The culmination of a decade of work, the first neutrino interactions have been detected at the Short-Baseline Near Detector, or SBND, at Fermi National Accelerator Laboratory. Los Alamos scientists have played a leading role in this international collaboration aimed at solving anomalies long besetting the Standard Model of physics.
"The detection of the first neutrino interactions represents a milestone moment. The Short-Baseline Near Detector is going to be an invaluable tool for shedding light on significant mysteries remaining within our understanding of physics," said William Louis, Los Alamos physicist and longtime project member. "The overall Short-Baseline Neutrino Program at Fermilab follows up on anomalies first observed at Los Alamos, and we are pleased to continue to have a significant role in this exciting line of physics research."
The Short-Baseline Neutrino Program includes the new SBND, nearer to the source of the neutrino beam at Fermilab; the MicroBooNE detector, an intermediate distance from the beam source; as well as the ICARUS detector, positioned farthest away and installed in 2017. The interspacing of detectors offers the chance to observe the oscillation of muon-neutrinos into different neutrino "flavors," such as electron-neutrinos or tau-neutrinos - and possibly something else.
Anomalies in the data accrued in past experiments, starting with the Liquid Scintillator Neutrino Detector at Los Alamos in the 1990s, may indicate the existence of a fourth neutrino flavor, the sterile neutrino. Studying neutrino oscillations with multiple detectors on the same neutrino beamline could provide the data needed to confirm this hypothetical particle. In addition, SBND will have world-leading sensitivity to new, beyond-the-Standard-Model particles associated with the dark sector - an explanation for the dark matter theorized to partially make up the universe.
Detecting photons
The SBND is a time-projection chamber filled with 112 tons of liquid argon. A time-projection chamber captures in 3D the electrons that result from interactions with a neutrino. More than 250 collaborators from around the world contributed to building the detector.
Los Alamos physicist Richard Van de Water took a lead role in building the innovative SBND photon-detection system. The detector's system includes 120 photomultiplier tubes. As the neutrino beam from Fermilab passes through the liquid argon in the SBND, the interaction of particles excites the argon molecules, which emit light, or photons, that are collected by the photon detectors in a nanosecond timescale. An early version of the photon-detection system has been employed in Van de Water's CAPTAIN Mills liquid-argon detector experiment at Los Alamos.
"Detecting the photons produced by the beam helps us determine the start time and duration of the particle interactions," Van de Water said. "That information complements the tracking of particles produced in these interactions, giving us a fuller picture of the particle physics occurring. The photon-detection system for SBND is one of the most advanced such systems ever built for a liquid-argon detector of this type."
Calibrating equipment
Sowjanya Gollapinni, neutrino physicist at Los Alamos, led the detector controls and monitoring effort for SBND's development and deployment in collaboration with Fermilab, providing the capability to control key systems of the experiment and continuously monitor the health and operations of the experiment during all phases of running. Gollapinni was joined by Los Alamos director's postdoctoral researchers William Foreman and Erin Yandel, who have served as experiment run coordinators, overseeing the day-to-day commissioning and startup operations of the detector. Los Alamos Oppenheimer postdoctoral fellow Mark Ross-Lonergan is also a key contributor in analysis efforts.
"The sensitive interplay of beam, detector system with multiple components and particle interactions inside the detector requires careful assembly and rigorous monitoring, especially during this critical setup and commencement phase," Gollapinni said. "Our contributions at Los Alamos also are in developing analysis frameworks that can allow joint analyses between the three detector experiments in the Short-Baseline Neutrino Program experiments, a challenging but necessary undertaking. As SBND will be situated much closer to the neutrino source, it will collect up to 15 times more data compared to MicroBooNE, which will allow us to explore a plethora of new physics signatures never investigated before."
Gollapinni has also been involved in the development of prototype detectors for a future near and far liquid-argon detector experiment, the Deep Underground Neutrino Experiment, or DUNE, which will see neutrinos sent from Fermilab to detectors a mile below the earth in South Dakota. Capturing more than 1 million neutrino-argon interactions every year with new levels of precision in the giga-electron volt energy range, the SBND will help assess the potential and pitfalls for the DUNE program.
