WoU-MMA: Cosmic-Ray Physics with IceCube

This award provides funding for scientists at six U.S. institutions (three of them in EPSCoR jurisdictions) to perform scientific analysis of cosmic-ray data from the IceCube Neutrino Observatory located at the U.S. Amundsen-Scott South Pole Station. The IceCube detector features one cubic kilometer of natural ice (at the depth from 1.4 to 2.4 km) that has been transformed into a giant particle detector.

This is complemented by a surface array of 162 particle detectors called IceTop. Both the in-ice and surface array detect ultra-short light flashes from particle cascades initiated when high-energy cosmic rays hit the atmosphere. Complementary to IceCube’s main mission as a neutrino observatory, this world-unique setup enables scientific progress in understanding the origin of Galactic cosmic rays of highest energies and the particle physics in the atmospheric cascades.

In addition to research aimed at fundamental progress in cosmic-ray physics, this award also supports scientific broader impacts through IceCube’s cosmic-ray measurements, such as heliospheric, solar, and atmospheric science, since stratospheric and solar events can impact the cosmic-ray flux measured by IceCube. Thus, this award addresses and advances the science objectives and goals of NSF's "Windows on the Universe: The Era of Multi-Messenger Astrophysics" program as well as those of the Polar program.

The mystique of the South Pole environment and the compelling science of IceCube are an alluring mix. Besides its extensive coverage in newspapers and publications popularizing science, IceCube has a significant presence on social media and the World Wide Web. Through successful high school MasterClasses as well as student engagement in the scientific research, this project contributes to the education of a diverse STEM workforce.

With its three-dimensional layout, the IceTop surface array above the cubic-kilometer deep optical grid, the IceCube Neutrino Observatory is also an excellent detector for cosmic-ray air showers. The combination of electromagnetic particles and low-energy muons at the surface and TeV to PeV muons by the deep in-ice grid makes IceCube a world-unique instrument for the most energetic Galactic cosmic rays.

Complementing IceCube’s multimessenger mission, this setup is used to measure the cosmic-ray energy spectrum and composition in the energy range where the still-enigmatic transition between Galactic and extragalactic cosmic rays is presumed to occur. IceCube is also providing the first and world’s most sensitive measurements of little understood cosmic-ray anisotropies in the Southern Hemisphere at TeV to PeV energies.

Combining data with the HAWC gamma-ray observatory in Mexico, IceCube has also compiled the first full-sky cosmic ray anisotropy observation at 10 TeV. Plans to provide combined observations at higher energies and with other experiments such as LHAASO are underway. IceCube also plays a leading role in scrutinizing hadronic interaction models of the particle physics in air showers, in particular, the forward production of leptons.

By this, IceCube will contribute to the understanding of the muon puzzle, a mismatch between the measured muon content of air showers and the one predicted by state-of-the-art hadronic interaction models. To continue IceCube’s leading role in this energy range of cosmic-ray physics, a number of improvements in the analysis will be implemented to increase both the statistics and accuracy of our data.

The significant increase in statistics results from additional data collection time with the complete detector, but also from expanding the analyzable data phase space to include uncontained and more inclined events. New event reconstructions applying machine-learning techniques will be developed and employed to analyze the surface and deep signals.

Higher accuracy will also result from implementing a better model to account for snow on the IceTop tanks, as well as cross-checks with prototype enhanced surface instrumentation. These improvements will drive IceCube’s cosmic-ray mission by providing higher sensitivity and resolution Taken together, they provide a path to follow to continue IceCube’s contribution to understanding the physics and origin of highest energy Galactic cosmic rays.

This project is jointly funded by the IceCube Research Support and Particle Astrophysics - Cosmic Phenomenon programs in MPS's Division of Physics, the Antarctic Astrophysics and Geospace Sciences program in GEO's Office of Polar Programs, the Established Program to Stimulate Competitive Research (EPSCoR), and the NSF Big Ideas: Windows on the Universe - Multimessenger Astrophysics.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.