Arunima Balan

Nima graduated with a Bachelor of Science degree in chemistry and physics from MIT in June. After graduation, she plans to attend UC Berkeley to pursue a PhD in chemistry. She has enjoyed her time at MIT, learning how to rock climb, participating in the Undergraduate Research Opportunities Program, and developing her love of science.

On some days, Gabriel Orebi Gann, a particle physicist at UC Berkeley, finds herself huddled with other scientists far removed from the sunny slopes of California. The two dozen particle physicists, dressed like miners in boots, overalls, and hard hats, shoot down 2,000 meters (about a mile and a quarter) into the rock, the elevator creaking and rattling all the way down before it opens onto a series of mine tunnels, dark and damp. After a winding two-kilometer hike, they arrive at their destination, SNOLAB, located beneath a nickel mine in Sudbury, Canada. Orebi Gann describes SNOLAB as a scene from a "science fiction movie from the '80s." SNOLAB is one of the most advanced laboratories in the world, hidden deep within the Earth. The experiment Orebi Gann is working on at SNOLAB, called SNO+, studies tiny particles called neutrinos, some of which travel more than 90 million miles from the Sun and through over a mile of rock before they reach the detector.

SNO+ is the successor to SNO, an experiment that was designed to solve a 50-year-old mystery in particle physics, called the "solar neutrino problem." Our Sun produces energy by bringing four hydrogen atoms together to form helium. This process, called fusion, also releases particles called electron neutrinos, which (like all neutrinos) rarely interact with anything and therefore can pass through space and much of the Earth unimpeded. In the 1960s, physicists had a very good idea of the properties of the Sun, and a solid model to predict the number of electron neutrinos expected to hit the Earth. Two physicists, Ray Davis and John Bahcall, tested that prediction with an experiment that counted the number of electron neutrinos coming from the Sun. Oddly, they discovered that only one third of the expected number of electron neutrinos were hitting their detector. Where were the rest of the electron neutrinos going? Some physicists thought that our understanding of the Sun was incorrect, but others thought that something unexpected might be happening to the electron neutrinos.

There are three types of neutrinos, called "flavors": electron, muon, and tau neutrinos. Physicists initially assumed that neutrinos could not change their flavors (an electron neutrino should stay an electron neutrino), but flavor-changing neutrinos were the only explanation for Davis and Bahcall's results—the missing electron neutrinos had changed into muon and tau neutrinos! Subsequent experiments, including snow, confirmed this phenomenon, termed neutrino oscillation.

SNO measured both the total number of all neutrinos and the total number of electron neutrinos coming from the Sun using an acrylic vessel filled with 1,000 tons of heavy water (D₂O). Heavy water is a form of water with deuterium, a heavier version of hydrogen. When electron neutrinos interact with the deuterium, the products include electrons, which produce light. Meanwhile, all three flavors of neutrinos can interact with deuterium in a different reaction that eventually produces a neutron. SNO used an array of 9,600 light detectors (called photomultiplier tubes or PMTs) surrounding the acrylic vessel to capture the signal from these particles. Physicists then determined how many and what flavors of neutrinos were hitting the detector. This was how SNO showed that indeed all of the "missing" electron neutrinos had changed into muon and tau neutrinos, unequivocally solving the solar neutrino problem.

SNO+ will be an upgraded version of SNO. SNO+ uses the SNO detector apparatus, but replaces the heavy water with a liquid scintillator called linear alkyl benzene, or LAB, to detect neutrinos. The liquid scintillator emits light when charged particles produced from neutrino interactions pass through it. Depending on the amount of light produced, physicists can determine information about the incoming neutrino. SNO+'s detection process produces 50 times more light than the processes in SNO, making SNO+ a more sensitive detector. In addition to changing the detection medium, SNO+ is improving the electronics and light detectors from the SNO setup.

Although SNO+ addresses a number of questions in neutrino physics, one part of SNO+ remains close to its roots in SNO and will study solar neutrinos for two purposes. First, SNO+ will look at neutrinos produced in certain solar fusion events to better understand the fundamental structure of the Sun. Physicists had assumed that their understanding of the Sun was accurate after SNO resolved the initial solar neutrino problem. However, a 2005 analysis of the Sun's atmosphere showed that the Sun has significantly smaller amounts of heavy elements (not hydrogen or helium) than models predict. Something is, once again, wrong either with the solar models or the experiment, and SNO+ is attempting to find the answer by studying solar neutrinos.

The abundance of heavy elements affects the neutrinos emitted by the sun. Consider, for example, the "CNO cycle," a fusion cycle that accounts for approximately 1% of the power generated from the Sun. The CNO cycle uses carbon, nitrogen, and oxygen as catalysts for hydrogen to helium fusion, releasing electron neutrinos throughout the process. Since the CNO fusion cycle is responsible for such a small fraction of the fusion occurring in the Sun, no experiment has yet been able to measure the number of CNO neutrinos—but SNO+ is aiming to be the first experiment to do this. According to Orebi Gann, if the findings confirm previous models of the Sun, then the 2005 measurements of heavy element abundances are likely incorrect. More interestingly, if SNO+ confirms the 2005 results, that means physicists are missing some component in the formation or operation of the Sun. Orebi Gann noted that the goal of this phase of SNO+ is "using neutrinos to probe the core of the Sun," an impressive feat from over a mile underground!

SNO+ will also look at solar neutrinos to better understand how neutrinos oscillate among flavors. In addition to using neutrinos to understand the workings of the Sun, physicists are using the Sun, which naturally produces neutrinos, to understand the workings of neutrinos. Physicists have a good grasp of how solar neutrinos oscillate when they are very high energy and when they are very low energy. However, at intermediate energies, neutrino oscillation is affected by their interaction with matter in the sun, which is very sensitive to new physics effects. Solar neutrinos can be used to probe possible new phenomena. SNO+ will focus on a set of intermediate energy neutrinos, called "pep neutrinos," which are emitted during a specific fusion process in the Sun. Since physicists can predict the number of pep neutrinos and subsequent neutrino oscillations, any deviations from that prediction would indicate the presence of novel physics. These solar neutrino studies are but one segment of the experiments that SNO+ scientists plan to carry out. Orebi Gann describes this as the trend in physics. Because particle physics experiments are so expensive and time-consuming to construct, they must be able to address a broad range of experimental questions to be approved for funding.

SNO+ is currently in a building and testing phase of the project. Current activity is focused on clearing the liquid scintillator of any contaminants such as heavy metals, which can emit radiation that creates false signals in the detector. For the past six months or so, SNO+ has been (slowly) filling the enclosure in which the detector sits with water to shield from radioactivity in the surrounding rock and other backgrounds. After this is completed later in 2014, the acrylic vessel will be filled with the LAB. SNO+ should start collecting data on solar neutrinos in early 2015, a milestone that SNO+ physicists are eagerly anticipating. Several years of data collection and analysis later, SNO+ might be able to resolve questions about the formation of the Sun billions of years ago as well as about the mechanism of neutrino oscillations, giving us fundamental new insights into both astrophysics and particle physics.