Theoretical physicists at Nikhef, together with colleagues from the U.S., have developed an improved calculation method for radioactive double-beta decay in atomic nuclei. This should help experiments gain a clearer picture.
The prominent journal Physical Review Letters recently accepted the new work by, among others, Jordy de Vries and Saad el Morabit of Nikhef for publication.
Precision calculations have long been possible for single beta decay, in which a neutron in an atomic nucleus spontaneously decays into a proton, an electron, and a neutrino. The calculation for two simultaneous beta decay events, however, is a first, says Ph.D. candidate El Morabit.
“We were able to show that electromagnetic corrections for double decay are not simply twice those of single decay, and we also found the correct corrections for this. The theoretical uncertainty is therefore smaller, making it easier to interpret experimental results.”
The precision calculations take into account a subtle effect that was previously often ignored: the electrical attraction between the positively charged atomic nucleus and the released electrons, along with the mutual electrical repulsion of those same electrons. A number of new computational techniques were introduced to account for this effect, which turns out to be far from negligible.
The additional details greatly complicate the calculations, says El Morabit. “Much of this is old-fashioned manual work—analytical equations with an enormous number of terms that all have to add up. We worked closely with the Americans to verify each other’s work down to the last plus and minus signs.”
The result is a model that can predict the energy spectrum of double beta decay with an accuracy of a few percent. With that level of precision, it becomes possible to physically interpret precise measurements of such spectra.
As a Ph.D. candidate, El Morabit is involved in the XENONnT experiment in Gran Sasso, Italy, in which Nikhef is a key partner. That underground experiment was built to detect dark matter from the universe, but in the tank of tonnes of xenon, a nucleus decays via double beta decay frequently enough to be measured.
The probability of this happening is once every 10^21 years, but a few metric tons of xenon contain enough atoms to detect a signal from time to time. El Morabit expects that the experiment in Gran Sasso will probably observe enough events to construct an energy spectrum.
Beta decay has long been a subject of great interest in particle physics. The process explains radioactivity, but it also revealed the existence of neutrinos, a previously unknown class of uncharged and nearly massless particles. Double beta decay is an extremely rare form of this radioactive decay.
Double beta decay is also studied because, in theory, the process can produce only two electrons without releasing any neutrinos. That would mean that neutrinos are their own antiparticles—a crucial new insight for theorists because it touches on the question of how neutrinos acquire their tiny mass.
Such neutrino-less double beta decay can only be detected in measured spectra if the predictions are as precise as possible. A new level of precision has now been achieved, says Saad el Morabits’ advisor Jordy de Vries. “We’re now really going to get the most out of a number of existing and future experiments.”
