Large atoms require a lot of energy to build. A new model of quantum interactions now suggests that some of the lightest particles in the universe may play a critical role in how at least some heavy elements form.
Physicists in the US have shown how subatomic ‘ghost’ particles known as neutrinos can force atomic nuclei to become new elements.
Not only would this be a completely different method of building elements heavier than iron, but it could also describe a long-hypothesized “in-between” pathway that lies at the boundary between two known processes, nuclear fusion and nucleosynthesis.
For most elements larger than hydrogen, the warm embrace of a big, bright star is enough for protons and neutrons to overcome their strong need to move far enough apart for other short-range interactions to take over. This thermonuclear embrace releases extra energy, helping the stars’ cores stay warm.
After atoms grow to a size of about 55 nucleons—the mass of an iron nucleus—adding extra protons requires more energy than the fusion process can pay off.
This change in fusion economics means that the heavyweights of the periodic table can only form when extra neutrons stick to the solidifying mass of nuclear particles long enough to decay and spin out an electron and neutrino, transforming it into the extra proton needed , to qualify as a new item.
Normally, this process is painfully slow, taking decades or even centuries as the cores in large stars collide, gaining and losing neutrons frequently, with few making it to the proton hood at the critical moment.
Given enough of a shock, this growth can also be surprisingly fast—in a matter of minutes in the hot mess of collapsing and colliding stars.
But some theoretical physicists wondered if there were other pathways intermediate between the slow or “s” process and the fast or “r” process.
“It’s not clear where the chemical elements are made, and we don’t know all the possible ways they could be made,” said the study’s lead author, University of Wisconsin-Madison physicist Baha Balantekin.
“We believe that some were created in supernova explosions or neutron star mergers, and many of these objects are governed by the laws of quantum mechanics, so then you can use the stars to study aspects of quantum mechanics.”
A solution may be found in the quantum nature of the streams of neutrinos – the most abundant particles with mass in the universe – that spill into cosmic environments.
Although virtually massless, with almost no means of revealing their presence, their sheer numbers mean that the emission and accidental absorption of these ephemeral “ghost particles” still affect the budget of protons and neutrons humming deep inside massive stars and cataclysmic cosmic events.
One strange quirk of the neutrino is its habit of oscillating within a quantum nebula, switching through several flavors of identity as it flies through empty space.
Modeling huge numbers of neutrinos flipping and flopping within a chaotic nucleon soup is easier said than done, so physicists will often treat them as a single system, where the properties of the individual particles are seen as a large, entangled superparticle.
Balantekin and his colleagues at George Washington University and UC Berkeley used the same approach to better understand how neutrino winds emitted by a newborn neutron star colliding with its environment can serve as an intermediate process of nucleosynthesis.
By determining the extent to which the quantum identity of individual neutrinos depends on the degree of this entangled state, the team discovered that a significant amount of new elements can be generated from this ghost storm.
“This paper shows that if neutrinos are entangled, then there is an enhanced new element production process, the i-process,” says Balantekin.
While the numbers add up in theory, testing the idea is another matter entirely.
The study of the interactions of “ghost” neutrinos on Earth is still in its infancy, leaving researchers staring into the distance of space in search of evidence of new ways in which the largest elements come together.
This research was published in The Astrophysical Journal.
