YSM Issue 90.4

COUNTERPOINT
GOOD PROSPECTS FOR NEUTRINO PHYSICS
►BY ISABEL SANDS
In the time it takes you to read this sentence, more than 100 billion
neutrinos from the sun will pass through your fingertip. You’re not
likely to notice—the neutrino is a tiny, subatomic particle with virtually
no mass. It interacts with matter through two of the four fundamental
forces of nature: gravity and the weak nuclear force, which causes
radioactivity. However, even this gravitational interaction is feeble due
to the neutrino’s infinitesimal mass. And yet, despite its imperceptible
nature, this elusive particle may be the key to unlocking some of the
deepest secrets of the physical universe.
Neutrinos have no electric charge and belong to a class of subatomic
particles called leptons. Leptons possess very little mass, do not
undergo strong interactions, and are characterized as the building
blocks of matter. According to our current understanding of particle
physics, there are three “flavors” of neutrino: the electron, muon,
and tau neutrino. Each of these uncharged leptons associates with
a specific type of charged leptin—the electron neutrino matches
with the electron, while the muon and tau neutrinos pair with the
electron’s heavier, less stable siblings: the muon and tau particles.
Some physicists, however, have hypothesized the existence of a fourth
flavor: a “sterile” neutrino that only interacts with gravity, making it
difficult to detect.
Recently, measurements from the Daya Bay power plant in China
generated excitement over the possibility of sterile neutrinos. Nuclear
reactors produce a flow of electron antineutrinos, which are the
antiparticles of electron neutrinos. Physicists can model this flow by
understanding the reactions that occur within the nuclear reactors.
But at the Daya Bay power plant, these models predicted a higher
number of flowing antineutrinos than were experimentally within the
IMAGE COURTESY OF YALE WRIGHT LABORATORY
►The result of a US-China-Russia collaboration, the Daya Bay reactor
neutrino experiment promises to shed light on the mysterious nature
of neutrinos.
reactor. This phenomenon, called the “reactor antineutrino anomaly,”
has also been measured at other sites. Neutrinos can spontaneously
change flavors in a phenomenon called “oscillation,” so is it possible
that some percentage of neutrinos are oscillating into undetectable,
sterile neutrinos?
Careful analysis of the data from Daya Bay initially seemed to
rule out sterile neutrinos. As nuclear reactors consume fuel, the
elemental composition of the fuel changes. Scientists found that the
degree of antineutrino deficit varies with the fuel’s composition. Thus,
they suspect that these elemental changes are responsible for the
antineutrino anomaly. Specifically, they hypothesize that isotopes of
uranium—forms of the atom with a different atomic mass—are the
primary culprits. They believe they are overestimating the number
of antineutrinos produced by uranium due to the variation between
isotopes. However, further analysis has revealed that uranium isotopes
cannot entirely explain the antineutrino anomaly.
Karsten Heeger, a Yale physics professor and the director of Yale’s
Wright Laboratory, is currently working on the Daya Bay experiment.
“The immediate conclusion people jumped to was, ‘Oh, this might
explain everything about the reactor antineutrino anomaly!’” Heeger
said. “But it does not. People have now taken a closer look and realized
that two effects could be taking place, wrong nuclear physics and
sterile neutrinos.” The combined effect of the two theories fits the
data better than that of either hypothesis alone, which leads Heeger to
believe that multiple causes contribute to the anomaly.
Heeger is now leading a new, Yale-led neutrino experiment called
PROSPECT at the Oak Ridge National Laboratory’s research reactor.
By using only one isotope in the fuel for the reactor at Oak Ridge,
PROSPECT will be able to test nuclear reactor models more precisely.
“We can see if over the distance of several meters, these neutrinos
oscillate into sterile neutrinos,” he said.
The implications of this research are certain to be profound. “Instead
of neutrinos being a byproduct, we’re now starting to see them as a
way to probe into nuclear reactions,” Heeger remarked. PROSPECT
will hopefully provide data that allows scientists to refine their nuclear
reaction models.
PROSPECT also has the potential to transform modern physics:
sterile neutrinos are not included in the Standard Model, the theory
that describes all known elementary particles. If PROSPECT finds
evidence for sterile neutrinos, “It would revolutionize particle
physics,” Heeger said. “It would require us to come up with something
completely new and different.”
Researchers and students at the Wright Laboratory will complete
construction on PROSPECT this fall, and installation at Oak Ridge
National Laboratory will take place by the end of the year. Heeger says
there may be a definitive answer to the antineutrino anomaly by 2018,
but for now, he remains cautious. “I have no predictions,” he said. “We
need to get the data.”
34 Yale Scientific Magazine October 2017 www.yalescientific.org