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 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 s 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.
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. More experimental input was needed and the process of designing, building and analysing a new experiment would have been costly and time consuming.
Almost uniquely, all these data are retained. The researchers have now re-analysed data from a CLAS experiment originally run for completely different purposes in 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 neutrons. Images for download on the MIT News office website are made available to non-commercial entities, press and the general public under a Creative Commons Attribution Non-Commercial No Derivatives license.
You may not alter the images provided, other than to crop them to size. A credit line must be used when reproducing images; if one is not provided below, credit the images to "MIT. Previous image Next image. MIT physicists now have an answer to a question in nuclear physics that has puzzled scientists for three decades: Why do quarks move more slowly inside larger atoms?
Quarks, along with gluons, are the fundamental building blocks of the universe. These subatomic particles — the smallest particles we know of — are far smaller, and operate at much higher energy levels, than the protons and neutrons in which they are found. Physicists have therefore assumed that a quark should be blithely indifferent to the characteristics of the protons and neutrons, and the overall atom, in which it resides.
But in , physicists at CERN, as part of the European Muon Collaboration EMC , observed for the first time what would become known as the EMC effect: In the nucleus of an iron atom containing many protons and neutrons, quarks move significantly more slowly than quarks in deuterium, which contains a single proton and neutron.
Therefore, the team concludes that the larger the atom, the more pairs it is likely to contain, resulting in slower-moving quarks in that particular atom.
In , Hen and collaborators, who has focused much of their research on SRC pairs, wondered whether this ephemeral coupling had anything to do with the EMC effect and the speed of quarks in atomic nuclei. They gathered data from various particle accelerator experiments, some of which measured the behavior of quarks in certain atomic nuclei, while others detected SRC pairs in other nuclei. The largest nucleus in the data — gold — contained quarks that moved 20 percent more slowly than those in the smallest measured nucleus, helium.
An adjacent vessel held liquid deuterium, with atoms containing the lowest number of protons and neutrons of the group.
This beam shot electrons at the deuterium cell and solid foil, at the rate of several billion electrons per second. While a vast majority of electrons miss the targets, some do hit either the protons or neutrons inside the nucleus, or the much tinier quarks themselves. When they hit, the electrons scatter widely, and the angles and energies at which they scatter vary depending on what they hit — information that the detector captures.
The experiment ran for several months and in the end amassed billions of interactions between electrons and quarks. But applying Feynman's parton model to lattice QCD requires knowing the properties of a proton with infinite momentum, which means that the proton particles must all be traveling at the speed of light.
Partially filling that knowledge gap, LaMET provides a recipe for calculating the parton physics from lattice QCD for large but finite momentum.
Running on supercomputers, lattice QCD calculations with LaMET are generating new and improved predictions about the structure of the speed-of-light proton. With deeper understanding of the 3D quark-gluon structure of matter using theory and EIC measurements, scientists are poised to reach a far more detailed picture of the proton.
We will then be entering a new age of parton physics. Original written by Joseph E. Note: Content may be edited for style and length. Science News. Large-momentum effective theory. ScienceDaily, 6 October Getting up to speed on the proton. Retrieved November 11, from www. The light particles, or photons, come directly Observing these squeezed
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