Protons in neutron-rich nuclei have a higher average energy than previously thought, according to a new analysis of electron scattering data that was first collected in 2004. The research appears to refute the conventional description of a nucleus in which neutrons and protons move independently of one another in a mean field. The results could have important implications for our understanding of nuclear structure and could also impact several other areas – including the physics of neutron stars.
Developed in the second half of the last century, the independent particle shell model of the nucleus assumes that nucleons (protons and neutrons) move independently in the mean field created by their mutual strong nuclear interaction –with negligible interactions between individual nucleons. Electron scattering experiments in the 1990s provided the first hints that this picture was inadequate and physicists have subsequently realized that nucleons can momentarily form high-energy pairs whose mutual interaction dominates over their interaction with the remaining nucleus.
Theory suggests a high-energy pair is much more likely to form between a neutron and a proton than between identical nucleons. This is backed-up by experimental work on light nuclei done by the CLAS collaboration – based at the Thomas Jefferson National Accelerator Facility in the US – and others. However, light nuclei normally contain almost equal numbers of protons and neutrons and the picture was murkier in heavier nuclei, which generally have a significantly more neutrons than protons.
Neutron outsiders
“You could have a core of protons and neutrons with correlations and some extra neutrons on the outside that don’t do anything,” explains Or Hen of Massachusetts Institute of Technology, a senior CLAS member, “or you could say these guys from the outside actually reach inside, find protons and correlate with them.” Different models gave different predictions: “Whenever there’s a big calculation of a nucleus like lead, these correlations are completely ignored,” says Hen.
The problem is that what we think is not interesting today might be fascinating tomorrow
Or Hen
More experimental input was needed and the process of designing, building and analysing a new experiment would have been costly and time consuming. But Hen and his colleagues came up with a better plan: “The CLAS spectrometer records literally every single interaction of an electron that hits the detector,” says Hen. Almost uniquely, all these data are retained. “In big, particle physics detectors like the LHC, people are experts at deciding in real time whether an interaction was exotic and interesting enough to be recorded on the computer for further analysis,” he explains. “The problem is that what we think is not interesting today might be fascinating tomorrow.”
The researchers have now re-analysed data from a CLAS experiment originally run for completely different purposes in 2004. They looked at the momenta of electrons that had scattered off targets made from various elements. The targets ranged from carbon (whose nucleus contains six neutrons and six protons) to lead (82 protons and around 125 neutrons). The momenta of the proton or neutron ejected in each collision was also recorded, allowing the team to work-out the momentum of that nucleon just before the collision occurred.
The conclusion of the study was clear: a nucleus contains almost identical numbers of high-momentum protons and high-momentum neutrons, regardless of its neutron/proton ratio. This means that adding extra neutrons to a nucleus increases the fraction of protons with high momentum.
Neutron stars
The team is preparing new experiments to explore these nucleon interactions in more detail. “We’re interested in understanding how you move from a quark-gluon picture to protons and neutrons and on to a full atomic nucleus,” says Hen. This could lead to a better understanding of neutron stars, which contain about 5% protons and could also impact how the next generation of neutrino experiments are interpreted.
Internal pressure of proton is measured for the first time
Commenting on the research, Willem Dickhoff of Washington University in St Louis, Missouri says: “What they document is not necessarily surprising, but it’s very useful to make the data quantitative at this stage,” says theoretical nuclear physicist. “There is a fraction of the community that prefers not to think about nucleons having high momentum.” Whether or not the results will have observable consequences for neutron star modelling, he says, remains “an open issue, but an interesting one – especially now that neutron star mergers have been observed with gravitational waves.”
The research is described in Nature.
