Feature - Computing the unseen: the search for dark matter

Researchers are using the Collider Detector at Fermilab detector to help in the search for dark matter, hoping to observe a Bs particle decay, which could provide evidence of supersymmetry and dark matter. Image courtesy of Fermilab

Think you’ve seen it all? Think again.

Everything we can see and detect makes up only four percent of the universe, according to Michael Weinberger, a physicist at Texas A&M University and member of the Collider Detector at Fermilab collaboration.

Weinberger and his CDF colleagues are conducting a search that could shed light on the universe’s “dark matter”—material that doesn’t emit or reflect radiation but is predicted by astrophysical observations to outnumber visible matter by nearly six-to-one—and they are using grid technology to expedite their hunt.

The elusive B_s decay

The key to the search is the ability to measure the rate at which B_s particles decay to form two muons. According to the Standard Model, a well-established theory that describes elementary particles and their interactions, this decay is extremely rare.

“It is comparable to you being picked randomly from the entire population of the U.S,” Weinberger explains.

At such a low rate, researchers cannot expect to identify this decay among the trillions of proton-antiproton collisions produced by Fermilab’s world-class accelerator, the Tevatron.

The limits for the frequency of Bs decay, as determined by the two Tevatron collaborations, CDF and DZero, has changed as measurements become more sensitive to supersymmetry and other new physics. The latest limits are closer to the Standard Model expectation. Image courtesy of the CDF Collaboration

An invisible solution

However, alternate models such as “supersymmetry” predict this decay is up to 1000 times more common.  An observation of the rare B_s decay could provide evidence for this theory, in which each known elementary particle has a heavier superpartner.

“One reason we like supersymmetry is that we haven’t seen any of the particles,” says Weinberger.

This makes the lightest uncharged superparticle, which would be stable if it exists, a good candidate for the dark matter scattered throughout the universe.

Grid technology enables push to new limits

To distinguish the decay from more abundant, look-alike processes, researchers employ sophisticated statistical tools developed using simulations of the particles as they travel through the CDF detector. Producing these simulations requires enormous computing power and the collaboration relies on the Open Science Grid and EGEE for the task.

“CDF is dependent upon grid technology,” says CDF co-spokesperson Rob Roser.

Donatella Lucchesi, University of Padova professor and one of the collaboration’s computing coordinators, estimates that 20 percent of all CDF’s computing currently uses grid resources. She expects that to double by the end of 2008.

The researchers have not observed the rare B_s decay, but they have set the world’s best limit on how often it can occur—no more than approximately ten times the Standard Model rate.
This result narrows the search for dark matter and is significantly more precise than previous measurements. Much of the improvement is attributable to a larger data set and refined techniques for identifying and distinguishing particles, but credit is also due to the speed gained from unprecedented computing capacity.

“We have been able to produce many of the competitive results in high-energy physics because we can exploit grid resources,” Lucchesi notes.

Apparently, that’s true even when the computers have to work in the dark.

- Susan Burke, Fermilab

This story also appeared as an OSG Research Highlight.