That big black ball is the CLAS detector
Photo: Douglas Higinbotham (JLAB)

Neutron stars are having a renaissance, as far as space objects go. These ultra-dense collapsed stars are the source of last year’s most important astrophysical discovery, and they could supply the universe with much of its gold and other heavier elements. But, confusingly, many of their most important properties may not come from the neutrons they are named for. Instead, protons might hold to key to many neutron star phenomena.

Scientists using data from an American particle accelerator compared how protons and neutrons behaved in collisions between electrons and atomic nuclei. It’s an important nuclear physics result that has interstellar implications when it comes to understanding neutron stars.

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“The analysis is very compelling,” Or Hen, assistant professor in physics at MIT, told Gizmodo. “It makes us think that the protons are much more important in determining the properties of neutron stars than we thought.”

Neutron stars are objects in space around 1.5 times to twice the mass of the Sun, but packed into a space less than 10 miles across. Scientists figure that these stars are comprised mostly of neutrons, with some small percentage of protons. Astronomers can’t study these stars up close—the nearest one observed is around 400 light-years away—so they need to create analogs in the lab.

Here, the analog was a decade of data from the continuous electron beam accelerator facility’s (CEBAF’s) CLAS detector at the Thomas Jefferson National Accelerator Facility in Virginia. This collider accelerates a beam of electrons to high energies before blasting it into a fixed target—in this case, atomic nuclei. These new results compare how protons versus neutrons shot out after the electron beam hit target nuclei of deuterium, carbon, aluminum, iron, and lead.

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The researchers were interested in the proportion of high-momentum protons that shot out of the atoms compared to high-momentum neutrons. Confusingly, they found that the higher a nucleus’ fraction of neutrons, the more high-momentum protons shot out—but the number of high-momentum neutrons stayed about the same, according to the paper published in Nature.

This study is interesting for its physics alone. The team used over a decade of old data that other particle physics experiments normally would have thrown out. The researchers were able to dig in and pick out the neutron signal for the first time, according to graduate student Meytal Duer, a graduate student at Tel Aviv University in Israel who led the study. It’s the first study to measure and compare the fraction of high-momentum protons and neutrons in these collisions, she said in a statement.

Why is this important? Well, if you extrapolate all the way out to something that’s mostly neutrons, like a neutron star, then all those neutrons might have a dramatic effect on the protons in the star, leading to changes in the behaviors we observe from Earth. Specifically, the high-energy protons could alter the rate that the neutron stars cool, or the relationship between their tiny radius and huge mass, said Hen.

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“The analysis seems to be solid,” Thomas Aumann, nuclear physicist from the Technische Universität Darmstadt who reviewed the work, told Gizmodo. He agreed that this is important information for understanding neutron stars, but noted that the comparison is just an extrapolation so far, and still needs theorists to develop just what the high fraction of protons would do to a neutron star.

Hen agreed that this result now needs to be looked at by theoretical physicists, and said the experiment was limited by the energy of the electrons. The CEBAF is soon getting an upgrade to accelerate electrons to higher energies.

The team would next like to research how the components of protons and neutrons, called quarks and gluons, contribute to their observations.

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The new result will be useful for the development of other experiments, like the upcoming DUNE neutrino experiment under construction at Fermilab. But it also demonstrates what particle colliders can tell us about the universe—physicists can recreate tiny, simpler neutron stars here on Earth.

“It’s a lot about understanding scales,” said Hen. “How we move from quarks and gluons to protons and neutrons to the atomic nuclei to the matter we see.”

[Nature]

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