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Particle Physics

Neutrino Physics

Neutrinos are similar to electrons but are electrically neutral, and they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a "weak" sub-atomic force and are able to pass through great distances in matter without being affected by it. Despite remarkable progress in understanding the fundamental properties of neutrinos, the scale of neutrino masses remains an open question. Knowledge of this scale will have significant impact on our understanding of both particle physics and cosmology, and hence it is the subject of inquiry for a number of experimental endeavors. Oscillation experiments conducted over the past 40 years have firmly concluded that neutrinos must possess a non-zero mass; yet, the mass scale itself remains elusive to experimental determination.

The 2015 Nobel Prize for Physics was awarded to Art McDonald and Takaaki Kajita for discovering that neutrinos have mass. The following PNNL researchers played a role in the Sudbury Neutrino Observatory (SNO) experiment led by McDonald: Andrew Hime, Dick Kouzes, John Orrell, and Brent VanDevender. These experiments also shared the 2015 Breakthrough Prize in Fundamental Physics. All of these researchers still are investigating neutrinos at PNNL.

Project 8

This spectrogram shows the first electron detected by the Project 8 collaboration. The horizontal axis is time, the vertical axis is frequency, and the color is power. Multiple separate "tracks" are seen because the electron scatters off of gas molecules and ends up emitting cyclotron radiation at slightly different frequencies; the slope of each track indicates that the electron is losing energy to the cyclotron radiation

Cyclotron radiation from single electrons measured directly for the first time

The frequency of that radiation encodes the energies of the magnetically trapped electrons. This new spectroscopy technique potentially enables a measurement of the mass of the neutrino by observing a small deviation in the energy spectrum of electrons emitted in tritium beta decays. Neutrinos are extremely lightweight, but neutrinos left over as relics of the Big Bang are also extremely abundant and might therefore contribute as much mass to the universe as all of the ordinary matter in stars, planets, dust, and gas. A precise value of neutrino mass is a necessary ingredient for our understanding of how the universe evolved to its current state.

Project 8 made the first observation of cyclotron radiation from individual electrons. The discovery is described in a PNNL news release and in the journal article in Physical Review Letters. Project 8 is a collaboration of physicists from the Massachusetts Institute of Technology (MIT), the Haystack Observatory of MIT, Karlsruhe Institute of Technology, Lawrence Livermore National Laboratory, National Radio Astronomy Observatory, Pacific Northwest National Laboratory, University of California at Santa Barbara, University of Washington, and Yale University. Learn more about Project 8.


Assembly of MicroBooNE, from moving the liquid nitrogen and argon dewars (left) to the placement of the cryostat. Photos courtesy of FermiLab

Cosmic ray tracks and electromagnetic showers in MicroBooNE


Searching for neutrino interactions with unprecedented resolution

This experiment uses unique germanium detectors to search for dark matter interactions and cryogenic bolometry to identify background events.

Located at Fermilab, the experiment is now collecting data with its 170-ton Liquid Argon Time Projection Chamber (LArTPC) located along the Booster neutrino beam (BNB) line. The experiment will measure low energy neutrino cross sections and investigate the low energy excess events observed by the MiniBooNE experiment. The detector serves as a next step in a phased program towards the construction of massive kiloton-scale LArTPC detectors.

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Short Baseline Neutrino Detector

Resolving neutrinos in the hunt for sterile neutrinos

The Short Baseline Neutrino Detector (SBND) is the next detector on the BNB at Fermilab. SBND, also a highly precise LArTPC linked to MicroBooNE, will resolve a very large sample of neutrinos (10 million events) to an ultra-fine precision. This will reduce uncertainties to definitively test the existence of the sterile neutrino. Along with the third detector, Icarus—the final piece in the BNB short baseline neutrino program—SBND will confirm or refute the existence of the sterile neutrino, whose existence has been invoked to explain not just MiniBooNE results, but those of the so-called Reactor and Gallium anomalies. Discovery of a sterile neutrino would be revolutionary and have paradigm-shifting implications on particle physics and cosmology.

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