September 2, 2014

Neutrinos Created By Energy-Generating Fusion Process Detected In Sun’s Core

Chuck Bednar for - Your Universe Online

Thanks to one of the most sensitive neutrino detectors on Earth, physicists have for the first time confirmed the existence of low-energy neutrinos created by the “keystone” proton-proton (pp) fusion process taking place in the core of the sun, ending a search that has been going on in the scientific community for decades.

The pp reaction, the researchers explain, is the first step of a reaction sequence responsible for roughly 99 percent of the sun’s power. Solar neutrinos are produced in nuclear processes and radioactive decays of various elements during fusion reactions occurring at the sun’s core, and these particles wind up shooting out of the star at roughly the speed of light, with up to 420 billion of them pelting every square inch of the Earth’s surface each second.

Since they only interact through nuclear weak force, the particles can pass through matter virtually unaffected, which has made it difficult to detect them and tell them apart from trace nuclear decays of other materials. Now, however, an international team of over 100 scientists write in the latest edition of the journal Nature that they have completed spectral observations of pp neutrinos.

According to Nature News writer Ron Cowen, the study is the first to confirm the existence of these low-energy neutrinos.

“While the detection validates well-established stellar fusion theory, future, more sensitive versions of the experiment could look for deviations from the theory that would reveal new physics,” he added.

The researchers used the Borexino detector, a particle physics unit designed to study low energy solar neutrinos that is housed deep beneath Italy’s Apennine Mountains at the Gran Sasso National Laboratory, Cowen said. The research helps ease some doubts about the multistep process through which the sun converts hydrogen into helium.

That process, he explained, is the source of 99 percent of the sun’s energy. It begins when the star’s hot, dense core squeezes two protons together to form the hydrogen isotope deuterium. One of those protons then transforms into a neutron and releases both a neutrino and a positron (the electron’s antimatter counterpart).

While physicists had a general understanding of the process, there were fears that they might have been mistaken about the precise reactions that take place and the relative importance of each, Cohen said. However, this study removes those doubts, and for this reason, University of California, Irvine neutrino physicist Michael Smy told Nature News that the Borexino collaboration's direct detection of the neutrinos was “a landmark achievement.”

“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,” University of Massachusetts Amherst physicist Andrea Pocar said in a statement. “While the light we see from the Sun in our daily life reaches us in about eight minutes, it takes tens of thousands of years for energy radiating from the sun’s center to be emitted as light.”

“By comparing the two different types of solar energy radiated, as neutrinos and as surface light, we obtain experimental information about the Sun’s thermodynamic equilibrium over about a 100,000-year timescale,” Pocar added. “As far as we know, neutrinos are the only way we have of looking into the Sun’s interior. These pp neutrinos, emitted when two protons fuse forming a deuteron, are particularly hard to study. This is because they are low energy, in the range where natural radioactivity is very abundant and masks the signal from their interaction.”


Particle Physics: A Beginner's Guide (Beginner's Guides) by Brian R. Martin