Neutrinos are often described as “ghost-like.” That’s because, with enough energy, these extremely lightweight particles can travel in space for light-years, uninterrupted and without disturbance, never betraying a sign of their presence. But, every so often, a neutrino knocks into a nucleon—a subatomic particle in the nucleus of an atom. Understanding these collisions with matter, called “interactions,” is essential for learning more about the mysterious neutrino itself.
The IceCube Neutrino Observatory detects neutrino interactions. The largest neutrino detector in the world, IceCube is an array of over 5,000 optical sensors buried in a cubic kilometer of ice at the South Pole. When a neutrino interacts with a nucleus in the ice, it produces charged particles that may generate accompanying cones of Cherenkov light. IceCube’s sensors detect this light—and thus, the passing neutrino is revealed.
The number of neutrinos that IceCube detects is dependent on many factors, including the “neutrino cross section”: how likely it is for neutrinos to interact with nuclei in the ice. In a paper being submitted soon to Physical Review D, the IceCube Collaboration reports a new cross section measurement obtained by using 7.5 years of IceCube data. This is the first such measurement to incorporate all three neutrino flavors. As expected, the results are consistent with the Standard Model.
“The cross section is a crucial piece in other IceCube measurements that attempt to characterize the astrophysical neutrino flux,” says Tianlu Yuan, a postdoctoral researcher at the University of Wisconsin–Madison and a lead on this paper. “IceCube measures an event rate, which is a product of the cross section and the flux, so we cannot measure the flux without knowing the cross section.” Plus, there is a possibility of finding new physics if the measured cross section disagrees with that predicted by the Standard Model.
Previously, IceCube measured neutrino cross sections for energies above a trillion electronvolts (TeV) using data that focused on individual neutrino types, or “flavors.” (Neutrinos come in three different flavors—electron, muon, and tau—each with different neutrino cross sections.) At these energies, interactions occur via “deep inelastic scattering” (DIS): high-energy neutrinos probe deep into the particle and scatter off individual quarks that make up the nucleon. It’s an inelastic collision because the nucleon absorbs some of the neutrino’s kinetic energy.
Using 7.5 years of data from high-energy neutrinos that originated in the IceCube detector—called high-energy starting events, or HESE—Yuan and his collaborators performed a measurement of the neutrino DIS cross section that combines information from all three neutrino flavors. This is the first publication of a measurement made with all three neutrino flavors, and it is consistent with the Standard Model cross section predictions.
The results do have large uncertainties, mostly because of the limited statistics in data. But with more statistics, a larger volume detector (like the proposed IceCube-Gen2), and multiple combined IceCube samples of high-energy neutrinos, the researchers think it should be possible to obtain increasingly more precise measurements of the neutrino-nucleon cross section in the future.
info “Measurement of the high-energy all-flavor neutrino-nucleon cross section with IceCube,” The IceCube Collaboration: R. Abbasi et al. Submitted to Physical Review D arxiv.org/abs/2011.03560