Neutrinos Offer Glimpse Into the Origins of the Universe

Each second, over a trillion neutrinos from the sun and other celestial objects pass through the average human body. With a name meaning “little neutral one” coined by italian physicist Enrico Fermi to reflect their absence of electric charge, neutrinos are a billion times more abundant than the particles that make up stars, planets and people. They so rarely interact with other particles that they are very difficult to detect.

The NOvA experiment uses a very intense neutrino beam generated at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, IL, and two massive detectors placed 500 miles apart, one at Fermilab and one at Ash River, in Northern Minnesota, to study Nature’s most elusive subatomic particle, the neutrino. By studying how neutrinos transform from one type into another over those 500 miles, NOvA will provide a new understanding of neutrino properties that may unravel the answers to the most fundamental questions in our understanding of the evolution of the universe.

Since construction was completed, NOvA is the most powerful accelerator-based neutrino experiment ever built in the US and the longest-distance one in operation in the world. Assistant Professor Alexandre B. Sousa of the UC Department of Physics and his team have been contributing to NOvA commissioning and data analysis efforts since 2012, as part of a collaboration of 208 scientists and engineers from 38 institutions from seven countries. He is serving as the coordinator in charge of development and validation of neutrino interaction simulations in the NOvA detectors. He spent 2013 at Fermilab (near Chicago) as an Intensity Frontier Fellow closely working on NOvA. Dr. Adam Aurisano, a UC postdoctoral fellow, is contributing to improvements in the neutrino simulations and has been instrumental in building a robust system to catalog and archive the massive amounts of data that will be generated by the experiment.

Six-year study

For six years, the NOvA detectors will capture and record the interactions of the neutrinos. The large amounts of data generated by the Far detector and 300-ton Near Detector will be combed through by researchers to find those neutrino interactions and to study neutrino properties, especially the transition of one type of neutrino into another. NOvA is specifically designed to study how muon neutrinos change into electron neutrinos. A precise understanding of this mechanism may explain the mystery of why the universe is composed of matter and why that matter was not completely annihilated by antimatter after the Big Bang.

Even with NOvA’s large detectors (the Far Detector is currently being certified by the Guiness Book as the largest free-standing plastic structure ever made), detecting actual neutrinos is very challenging. Fermilab will shoot 50 trillion neutrinos toward northern Minnesota every 1.3 seconds, but, on average, the 14,000-ton Far Detector will register only one beam neutrino interaction every other day. Researchers then have to turn to simulations built upon sparse actual results to estimate outcomes for larger numbers of neutrinos. These simulations enable the comparison of the observed data with the existing theoretical knowledge to precisely measure the neutrino behavior and will perhaps uncover unexpected surprises, which have been very frequent in neutrino physics.

“The generation of NOvA simulated data involves several steps and multiple software packages,” says Sousa. “First, we simulate the neutrino beam as it travels along its path and through the detectors. Second, we simulate the neutrino interactions for each parent neutrino and its daughter particles. And, third, we convert the detector energy depositions into digital signals analogous to the ones that will be produced during real data taking.” Large quantities of these simulated data are required, and their generation consumes enormous amounts of computing resources. Sousa has been working with staff members from the Ohio Supercomputing Center (OSC) since 2013 and has successfully leveraged the available computing and storage resources to turn OSC into one of NOvA’s primary locations for developing and generating simulations.

Doctoral student Shaokai Yang has helped assemble NOvA’s Near detector at Fermilab and is the primary expert in charge of the cooling systems that allow the detector to operate at a temperature of minus 15 C (5 F). He is working with Sousa in searching for an even more elusive particle, the sterile neutrino, hinted at by several anomalous results collected over the least 17 years, but still unconfirmed. If sterile neutrinos do exist, they will have profound implications in our understanding of the universe, as they may clarify the origin of neutrino mass, explain the mechanism for supernovae explosions and explain partially or completely the nature of the missing dark matter.

Bigger experiments planned

Even though NOvA has just started its six-year run, the neutrino community is already planning its successor, which will shape the future of neutrino physics research for the next decade and beyond. Sousa, together with Professor of Physics Emeritus Randy Johnson, has joined a large international collaboration of more than 700 physicists who are planning the Long-Baseline Neutrino Facility (LBNF) project, which entails building a new 34,000 detector 800 miles away from Fermilab at the Homestake mine, South Dakota. Under Sousa’s supervision, UC Physics undergraduate Micah Groh spent the summer at Fermilab working on a 1:1000 scale prototype that will be testing the components for this gigantic detector. Its size dwarfs NOvA’s Far detector and even the ATLAS and CMS detectors that discovered the Higgs boson at CERN, in Switzerland. Micah is continuing his studies of cosmic-ray interactions in the prototype detector during his senior year and capstone project. His work on LBNF is supported by funds from the ORAU Ralph E. Powe Junior Faculty Enhancement award Sousa received in June 2014.

LBNF was granted the highest priority in the report from the US Particle Physics Project Prioritization Panel (http://www.usparticlephysics.org/p5/), which set research directions for the US High Energy Physics program for the next 20 years.

“This is a very exciting time to be doing neutrino physics in the US”, Sousa says. “The first results from NOvA are just around the corner, and we are laying down the groundwork for LBNF, which will be the LHC-equivalent experiment for neutrino physics. I am looking forward to seeing UC Physics play a significant role at the forefront of US neutrino research."

For more information, visit the NOvA

experiment’s website

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NOTE: NOvA stands for NuMI Off-Axis Electron Neutrino Appearance. NuMI is itself an acronym, standing for Neutrinos from the Main Injector, Fermilab’s flagship accelerator.
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

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