The force is strong at the heart of neutron stars

Researchers have developed a new way to probe the elusive strong force — which binds protons and neutrons together within atoms — at close distances such as in the extreme conditions found at the core of neutron stars.

The strong nuclear force is the invisible subatomic glue that holds protons and neutrons together in all the atoms that form the world around us, but this force is only detectable over an incredibly small distance, making it notoriously difficult to investigate. Using data from the Thomas Jefferson National Accelerator Facility, a team nuclear physicists — including researchers from the Massachusetts Institute of Technology (MIT) led by Or Hen, assistant professor of physics — have devised a novel method of investigating the space between the protons and neutrons that make up the atomic nucleus. The team has, for the first time, characterised the strong force and the interactions between protons and neutrons it mitigates at small distances.

“This is the first very detailed look at what happens to the strong nuclear force at very short distances,” says Hen.

“This has huge implications, primarily for neutron stars and also for the understanding of nuclear systems as a whole.”

Thus, their research, published in the journal Nature, could herald a new era of experimentally probing the strong force and possibly, a better understanding of the structure of neutron stars.

The four forces — holding it together

The behaviour of all matter in the universe and the interactions that they undergo are defined by four fundamental forces of nature; gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. On a day to day basis, we are most familiar with the first two of these forces, but this shouldn’t downplay the importance of the latter two. In fact, without the strong nuclear force, the matter we see around us simply couldn’t exist.

Searching for the strong force (MIT)

“The strong force holds the nucleus of an atom — protons and neutrons — together at a scale of around 10^–15 m, just as the electromagnetic force holds the electron cloud around an atom at a scale of 10^-10 m, and the force of gravity holds us to the ground,” explains Douglas Higinbotham, professor of nuclear physics at Jefferson Labs. “At these short distances, the strong force is many times greater than the electromagnetic and many many many times greater than gravity.

“Without the strong force, there would be no nucleus and, to put it simply, the universe as we know it wouldn't exist.”

In the atomic nuclei that form the matter around us on a day-to-day basis, protons and neutrons are far enough apart to allow physicists to accurately predict their interactions. But, things become more challenging when these subatomic particles — labelled nucleons — are so close together as to be practically on top of each other. Though rare in the matter found on Earth, these ultrashort distances and the conditions cause them literally define the cores of neutron stars and other extremely dense astrophysical objects.

“The question is how do you do the calculation of the system as you cannot do experiments on a neutron star,” Higinbotham adds.

Another issue with investigating the strong force at close distances has historically involved the mathematics which governs it. “The equation that governs the strong force is simple to write down but can be difficult to employ for making useful calculations,” says Axel Schmidt, co-author of the paper and Assistant Professor of Nuclear Physics at George Washington University (GW). “As an example, the mathematics is relatively simple if one wants to calculate a reaction at extremely high energies — enough to obliterate a nucleus thousands of times over — but becomes intractable if one wants to merely calculate the force between a proton and neutron sitting side-by-side.”

Fortunately, a new theoretical breakthrough allowed the team to begin making approximate calculations and granted them a new way to interpret data already collected from particle accelerators. “Rather than trying to calculate the strong forces between all of the protons and neutrons in a nucleus in a reaction, this formalism let us calculate how a single pair of nucleons in very close proximity could be dislodged from the nucleus,” Schmidt tells me.

Snapshot of a neutron star droplet

To detect interactions between protons and neutrons at small distances, atoms have to be bombarded with a huge number of high-energy electrons. A small fraction of these collisions has the chance of ejecting a pair of nucleons at high speed, giving researchers a clue that the particles must be interacting at short distances. These kinds of collisions are only achievable and recordable at high-current particle accelerators and detectors such as the JLab accelerator and CLAS particle detector which operated at Jefferson Labs between 1988 and 2012.

Jefferson Lab’s CEBAF Large Acceptance Spectrometer collected the data in an experiment that found that protons and neutrons that have briefly paired up in the nucleus have higher-average momentum, which allows a greater fraction of the protons than neutrons to have high momentum in relatively neutron-rich nuclei. The CLAS detector completely surrounds an experimental target and is roughly spherical, measuring 30 feet across. As particles flying out of the target enter the detector, their paths are bent by a magnet and are measured by successive layers of different types of particle detectors. ( DOE’s Jefferson Lab)

The data collected by CLAS, which resulted from an electron beam aimed at foils made from carbon, lead, aluminium, and iron, each with atoms of varying ratios of protons to neutrons, has been available for scientists to analyse for years. Yet, the MIT team discovered something new in data from experiments conducted in 2004 that equated to quadrillions of electrons hitting atomic nuclei in the CLAS detector.

“CLAS was able to generally measure and record the trajectories of any particles emerging from collisions. It was this outstanding ‘generality’ that allowed us to do ‘data mining,’ essentially use this old data to answer questions that weren’t even considered when the experiment was conducted,” Schmidt explains.

When an electron collides with a proton or neutron in an atom, the energy at which it scatters away is proportional to the energy and momentum of the corresponding nucleon. Hen makes the analogy: “If I know how hard I kicked something and how fast it came out, I can reconstruct the initial momentum of the thing that was kicked.”

