Thursday, August 28, 2014
AMHERST — Scientists are a step closer to understanding the interior of the sun and the process that creates the light and energy that it sends to Earth after physicists, including two at the University of Massachusetts, have directly detected neutrinos from the sun’s core for the first time.
An international team that includes UMass physicists Andrea Pocar and Laura Cadonati and doctoral student Keith Otis explains in an article published Wednesday in the scientific journal Nature how they detected these neutrinos using the Borexino experiment, one of the world’s most sensitive neutrino detectors located deep below the Apennine Mountains in Italy at the Gran Sasso Laboratories.
While neutrinos from the sun are plentiful, they are a significant challenge for scientists to detect, said Pocar, who is a lead author of the Nature article. These subatomic particles stream out of the sun at nearly the speed of light and about 420 billion neutrinos hit each square inch of the Earth’s surface every second.
Detecting these neutrinos is important, Pocar said, because they increase the understanding of what is going on at the center of the star and the fusion process that creates them. “Neutrinos are the only particles that can escape the core of the sun unaffected, and tell us what happens there directly,” Pocar said.
The light that shines from the sun only informs scientists about the surface of the sun, not about what’s happening inside it, he said. Unlike the light from the sun, which takes eight minutes to get to Earth, the energy radiating from the sun’s center can take tens of thousands of years to be emitted as light.
Pocar said the understanding of the sun dates back centuries and this experiment gives important information about the properties of the neutrinos.
“We now have a very detailed model of the sun, and our measurement, made detecting the most abundant neutrino type produced in the sun, is compatible with the predictions and tells us we do really understand our star,” Pocar said.
These neutrinos are created by what is known as the “keystone” proton-proton, or pp fusion process. This process is first step of a reaction sequence responsible for about 99 percent of the sun’s power, Pocar said.
“With these latest neutrino data, we are directly looking at the originator of the sun’s biggest energy-producing process, or chain of reactions, going on in its extremely hot, dense core,” Pocar said.
Pocar began working with Borexino as a graduate student in 1998. Borexino is an international collaboration funded by the National Science Foundation, the Italian National Institute for Nuclear Physics, which manages the Gran Sasso labs, and similar organizations in Germany, Russia, Poland and France.
Borexino works by detecting neutrinos as they interact with the electrons of an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 1,000 tons of water. Besides being deep underground, its many protective layers maintain the core as the most radiation-free medium on Earth.
Borexino is also the only detector on Earth capable of observing the entire spectrum of solar neutrinos simultaneously. Neutrinos come in three types, or flavors, with the ones from the sun’s core considered electron flavors. As they travel to Earth, they can change between two other flavors, muon and tau. Borexino has confirmed this behavior of neutrinos.
One issue in detecting neutrinos is known as “pileup,” which happens when two C14 atoms in the scintillator decay at the same time and can look like a pp solar neutrino interaction.
Pocar credits Otis, the UMass graduate student, with determining how to address this concern.
“Keith Otis figured out a way to solve the problem of statistically identifying and subtracting these pileup events from the data, which basically makes this new pp neutrino analysis process possible,” Pocar said.
Though detecting pp neutrinos was not part of the original Borexino experiment, Pocar said “it’s a little bit of a coup that we could do it. We pushed the detector sensitivity to a limit that has never been achieved before.”
Scott Merzbach can be reached at firstname.lastname@example.org.