IceCube detector searches for phenomena beyond the Standard Model
For decades, physicists have assumed that the currently best theory for describing particle physics – the Standard Model – is not sufficient to explain our universe with all its facets. In the search for new physics beyond the Standard Model, neutrinos could point the way. They rarely interact with matter and penetrate just about everything. However, on their way through matter, they can be slowed down via the matter effect, depending on the type of neutrino. Many models beyond the Standard Model (BSH) predict additional, previously unknown interactions with matter for neutrinos. Different neutrino species may be affected differently by these interactions. Also, the strength of the resulting matter effects depends on the density of the matter through which the neutrinos pass. If researchers observe matter effects that can be explained as “non-standard interactions” (NSI), this could point to new physics.
The IceCube neutrino telescope consists of numerous light sensors deep in Antarctic glacial ice near the South Pole. IceCube was built to measure the light signatures of neutrinos from space and to gain new insights into their properties and sources. At the center of the huge detector, which encompasses a total of one cubic kilometer of glacial ice, is a group of more densely arranged sensors called DeepCore. This part of IceCube is sensitive to lower-energy neutrinos generated in Earth’s atmosphere, which are used to more clearly see the effects of NSI. Now, the IceCube collaboration reports on an analysis in which it examined DeepCore data from three years of measurements to determine whether atmospheric neutrinos have additional interactions with matter. The new analysis sets limits for the first time on all parameters used to describe NSI. This is an improvement over previous analyses that were limited to only one NSI parameter, to which IceCube is most sensitive.
“Atmospheric neutrinos provide us ine great opportunity to test whether neutrino interactions exist beyond the Standard Model because neutrinos fly through the Earth, including its center, which has a very high matter density,” said Elisa Lohfink, a doctoral student at Johannes Gutenberg University in Mainz, Germany. Changes in matter density directly affect the oscillation patterns of neutrinos – the way neutrino varieties transform, or oscillate, into each other – and thus which varieties of neutrinos arrive at the detector. DeepCore is sensitive to these matter effects because of the large number of atmospheric neutrinos it detects each year.
In the analysis led by graduate student Thomas Ehrhardt, the research team examined the oscillation patterns of neutrinos arriving at DeepCore from all directions. They analyzed whether the patterns were in better agreement with the predictions of the Standard Model – or better with models that envision new interactions. Specifically, they checked this against five effective parameters that represent the effects of the additional interactions on the individual neutrino species. By testing numerous hypotheses, the researchers were thus able to narrow down the ranges for the effective NSI parameters.
First, Ehrhardt and his colleagues examined each of the effective parameters separately. Completely free NSI – where all parameters can play a role simultaneously – were then additionally examined. Because the analysis was largely independent of the underlying models, the researchers were able to constrain NSI without relying on the accuracy of any particular model. As a result, the IceCube collaboration was able to constrain each of the five NSI parameters individually with a sensitivity at least comparable to thresholds from global analyses – a feat Lohfink calls unprecedented.
Even more important, the researchers say, is the finding that IceCube can now test models in which all NSI parameters play a role and must be considered simultaneously. “As far as we know, there is no other experiment in the world that can do this with a single measurement,” said Sebastian Böser, a member of the PRISMA+ Cluster of Excellence. “This allows us to test an unprecedented range of models for new physics in the neutrino sector.” The result is a significant improvement over previous IceCube analyses, which examined only one NSI parameter.
The research team hopes the rest of the neutrino community will pick up on their findings and incorporate them into global analyses. Lohfink and her colleagues are already working on a follow-up analysis with a much larger data set – drawn from eight years instead of three – and much better sensitivity. They hope to soon present even more accurate limits for all NSI parameters. “In the long term, the IceCube upgrade will open up completely new possibilities for this type of analysis,” Böser says. “Not only will the upgrade provide better calibration and reduce systematic uncertainties, but it will also allow us to resolve neutrino oscillations much better and thus see possible deviations from the standard model much more clearly.”