The Carbon-Nitrogen-Oxygen energy-production mechanism in the universe is detected
University of Massachusetts Amherst
November 25, 2020 -- AMHERST, Mass. – An
international team of about 100 scientists of the Borexino Collaboration,
including particle physicist Andrea Pocar at the University of Massachusetts
Amherst, report in Nature
this week detection of neutrinos from the sun, directly revealing for the first
time that the carbon-nitrogen-oxygen (CNO) fusion-cycle is at work in our sun.
The CNO cycle is the dominant energy
source powering stars heavier than the sun, but it had so far never been
directly detected in any star, Pocar explains.
For much of their life, stars get energy
by fusing hydrogen into helium, he adds. In stars like our sun or lighter, this
mostly happens through the ‘proton-proton’ chains. However, many stars are
heavier and hotter than our sun, and include elements heavier than helium in
their composition, a quality known as metallicity. The prediction since the
1930’s is that the CNO-cycle will be dominant in heavy stars.
Neutrinos emitted as part of these
processes provide a spectral signature allowing scientists to distinguish those
from the ‘proton-proton chain’ from those from the ‘CNO-cycle.’ Pocar points
out, “Confirmation of CNO burning in our sun, where it operates at only one
percent, reinforces our confidence that we understand how stars work.”
Beyond this, CNO neutrinos can help
resolve an important open question in stellar physics, he adds. That is, how
the sun’s central metallicity, as can only be determined by the CNO neutrino
rate from the core, is related to metallicity elsewhere in a star. Traditional
models have run into a difficulty – surface metallicity measures by
spectroscopy do not agree with the sub-surface metallicity measurements
inferred from a different method, helioseismology observations.
Pocar says neutrinos are really the only
direct probe science has for the core of stars, including the sun, but they are
exceedingly difficult to measure. As many as 420 billion of them hit every
square inch of the earth’s surface per second, yet virtually all pass through
without interacting. Scientists can only detect them using very large detectors
with exceptionally low background radiation levels.
The Borexino detector lies deep under
the Apennine Mountains in central Italy at the INFN’s Laboratori Nazionali del
Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos
collide with electrons in 300-tons of ultra-pure organic scintillator. Its
great depth, size and purity make Borexino a unique detector for this type of
science, alone in its class for low-background radiation, Pocar says. The
project was initiated in the early 1990s by a group of physicists led by
Gianpaolo Bellini at the University of Milan, Frank Calaprice at Princeton and
the late Raju Raghavan at Bell Labs.
Until its latest detections, the
Borexino collaboration had successfully measured components of the
‘proton-proton’ solar neutrino fluxes, helped refine neutrino flavor-oscillation
parameters, and most impressively, even measured the first step in the cycle:
the very low-energy ‘pp’ neutrinos, Pocar recalls.
Its researchers dreamed of expanding the
science scope to also look for the CNO neutrinos – in a narrow spectral region
with particularly low background – but that prize seemed out of reach. However,
research groups at Princeton, Virginia Tech and UMass Amherst believed CNO
neutrinos might yet be revealed using the additional purification steps and
methods they had developed to realize the exquisite detector stability
required.
Over the years and thanks to a sequence
of moves to identify and stabilize the backgrounds, the U.S. scientists and the
entire collaboration were successful. “Beyond revealing the CNO neutrinos which
is the subject of this week’s Nature article,
there is now even a potential to help resolve the metallicity problem as well,”
Pocar says.
Before the CNO neutrino discovery, the lab
had scheduled Borexino to end operations at the close of 2020. But because the
data used in the analysis for the Nature paper was frozen,
scientists have continued collecting data, as the central purity has continued
to improve, making a new result focused on the metallicity a real possibility,
Pocar says. Data collection could extend into 2021 since the logistics and
permitting required, while underway, are non-trivial and time-consuming. “Every
extra day helps,” he remarks.
Pocar has been with the project since
his graduate school days at Princeton in the group led by Frank Calaprice,
where he worked on the design, construction of the nylon vessel and the
commissioning of the fluid handling system. He later worked with his students
at UMass Amherst on data analysis and, most recently, on techniques to
characterize the backgrounds for the CNO neutrino measurement.
This work was supported in the U.S. by
the National Science Foundation. Borexino is an international collaboration
also funded by the Italian National Institute for Nuclear Physics (INFN), and
funding agencies in Germany, Russia and Poland.
vhttps://www.umass.edu/newsoffice/article/neutrinos-yield-first-experimental
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