Isolating several hundred pairs of ejected high-momentum nucleons — which Hen describes as ‘neutron stars droplets’ — and calculating their momentum allowed the team to infer what the distance between the protons and neutrons must be. They discovered that as the distance between the particles reduces an unexpected transition occurs in their interactions.

“Early neutron star models considered proton and neutrons basically as independent systems, but we have learned from doing scattering experiments on nuclear target that the nucleons in the nucleus like to pair up,” Higinbotham says. “We have managed to take snapshots (below) of what happens when two protons in the atomic nucleus strongly overlap.”

Using Electron Scattering Measurements to Test Nuclear Interaction (A. Schmidt et al. (CLAS Collaboration)

The team collected several hundred of these snapshots and organised them according to their momentum. At the low-end of this momentum spectrum, the team spotted what appears to be a suppression of proton-proton pairs. This implies that at short distances and less than high-momentums the strong force mostly pairs protons with neutrons.

Moving along the spectrum to the higher momentum end, the team observed a change in effect. More proton-proton and neutron-neutron parings began to emerge. This told the researchers that at higher momentum, or increasingly short distances, the strong nuclear force acts not just on proton-neutron pairings, but also on protons and protons and neutrons and neutrons. This pairing force is understood to be repulsive in nature, meaning that at short distances, neutrons interact by strongly repelling each other.

“We discovered that when two protons get closer and closer together, the force between them transitions from being attractive to becoming tensor-dominated — neither attractive nor repulsive — to finally becoming repulsive. We’ve found clear signatures of a ‘repulsive core’ in this interaction,” Schmidt says. “I did not expect the signatures of the repulsive core to be so clear. There are plenty of reasons to expect that it should occur, but I didn’t expect it to be staring at me in the face, so to speak.”

Another surprise arose from the fact that even though protons and neutrons could be treated as if they overlapped in interactions, their behaviour can still be described by models that treat them as individual particles.

This hard, repulsive core of the strong nuclear force has never before been directly accessed experimentally inside a nucleus. Possibly, because attempts to create these short distance scales in particle accelerators requires using higher and higher energies caused the data to become obscured by the production of other particles.

Neutron stars and new horizons

One of the most important aspects of the team’s research is in understanding neutron stars, which are composed of the second densest matter in the known universe outside of black holes. Neutron stars are formed when stars reach the end of their lives and undergo gravitational collapse, but aren’t quite massive enough to form a black hole. It results in a core of matter so ultradense that a single teaspoon of it would weigh as much as the entire human race.

A neutron star (in yellow) lies at the centre of a supernova’s remains. MIT researchers have now characterized the nuclear force between neutrons at extremely short distances, such as in the dense cores of neutron stars. ( NASA/JPL-Caltech/CXC/ESA/NRAO/J. Rho (SETI Institute))

“One big direct application of this work is understanding what’s happening inside the cores of neutron stars, which have neutrons packed even more densely than in nuclei” Schmidt points out. “Our lack of understanding of the forces between neutrons at very, very short distances has hampered our ability to understand the pressure, size, cooling rates, etc. of neutrons stars.

“I hope this work will lead to newer and better neutron star calculations.”

Our current understanding of neutron star stuff suggests that the only thing that remains to prevent the neutron star core from complete collapse are the rules of quantum mechanics. In his work, Or Hen has previously suggested that in the outer core neutrons pair mostly with protons via interactions mitigated by the strong force. This new research implies that when particles are forced into much denser configurations and separated by shorter distances, the strong nuclear force creates a repulsive force between neutrons. Thus, this could be what is keeping the neutron star from collapsing in on itself.

The team compared their findings with several existing models finding that they match incredibly well with a model developed by a research group at Argonne National Laboratory, that considered 18 different ways nucleons may interact, as they are separated by shorter and shorter distances — named Argonne V18. This means that if the properties of a neutron star’s core need to be calculated, the strong force between nucleons can be estimated using just this model.

This does away with the need to explicitly account for more complex interactions between the quarks and gluons that make up individual nucleons going against assumptions physicists had previously held about the dense, chaotic environments at the heart of neutron stars. Hen adds:

“The cores of neutron stars could be much simpler than people thought. That’s a huge surprise.”

“ I think we are all surprised at how well the AV18 nucleon-nucleon potential did at describing the data,” Higinbotham says. “I don’t think many thought this nucleon-nucleon potential would work as well in such extreme conditions and now the question is why does it work so well.”

The results discussed in the paper have consequences closer to home than the average neutron star as well. Schmidt brings the results down to earth: “Understanding the strong force more generally is hugely beneficial to nuclear physics. Most nuclear physics calculations require an approximate model of the forces between protons and neutrons.

“This work helps firm up our knowledge of these forces, and that will help improve all sorts of calculations.”

Schmidt explains that several new experiments will collect fresh data very soon that could revolutionize efforts to understand the strong force. “A few years ago, CLAS was replaced by a completely new and improved particle detector, dubbed CLAS-12, and I am thrilled to be conducting an experiment with it next year.

“My collaborators and I expect to achieve orders of magnitude more collisions with better detector resolutions and, best-of-all, enhanced capabilities for neutron detection.