Neutrinos are as mysterious as they are ubiquitous. One of the most abundant particles in the universe, they pass through most matter unnoticed. Their masses are so tiny that so far no experiment has succeeded in measuring them, while they travel at nearly the speed of light.
The MicroBooNE neutrino experiment at the Department of Energy’s Fermilab has published a new measurement that helps paint a more detailed portrait of the neutrino. This measurement more precisely targets one of the processes arising from the interaction of a neutrino with an atomic nucleus, one with a fancy name: charged-current quasielastic scattering.
Physicists have spent a lot of time exploring the properties of these invisible particles. In 1962, they discovered that neutrinos come in more than one type, or flavor. By the end of the century, scientists had identified three flavors and also discovered that neutrinos could switch flavor through a process called oscillation. This surprising fact represents a revolution in physics: the first known evidence of physics beyond the extremely successful Standard Model.
This shows the tracks of particles resulting from a candidate CCQE interaction of a neutrino with an argon nucleus inside the MicroBooNE detector. The long trail of a muon is seen shooting to the upper right, and the shorter trail of a proton is heading to the lower right. Image: MicroBooNE
Given the abundance of unanswered questions related to these elusive particles, neutrino physics is about to enter a new era of high-precision measurements, where forthcoming experiments will try to extract the oscillation parameters with unprecedented accuracy. These experiments will use state-of-the-art detectors to measure neutrino interactions. For the experiments to be a success, accurate modeling of neutrino-nucleus interactions in their simulations is a must.
Liquid-argon time projection chambers are powerful particle detectors that allow us to study neutrino interactions in detail, and these measurements can be used to benchmark the validity of neutrino interaction models in current simulations. The MicroBooNE neutrino experiment is the first large-scale operating experiment at Fermilab to use this novel detector technology. It has already collected a wealth of neutrino scattering events over the course of the past five years.
When a neutrino interacts with a nucleus, it can produce a muon (a cousin of the electron) and a proton through charged-current quasielastic scattering, or CCQE scattering. MicroBooNE published in Physical Review Letters the first measurement of CCQE-like interactions on argon for events that produce a single muon and a single proton, but no charged pions — another kind of subatomic particle that often arises from neutrino interactions with matter. This measurement constrains calculations essential for future measurements and identifies regions where improvement of theoretical models is required.
This result is of great importance for all future neutrino oscillation experiments that will use argon-target detectors, such as experiments of the Short-Baseline Neutrino program and the international Deep Underground Neutrino Experiment, both hosted by Fermilab, which will rely on precise modeling of neutrino interactions on argon to reach their projected sensitivities.
This work is supported by the DOE Office of Science and the Visiting Scholars Award Program of the Universities Research Association.
Adi Ashkenazi will be starting her appointment as assistant professor at the University of Tel Aviv. Or Hen is assistant professor of physics at MIT. Afroditi Papadopoulou is a graduate student at MIT.
The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
The U.S. Department of Energy has formally approved the scope, schedule and cost of the PIP-II project at DOE’s Fermilab.
The approval, known as Critical Decision 2 or CD-2, is an endorsement of Fermilab’s detailed, formal plan for building the PIP-II accelerator, a high-power, superconducting machine that will become the heart of the laboratory accelerator complex.
PIP-II, the only particle accelerator project in the United States with significant contributions from international partners, will send megawatt-scale proton beams — 60% higher than what Fermilab currently provides — to the lab’s experiments. The high beam power is especially important for the international Deep Underground Neutrino Experiment, hosted by Fermilab, where scientists will study difficult-to-detect subatomic particles called neutrinos, which could provide clues about the evolution of the universe. PIP-II’s powerful beams will provide researchers with an abundance of these mysterious particles to study for decades to come.
The PIP-II project received CD-2 approval from the U.S. Department of Energy this month. When complete, it will provide more powerful beams of protons to Fermilab experiments. This rendering shows the site of the PIP-II complex, just above and to the left of the 15-story Wilson Hall. Image: Fermilab
The PIP-II team designed the program to be versatile, capable of providing customized proton beams to multiple experiments and thus serving a broad range of particle physics research.
“This major approval milestone is the culmination of years of hard work by a large group of excellent people across 11.5 time zones,” said PIP-II Project Director Lia Merminga of Fermilab. “It is tremendously gratifying to see their efforts being recognized and thrilling to dream about all the great science PIP-II will enable.”
The centerpiece of the PIP-II project is its superconducting linear accelerator. As the proton beam bolts down its 215-meter length, it picks up energy until it reaches 800 million electronvolts, about 84% of the speed of light. It then hands off the beam to the next accelerator in the lab’s accelerator chain or to one of the lab’s experiments.
“Fermilab and its partners are building a state-of-the-art machine with PIP-II that will ensure that the U.S. remains at the forefront of discovery in particle physics for decades to come,” said Chris Fall, director of DOE’s Office of Science. “By fostering international collaboration, we’re realizing the value of global partnerships in science today and for future generations.”
PIP-II institutional partners are contributing both components and expertise to the accelerator’s construction. These include institutions in France, India, Italy, Poland and the United Kingdom.
“PIP-II is a truly global scientific undertaking that will usher in a new era of research and discovery in particle physics,” said Fermilab Director Nigel Lockyer. “Every member in the international collaboration played a part in creating the PIP-II plan, and their collective efforts are what made this CD-2 approval possible. I congratulate Lia Merminga and the absolutely superb PIP-II team on this achievement.”
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC, a joint partnership between the University of Chicago and the Universities Research Association, Inc. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.