A lot of energy is required to form large atoms. A new model of quantum interactions now suggests that some of the lightest particles in the universe could play a crucial role in the creation of at least some of the heavy elements.
Physicists in the USA have shown how subatomic “ghost particles” – so-called neutrinos – can force atomic nuclei to form new elements.
Not only would this be a completely different method for building elements heavier than iron, but it could also describe a long-suspected “intermediate pathway” that lies on the border between two known processes, nuclear fusion and nucleosynthesis.
For most elements larger than hydrogen, the warm embrace of a large, bright star is enough for protons and neutrons to overcome their strong urge to push apart long enough for other short-range interactions to take over. This fusion embrace releases extra energy and helps keep stars’ cores comfortably warm.
Once atoms reach a size of about 55 nucleons—the mass of an iron nucleus—adding more protons requires more energy than the fusion process can ever recover.
This shift in thermonuclear economics means that the heavyweights of the periodic table can only be created if extra neutrons cling to the coagulating mass of nuclear particles long enough for one to decay and spit out an electron and a neutrino, converting it into the extra proton needed to be considered a new element.
Normally, this process is painfully slow, dragging on for decades or even centuries, as the nuclei in large stars jumble up and down, frequently gaining and losing neutrons, with only a few making the transition to protons at the crucial moment.
With sufficient force, this growth can also occur surprisingly quickly – in the chaos of collapsing and colliding stars, within minutes.
However, some theoretical physicists have wondered whether there are other paths, intermediate paths between the slow or ‘s’ process and the fast or ‘r’ process.
“It is not clear where the chemical elements are produced, and we do not know all the possible production routes,” says the study’s lead author, physicist Baha Balantekin of the University of Wisconsin in Madison.
“We believe that some of them are formed by supernova explosions or neutron star mergers and that many of these objects are governed by the laws of quantum mechanics. So you can use the stars to explore aspects of quantum mechanics.”
One solution could lie in the quantum nature of the floods of neutrinos that flow into the cosmic environment – the most common particles with mass in the universe.
Although they are virtually massless and have little way of making their presence felt, their sheer numbers mean that the emission and occasional absorption of these fleeting “ghost particles” still has an impact on the quantities of protons and neutrons floating around deep inside massive stars, and on huge cosmic events.
A bizarre peculiarity of the neutrino is its habit of oscillating within a quantum uncertainty, switching between different types of identity as it flies through empty space.
Modeling enormous numbers of neutrino flairs in a chaotic soup of nucleons is easier said than done, so physicists often treat them as a single system, considering the properties of individual particles as one big, entangled superparticle.
Balantekin and his colleagues at George Washington University and the University of California, Berkeley, used the same approach to better understand how neutrino winds emitted by a newborn neutron star colliding with its surroundings could serve as an intermediate process in nucleosynthesis.
By determining the extent to which the quantum identity of individual neutrinos depends on the extent of this entangled state, the team found that a significant amount of new elements could be created by this ghostly storm.
“This work shows that when neutrinos are entangled, an enhanced new process of element production occurs, the i-process,” says Balantekin.
While the numbers are correct in theory, testing the idea is a whole different matter.
The study of how “ghostly” neutrinos interact with Earth is still in its infancy, so researchers must look far into space for evidence of new ways in which the largest elements come together.
This research was published in The Astrophysical Journal.