Findings on short-range nuclear interactions will help scientists study neutron stars and heavy radioactive nuclei

Atoms in a gas can look like revelers in a nanoscopic rave, with particles moving, associating, and flying in seemingly random ways. And yet, physicists have come up with formulas that predict this behavior, even when atoms are extremely close to each other and can pull and pull in complicated ways.

The environment in the nucleus of a single atom looks similar, with protons and neutrons dancing as well. But because the nucleus is such a compact space, scientists have struggled to determine the behavior of these particles, called nucleons, in the nucleus of an atom. Models that describe the interactions of distant nucleons break down when particles couple and interact at close range.

Now, a team led by MIT has simulated the behavior of protons and neutrons in several types of atomic nuclei, using some of the most powerful supercomputers in the world. The team explored a wide range of nuclear interaction models and surprisingly found that formulas describing the behavior of atoms in a gas can be generalized to predict how protons and neutrons interact at close range in the gas. core.

When the nucleons are less than a femtometer – 1 quadrillionth of a meter – the researchers found another surprise: the particles couple in the same way, whether they inhabit a small nucleus like helium or a larger nucleus. congested like calcium.

“These close-range pairs don’t really care about their surroundings – whether it’s in a big party or in a party of five, whatever – they’re going to pair up in the same universal way,” says Reynier Cruz Torres, who has co-directed the work as a graduate student in physics at MIT.

This short-range behavior is probably universal for all types of atomic nuclei, such as the much denser and more complex nuclei of radioactive atoms.

“People didn’t expect this type of model to capture nuclei, which are among the most complex objects in physics,” says Or Hen, assistant professor of physics at MIT. “Despite a density difference of over 20 orders of magnitude between an atom and a nucleus, we can still find this universal behavior and apply it to many open problems in nuclear physics.”

The team published their results today in the journal Physics of nature. MIT co-authors include Axel Schmidt, a research affiliate at the Nuclear Science Laboratory, as well as collaborators from the Hebrew University, Los Alamos and Argonne National Laboratories, and various other institutions.

Party pairs

Hen seeks to understand the disordered interactions between protons and neutrons at very short distances, where the pull and pull between nucleons in the very small and dense environment of the nucleus has been notoriously difficult to pin down. For years he wondered if a concept in atomic physics known as contact formalism could also apply to nuclear physics and the inner workings of the nucleus.

Very broadly, the contact formalism is a general mathematical description which proves that the behavior of atoms in a cloud depends on their scale: those which are far from each other follow a certain physics, while atoms very close to each other follow an entirely distinct set of physics. Each group of atoms goes about its interactions without taking into account the behavior of the other group. Depending on the contact formalism, for example, there will always be a certain number of ultraclose pairs, regardless of what other atoms farther away in the cloud are doing.

Hen wondered if the contact formalism could also describe interactions within the nucleus of an atom.

“I thought you couldn’t see this beautiful formalism, which has been a revolution in atomic physics, and yet we can’t make it work for nuclear physics,” says Hen. “It was just too much connection.”

“On a human scale”

The researchers first teamed up with Ronen Weiss and Nir Barnea of ​​the Hebrew University, who led the development of a theoretical generalization of the atomic contact formalism, to describe a general system of interacting particles. They then set out to simulate particles in a small dense nuclear environment, to see if behavioral patterns would emerge among short-range nucleons, in a way entirely distinct from that of long-range nucleons as predicted by the contact formalism. generalized.

The group simulated particle interactions within several light atomic nuclei, ranging from three nucleons in helium to 40 in calcium. For each type of atomic nucleus, they ran a random sampling algorithm to generate a video of where each of the protons and neutrons in a given nucleus might be found over time.

“At some point, these particles can be distributed in one direction, interacting with each other with a given pattern, where this one pairs with that one, for example, and a third particle gets kicked instead. , at another time, they distributed differently, “says co-lead author Diego Lonardoni, a physicist at Los Alamos National Laboratory and Michigan State University.” So we repeat these calculations over and over to achieve balance. “

To see some kind of equilibrium or pattern emerge, the team had to simulate all the physics possible between each particle, generating thousands of snapshots for each type of nucleus. To perform this number of calculations would normally require millions of processing hours.

“It would take my laptop more than the age of the universe to complete the calculation,” says Hen. “If you spread the calculation among 10,000 processors, you can get your result in one time on a human scale.”

So the team used supercomputers at Los Alamos and the Argonne National Laboratory – some of the most powerful computers in the world – to distribute the work in parallel.

After running the simulations, they plotted a distribution of nucleons for each type of nucleus they simulated. For example, for an oxygen nucleus, they found a certain percentage of nucleons within 1 fermi, and another percentage slightly closer, and so on.

Surprisingly, they found that, for long-range nucleons, the distribution varied considerably from one type of nucleus to another. But for short-range nucleons less than 1 femtometer apart, the distributions between atomic types were exactly the same regardless of whether the nucleons inhabited an ultralight helium nucleus or a denser carbon nucleus. In other words, short-range nucleons behaved independently of their larger-scale environment, in the same way that atomic behavior is described by the contact formalism.

“Our discovery offers a new and simple way to determine the short range part of the nuclear distribution which, together with existing theory, essentially achieves the complete distribution,” says Hen. “With this, we can test the nature of the neutrino and calculate the cooling rates of neutron stars, among other open questions.”