contains the detector's solenoid magnet, weighs as much as five jumbo jets and is 16 meters tall, 17 meters wide and 13 meters long. Its descent took over 10 hours.

"This is a challenging feat of engineering, as there are just 20 centimeters of leeway between the detector and the walls of the shaft," said CERN physicist Austin Ball, technical coordinator of CMS. "The detector is suspended by four massive cables, each with 55 strands, and attached to a step-by-step hydraulic jacking system, with sophisticated monitoring and control to ensure the object does not sway or tilt."

When LHC construction is completed late this year, scientists will send two beams of high-energy protons hurtling around the circular track in opposite directions at speeds just shy of the speed of light. At four points on the circle, the beams cross through each other, smashing the protons together and sending their parts flying. The pattern of quarks, leptons, bosons, electrons and other elementary particles that results will allow physicists to one day answer such questions as:

• How is mass generated?
• Why does there appear to be more matter in the universe than antimatter?
• What is the nature of dark matter and dark energy?
• Can we "see" the extra dimensions in space, predicted in certain physics models?

Greg Landsberg, associate professor of physics at Brown University, says "the process is a little like a curious child smashing two alarm clocks together and collecting the parts to see how a clock works."

Of the CMS collaboration's approximately 1,500 physicists, about one-third are U.S. scientists. Brown physicists David Cutts, Meenakshi Narain, Landsberg and their students have a role in several key components of the international collaboration. As members of the critical path project, they participate in the assembly of the tracker detector, a series of densely-packed silicon wafers and circuit boards in a multilayered cylinder that will record the exact path of any charged particles generated by proton collisions.

Brown physicists also are developing algorithms for deciding which data to keep for closer examination. When it is running at full power in spring or summer 2008, the CMS detector will see 40 million collisions per second. Because it is only practical to record about one event in a million, deciding which events to retain is critical to the success of the experiment. Beyond knowing what kinds of events are most interesting, the data collection algorithms, known as the "triggering" system, also need to account for results that may be unexpected. Otherwise, researchers would only be observing what they expect to see, while the surprises that could generate new insights slip down the data drain.

Brown physicists are also involved in identifying which of the fragments produced in the violent LHC collisions can be attributed to the so-called bottom quarks, cousins of the heaviest elementary particle discovered – the top quark. Scientists believe that the presence of bottom quarks among the produced particles may indicate very interesting physics, so it's of crucial importance to retain these events for further analysis. Particles that contain bottom quarks have relatively long lifetimes, so the point at which they decay is a fraction of a millimeter away from where the collision takes place. With the precision tracker the Brown group helped to build, researchers can see such "delayed" decays and consequently infer the presence of bottom quarks in the event.

The Department of Energy's Fermi National Accelerator Laboratory (Fermilab) is the host laboratory for U.S. scientists working on the CMS detector, and many of Brown's graduate students working on the project spend more time there than they do on campus. When the collider is active, the tunnels are sealed and researchers control and observe operations from above-ground laboratories. With the speed of today's computer networks, a control room at Fermilab outside of Chicago functions almost identically to a control room on site at CERN outside of Geneva. This allows U.S.-based researchers to gather at a central location where they can conduct science, hold seminars, exchange ideas and have the same kind of high-energy atmosphere available at CERN – and still get back to teach classes on Monday.

Experimenters have already lowered the first seven of 15 pieces of the CMS detector, with the first piece arriving in the experimental cavern last November 30. The giant section placed February 28 marks the halfway point in the lowering process, with the last piece scheduled to make its descent in summer 2007. Particle detectors are typically assembled underground, where the accelerator tunnel is located. CMS has broken with tradition by starting assembly before completion of the underground cavern, taking advantage of a spacious surface assembly hall to preassemble and pretest the detector's myriad components and systems.

There's a great deal to do before the collider becomes operational, but researchers eagerly await the first data. It might take several months after powering up to complete the first experiments and get results, said Landsberg, or "we could see completely surprising things right away that would just be mind-boggling."