Synopsis: Rare Fusion Reactions Probed with Solar Neutrinos

An underground neutrino detector has found the first evidence of a nuclear reaction that produces deuterium in the sun.
Synopsis figure
Courtesy Borexino Experiment

Neutrinos coming from the sun offer a window into the inner workings of our star. Scientists have detected the neutrino signature of several different solar nuclear reactions, but other steps in the fusion process have remained elusive. Now, the Borexino Collaboration, which runs a neutrino detector that lies a kilometer below the Gran Sasso mountain in Italy, reports in Physical Review Letters that they have obtained the first evidence of a relatively rare fusion reaction in the sun, while also placing strong limits on another.

Deep in their cores, most stars get their energy by fusing hydrogen into helium. This has been verified by Earth-bound measurements of solar neutrinos, which are one of the by-products of nuclear fusion. For our sun, the dominant pathway is the proton-proton, or pp, reaction chain.

Solar models predict that other reaction pathways occur in the sun. The proton-electron-proton, or pep, reaction produces deuterium that can feed into the pp chain, but only 1 out of 400 deuterium atoms are made through pep. The signature for the pep reaction is a neutrino with a distinct energy of 1.44 mega-electron-volts, and the Borexino experiment was designed to detect neutrinos in this energy range. By carefully removing background signals from cosmic rays and other sources, such as gamma rays from the rocks surrounding the detector and from detector materials, the Borexino Collaboration (Bellini et al.) claims to have seen 3.1 pep neutrinos per day per 100 tons of detector. The team also looked for neutrinos from a separate reaction network, the carbon-nitrogen-oxygen, or CNO, cycle, but was only able to set a stringent upper limit on the flux of these neutrinos. As more data are collected, the researchers may be able to discriminate between competing models of the sun as well as disentangle the different ways neutrino flavors can mix. – Michael Schirber


Features

More Features »

Announcements

More Announcements »

Subject Areas

AstrophysicsParticles and Fields

Previous Synopsis

Materials Science

Giant Nernst Effect in a 1D Metal

Read More »

Next Synopsis

Astrophysics

Tidal Disruption of a Star

Read More »

Related Articles

Viewpoint: Black Hole Evolution Traced Out with Loop Quantum Gravity
Particles and Fields

Viewpoint: Black Hole Evolution Traced Out with Loop Quantum Gravity

Loop quantum gravity—a theory that extends general relativity by quantizing spacetime—predicts that black holes evolve into white holes. Read More »

Viewpoint: The Plot Thickens for a Fourth Neutrino
Particles and Fields

Viewpoint: The Plot Thickens for a Fourth Neutrino

Confirming previous controversial results, the MiniBooNE experiment detects a signal that is incompatible with neutrino oscillations involving just the three known flavors of neutrinos. Read More »

Viewpoint: Dissecting the Mass of the Proton
Nuclear Physics

Viewpoint: Dissecting the Mass of the Proton

A calculation determines four distinct contributions to the proton mass, more than 90% of which arises entirely from the dynamics of quarks and gluons. Read More »

More Articles