Parker leads IceCube study detailing most precise search yet for nonstandard neutrino interactions

The neutrino is one of the fundamental particles which comprise the universe, but to scientists it remains a mystery in many ways. It has no electric charge and its mass is so small that it was long thought to have none at all.

This last point has provided a perplexing challenge for physicists and means that the Standard Model of particle physics, while highly successful in predicting how the basic building blocks of matter interact, is incomplete. Neutrinos come in three types, or “flavors”: electron, muon, and tau. As neutrinos travel through space, their flavor changes in ways that are prescribed by the laws of quantum physics. The way in which neutrinos travel, or oscillate, could also be influenced by new physics which is not currently part of the Standard Model.

A new analysis led by Grant Parker, Ph.D. student in physics at The University of Texas at Arlington, has searched for contributions to these oscillations from new physics associated with non-standard neutrino-nucleus interactions, or NSI. Using data from the IceCube South Pole Neutrino Observatory, Parker constrained the type of NSI that changes a neutrino’s flavor (type) between the muon type and the tau type.

The study sets the strongest constraint from any experiment to date on nonstandard neutrino nucleus interactions, which are widely believed to be one of the most promising windows through which to seek evidence of physics beyond the Standard Model.

“In the analysis, I constrain how strong this type of NSI is, or in other words, how much does this hypothetical interaction take place among the neutrino's other interactions,” Parker said.

IceCube is an international collaboration of 300 scientists that operates and analyzes data from the IceCube Neutrino Observatory at the South Pole, where this work was carried out. Parker’s faculty advisor is Ben Jones, UTA associate professor of physics and leader of the Neutrino Oscillations Physics Working Group at IceCube.

“Grant's results advance the frontier of our knowledge about how neutrinos may be connected with new physics beyond the Standard Model,” Jones said. “The mission of the IceCube team at UTA is to make sensitive searches for new physics effects in high energy neutrino oscillations, an area where many suspect that new discoveries may lie just around the corner. The group has not only played a leading role in the world’s most sensitive search for sterile neutrinos, but now via Grant’s work the world’s most sensitive search for nonstandard neutrino nucleus interactions as well."

The study, “Strong constraints on neutrino nonstandard interactions from TeV-scale νµ disappearance at IceCube”, was published on arXiv, an open-access archive, on January 10 and has been submitted to the journal Physical Review Letters. 

“Using IceCube data, we took a sample of over 300,000 neutrinos produced in the atmosphere by cosmic rays, and as they mainly oscillate between muon and tau flavors, we looked at the number of muon neutrinos detected by IceCube and compared that to the number of muon neutrinos we predicted IceCube would see,” Parker said.

By comparing what they see versus what they predict by using extremely precise models, Parker and his colleagues can comment on if something is making muon neutrinos unexpectedly oscillate into tau neutrinos prematurely, which is called “muon disappearance”, he explained.

“The important part is that we set limits on the values of the NSI parameter – this tells us that if NSI exists, say through a new force with a new force-carrying particle, the strength of the interaction must be small – or equivalently, the energy scale of the new physics must be large,” Parker said. “The limits on this parameter improve as the number of neutrinos in the sample gets larger, when a wider energy spectrum is observed, or when systematic detector uncertainties are more precisely understood. To-date this is the largest-sample analysis for high-energy atmospheric neutrinos of its kind.”

In 2021, Parker was one of three UTA students honored with the IceCube Impact Award, along with UTA Ph.D. student Ben Smithers and Ibrahim Safa, a former UTA undergraduate and now a Ph.D. student at the University of Wisconsin. They received the award for their analysis of data using software which was designed to properly describe the optical properties of the glacial ice below the South Pole and is now being applied to other analyses in IceCube. The Impact Award is given annually to junior researchers to recognize efforts dedicated toward improving IceCube’s particle detector performance.

“I am very proud of Grant’s contributions, and also excited about the path they have laid for the next steps in these difficult analyses,” Jones said. “It is truly gratifying to see UTA graduate students continuing to make such a strong impact in the field of neutrino physics.”

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