New Research Sheds Light on Neutrino-Nucleus Interactions | Physics

A team of physicists from the United States and Italy has developed an accurate model to explain how neutrinos interact with atomic nuclei, complicated systems made of protons and neutrons (nucleons) bound together by the strong force. This knowledge is essential to unravel an even bigger mystery — why during their journey through space or matter neutrinos magically morph from one into another of three possible types or flavors.

Blazars may beam super-energetic neutrinos at us. Image credit: DESY / Science Communication Lab.

Neutrinos — often called ‘ghost particles’ because they pass through matter, and our bodies, unnoticed — are shrouded in mystery.

They were one of the most abundant particles at the origin of the Universe and remain so today.

Despite nearly a century of investigations, physicists still do not fully understand the masses of neutrinos or the parameters that characterize a bizarre behavior known as flavor oscillations — the ability to morph from one flavor (or type) to another.

To measure these flavor oscillations, physicists have conducted two sets of experiments — MiniBooNE (Mini Booster Neutrino Experiment) and NOvA (NuMI Off-axis νe Appearance) — at DOE’s Fermi National Accelerator Laboratory.

In these experiments, they generate an intense stream of neutrinos in a particle accelerator, then send them into particle detectors over a long period of time or five hundred miles from the source, respectively.

Knowing the original distribution of neutrino flavors, they then gather data related to the interactions of the neutrinos with the atomic nuclei in the detectors.

From that information, they can calculate any changes in the neutrino flavors over time or distance.

In the case of the MiniBooNE and NOvA detectors, the nuclei are from the isotope carbon-12, which has six protons and six neutrons.

Cross sections of neutrino-nucleus interactions versus energy; improved agreement between experiment and model calculations clearly shown for case of nucleon pair rather than single nucleon; inset shows neutrino interacting with nucleus and ejecting a lepton. Image credit: Argonne National Laboratory.

Cross sections of neutrino-nucleus interactions versus energy; improved agreement between experiment and model calculations clearly shown for case of nucleon pair rather than single nucleon; inset shows neutrino interacting with nucleus and ejecting a lepton. Image credit: Argonne National Laboratory.

Interpretation of these experiments depends strongly on a detailed understanding of how neutrinos interact with atomic nuclei over a broad range of energies.

Dr. Alessandro Lovato and colleagues from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the INFN-TIFPA Trento Institute of Fundamental Physics and Applications, Los Alamos National Laboratory, Jefferson Lab and Old Dominion University addressed this problem in the regime where protons and neutrons are the dominant players in the interaction.

“While first postulated almost a century ago and first detected 65 years ago, neutrinos remain shrouded in mystery because of their reluctance to interact with matter,” Dr. Lovato said.

“Our team came into the picture because these experiments require a very accurate model of the interactions of neutrinos with the detector nuclei over a large energy range,” added Dr. Noemi Rocco, a postdoctoral researcher at Argonne National Laboratory and Fermilab.

The team’s nuclear physics model of neutrino interactions with a single nucleon and a pair of them is the most accurate so far.

“Ours is the first approach to model these interactions at such a microscopic level,” Dr. Rocco said.

“Earlier approaches were not so fine grained.”

One of the team’s important findings, based on calculations carried out on the now-retired Mira supercomputer at the Argonne Leadership Computing Facility (ALCF), was that the nucleon pair interaction is crucial to model neutrino interactions with nuclei accurately.

“The larger the nuclei in the detector, the greater the likelihood the neutrinos will interact with them,” Dr. Lovato said.

“In the future, we plan to extend our model to data from bigger nuclei, namely, those of oxygen and argon, in support of experiments planned in Japan and the U.S.”

The team’s paper was published in the journal Physical Review X.

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A. Lovato et al. 2020. Ab Initio Study of (νℓ,ℓ) and (ν¯ℓ,ℓ+) Inclusive Scattering in 12C: Confronting the MiniBooNE and T2K CCQE Data. Phys. Rev. X 10 (3): 031068; doi: 10.1103/PhysRevX.10.031068

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