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Charged particles, like protons and electrons, can be characterized by the trails of atoms these particles ionize. In contrast, neutrinos and their antiparticle partners almost never ionize atoms, so their interactions have to be pieced together by how they break nuclei apart.
But when the breakup produces a neutron, it can silently carry away a critical piece of information: some of the antineutrino’s energy.
Fermilab’s MINERvA collaboration recently published a paper to quantify the neutrons produced by antineutrinos interacting on a plastic target.
The way antineutrinos change between their various types could help explain why the modern universe is dominated by matter. The most promising model of how this behavior relates particles and antiparticles depends on antineutrino energy. However, neutrons can leave holes in the puzzle of an antineutrino’s identity because they carry away energy and are produced in different quantities by neutrinos and antineutrinos. This MINERvA result is aimed at improving predictions of how neutrons could affect current and future neutrino experiments, including the international Deep Underground Neutrino Experiment, hosted by Fermilab.
The MINERvA detector at Fermilab helps scientists analyze neutrino interactions with atomic nuclei. Photo: Reidar Hahn
In this study, MINERvA looked for antineutrino interactions that produce neutrons. The antineutrino interactions that MINERvA studies look like one or more trails of ionized atoms all pointing back to a single nucleus. Unlike charged particles, neutrons can travel many tens of centimeters from an antineutrino interaction before being detected. So, the MINERvA collaboration characterized neutron activity as pockets of ionized atoms spatially isolated from both charged particle tracks and the interaction point.
An antineutrino interaction can produce other types of neutral particles, which can fake a neutron interaction, and charged particles, which can confuse a neutron counting measurement by themselves ejecting neutrons from nuclei. In addition, when these charged particles have low momentum, they can end up in a mass of ionization too close to the interaction point to be counted separately that also masks evidence for neutral particles. So, neutrons can be counted more accurately in antineutrino interactions that produce few additional particles. MINERvA scientists used conservation of momentum calculations to avoid interactions that produced many charged particles.
This graphic illustrates a neutrino interaction in the MINERvA detector. The rectangular box highlights the spot where a neutrino interacted inside the detector. The square box just above it highlights the appearance of a neutron resulting from the neutrino interaction. Image: MINERvA
Other experiments’ measurements of neutrons from antineutrinos have waited for each neutron to lose most of its energy before it can be counted. However, neutrons from MINERvA’s antineutrino sample have enough energy to knock other neutrons out of nuclei they collide with. This chain reaction changes both the original neutrons’ energies and the number of neutrons detected. This result focuses on signs of neutrons within tens of nanoseconds of an antineutrino interaction.
By understanding neutron production in concert with MINERvA’s characterization of antineutrino interactions on many nuclei, future oscillation studies can quantify how undetected neutrons could affect their conclusions about the differences between neutrinos and antineutrinos.
Andrew Olivier is a physicist at the University of Rochester and member of the MINERvA collaboration.
Why is our universe accelerating in its expansion? If Einstein’s theory of general relativity is correct, then the dark energy that drives this expansion accounts for nearly 70% of the total energy in the universe. However, precise measurements of the history of this expansion may reveal that new dynamic forces are in play. The Dark Energy Survey has combined its four primary cosmological probes for the first time in order to constrain the properties of dark energy. These first combined constraints are competitive with previous experiments and will improve as more data is analyzed.
The Dark Energy Survey is the first experiment to demonstrate the immense power and promise of this combined-probes approach to survey design. The combined-probes approach is the basis for all major next-generation dark energy experiments in the 2020s including the Large Synoptic Survey Telescope. It enables scientists to make the most precise measurement of dark energy possible while protecting against measurement bias.
Researchers used the Blanco telescope in conducting the Dark Energy Survey. The Milky Way is on the left of the sky, with the Magellanic clouds in the center. Photo: Reidar Hahn
Dark energy is the mysterious phenomenon that is accelerating the universe’s expansion. To get a firmer grasp on dark energy’s nature, scientists take various measurements of celestial objects, analyzing the data to determine how dark energy affects the growth of our universe.
Researchers model dark energy with an equation of state. This is related to the rate at which the universe grows over time. That this equation of state is constant in time (with a value of -1) is the prediction of a cosmological constant in Einstein’s field equations in general relativity.
For the first time, the Dark Energy Survey has combined four approaches to inform the dark energy equation of state. The four approaches measure the distances to the explosions of dying stars called supernovae, the regular variations in the density of galaxies called baryon acoustic oscillations, the way galaxies cluster together, and the way light from distant galaxies is distorted (lensed) by structure in the universe. This combination is one of the most powerful measurements ever made by a dark energy experiment. This combined result agrees with the result obtained by combining many previous cosmological data sets: that the dark energy equation of state appears consistent with a cosmological constant. The Dark Energy Survey also demonstrates for the first time that researchers can use similar surveys to independently constrain the amount of ordinary matter in the universe, an important check against measurements from the primordial universe nearly 14 billion years ago.
Arguably, the most important aspect of this measurement is that it is the first time scientists have confirmed this result to such precision in an analysis that was protected against observer bias. This is important because the Dark Energy Survey is now making the most precise measurements of dark energy ever, and when the standard cosmological model and all previous evidence suggests a cosmological constant explanation for dark energy, researchers must do everything they can to limit the possibility of unconscious biases in their analyses.
Michael Troxel is a Duke University physicist on the Dark Energy Survey.
Leon Lederman stands outside Wilson Hall at Fermilab on the day he learned he was awarded the 1988 Nobel Prize.
Leon Lederman was one of a kind.
He was a brilliant physicist, winning the Nobel Prize in 1988 for the discovery of the muon neutrino. He was one of the founders and served as the second director of the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Batavia. As a champion for science education, he helped start the Illinois Math and Science Academy (IMSA) in Aurora. As an educator himself, he spent decades teaching at the Illinois Institute of Technology, inspiring young minds to consider physics as a career.
He was also, according to the people who knew him, a character in the best sense of the word. Many of those people will be on hand on Wednesday, Sept. 25, at 6 p.m. for a special event celebrating Lederman’s life and legacy and looking forward to the future of particle physics. Presented by the Chicago Council on Science and Technology and Fermilab, in conjunction with the Chicago Public Library, the program will include presentations, a question-and-answer panel with physicists and a miniature physics slam featuring students from IMSA.
On hand to share stories and discuss Lederman’s life will be two scientists who worked closely with him, Rocky Kolb and Michael Turner of the University of Chicago, who started Fermilab’s astrophysics program under Lederman’s leadership. Fermilab Deputy Director Joe Lykken will give an overview of the laboratory’s work from Lederman’s era to the present and the future, and Fermilab scientists Kirsty Duffy, Jessica Esquivel and Don Lincoln will join Turner on the ask-a-physicist panel.
As a special treat, three IMSA students will join three Fermilab researchers in a physics slam, a fun competition that pits teams against one another to see which one can explain a scientific concept or principle in the most entertaining and informative way. The audience will then choose the winner. Fermilab hosts a physics slam every year as part of its Arts and Lecture Series, and it is one of the lab’s most popular events.
This event is free of charge and will be held in Pritzker Auditorium at the Harold Washington Library, 400 State Street, in Chicago. Registration is required for this event. For more information please visit the C2ST website.
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. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
Chicago Council on Science and Technology (C2ST) is a not-for-profit organization that brings researchers, scientists and STEM professionals to you. Our goal is to reignite excitement and passion for science and technology and highlight their relevance in our lives. Visit our website, c2st.org, for a complete listing of our program offerings.
The DOE 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 https://energy.gov/science.