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Home > iSGTW 02 May 2007 > iSGTW Feature - Little Particles, Big Implications


Feature - Little Particles, Big Implications

Schematic picture of the MiniBooNE detector.
Image courtesy of MiniBooNE, Fermi National Accelerator Laboratory

Neutrinos—tiny, elusive particles that abound in nature. They’re hard to see and they rarely interact with other particles, yet they have huge implications for particle physicists who are trying to understand matter and the origins of the Universe.

Researchers for the MiniBooNE experiment at Fermilab, which announced its first results on April 11, are trying to find evidence of muon-to-electron-neutrino oscillation—one neutrino turning into another. More specifically, MiniBooNE is testing results by the Liquid Scintillator Neutrino Detector, or LSND, experiment that suggested the existence of a new type of neutrino.

The hypothetical “sterile” neutrino is a non-interacting particle with a distinctly different mass than the three neutrinos known to exist. Sterile neutrinos are outside the Standard Model, the well-established particle physics theory that describes the basis of all matter. The discovery that neutrinos had mass put significant dents in the Standard Model. The existence of sterile neutrinos would be like a meteor impact.

Alas, MiniBooNE’s results indicate no evidence for oscillations in the range of energies expected from the sterile neutrino explanation of the LSND results. But that doesn’t mean there aren’t other explanations that would reconcile the two conflicting experiments, or that there isn’t more work to do.

“We know neutrinos have mass. Now we have to start asking what other properties neutrinos have,” says Janet Conrad of Columbia University, Co-spokesperson for the MiniBooNE Collaboration.

In a nutshell, MiniBooNE studies neutrinos by analyzing patterns of light created on the detector wall following collisions. Each oscillation produces a certain signature.

“The trick is to tell the different patterns apart, even though they look similar. That’s what takes computing time,” says Conrad, which is where the Open Science Grid comes in.

MiniBooNE cospokesperson Janet Conrad, professor of physics at Columbia University, holds one of the 1520 light sensors, called photomultiplier tubes, installed inside the MiniBooNE detector.
Image courtesy of Reidar Hahn, Fermi National Accelerator Laboratory

Physicists must run computer simulations of the light patterns they expect to see for various types of interactions and compare their predictions to the patterns actually observed. In addition, they run programs reconstructing the interactions, taking the light patterns and performing fits to determine the type of particle, position, direction and energy of the neutrino. Comparing the simulations with the reconstruction data checks how well the reconstruction code performs before it is used on actual experimental data.

In terms of computing power, this is anything but “Mini.”

According to Chris Green, a former MiniBooNE collaborator, MiniBooNE has used upwards of 800K CPU hours on Fermilab sites and a little less than 1K CPU hours on other sites. All of these sites, including the ones at Fermilab, were accessed by MiniBooNE via the Open Science Grid infrastructure.

Regardless of the sterile neutrino, researchers are optimistic the fruits of their labor will yield new insights into neutrino behavior.  One new avenue of investigation for MiniBooNE is the unexpected existence of an excess of electron neutrinos at lower energies.

“There are many other more exotic non-Standard Model theories that would allow for some of the results we’ve seen,” says Chris Polly, a postdoctoral researcher from Indiana University. “That’s why it’s our goal to impartially record the facts, so that they can then be woven into a larger theoretical framework.”

- Marcia Teckenbrock, OSG

This article originally appeared as an OSG Research Highlight.


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