For decades, physicists have theorized that the current best theory describing particle physics—the "Standard Model"—was not sufficient to explain the way the universe works. In the search for physics beyond the Standard Model (BSM), elusive particles called neutrinos might point the way.
Neutrinos are sometimes called "ghost particles" because they so rarely interact with matter that they can travel through just about anything. However, while traveling through matter, they may be "slowed down", depending on the neutrino's type (or "flavor"), in what is known as a "matter effect".
In many BSM models, neutrinos have extra interactions with matter due to new and thus far unknown forces of nature. Different neutrino flavors might be affected to varying extents by these interactions, and the strength of the resulting matter effects depends on the density of matter the neutrinos are passing through. If researchers observe matter effects that can be explained as "nonstandard interactions" (NSI), it might point to new physics.
The IceCube Neutrino Observatory, an array of sensors embedded in the South Pole ice, was built to detect and study neutrinos from outer space. But in IceCube's center is a subset of more densely packed sensors called DeepCore; this region is sensitive to lower energy neutrinos formed in Earth's atmosphere that are potentially more strongly affected by nonstandard matter effects. In a paper published today in Physical Review D, the IceCube Collaboration discusses an analysis in which they examined three years of DeepCore data to see whether atmospheric neutrinos have extra interactions with matter. This analysis puts limits on all the parameters used to describe NSI, an improvement upon earlier analyses that were restricted to only the NSI regimes to which IceCube is most sensitive.
