Feb 21 2019
A potential link between correlated neutrons and protons in the nucleus, together with a 35-year-old mystery, has been unraveled by a meticulous re-analysis of data obtained from the Department of Energy’s Thomas Jefferson National Accelerator Facility.
Short-range correlations among protons and neutrons in the nucleus may account for the EMC Effect. (Image credit: Jefferson Lab)
The data have yielded the extraction of a universal function describing the EMC Effect, the once-astonishing finding that quarks within nuclei have lower average momenta compared to the estimated values, and support a description for the effect. The research has been reported in the Nature journal.
Just about 35 years ago, the EMC effect was first discovered by the European Muon Collaboration using data obtained from CERN. The collaboration discovered that when quarks within a nucleus were measured, they looked different than those found in free neutrons and protons.
There are currently two main models that describe this effect. One model is that all protons and neutrons in a nucleus [and thus their quarks] are modified and they are all modified the same way. The other model, which is the one that we focus on in this paper, is different. It says that many protons and neutrons are behaving as if they are free, while others are involved in short-range correlations and are highly modified.
Douglas Higinbotham, Staff Scientist, Jefferson Lab
Short-range correlations are transient associations formed between neutrons and protons within the nucleus. In a correlation, when a neutron and a proton pair up, their structures tend to briefly overlap. This overlap lasts only for a few moments before the particles disintegrate.
A meticulous re-analysis of data obtained from an experiment performed in 2004 using Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science User Facility, led to the development of the universal modification function. A 5.01 GeV electron beam was produced by CEBAF to investigate the nuclei of aluminum, carbon, lead, and iron in comparison with deuterium (an isotope of hydrogen that contains a neutron and a proton in its nucleus).
Upon comparing the data from each of the nuclei to deuterium, the authors observed the same pattern emerge. From this information, the nuclear physicists derived a universal modification function for short-range correlations in nuclei. Then, they applied the function to the nuclei used for measuring the EMC Effect and found that it was similar for all the measured nuclei that they considered.
“Now we have this function, where we have neutron-proton short-range correlated pairs, and we believe that it can describe the EMC Effect,” stated Barak Schmookler, a former MIT graduate student who is now a postdoctoral scientist at Stony Brook University and led this research effort. Schmookler is also the lead author of the paper.
According to Schmookler and his colleagues, the approximately 20% of the nucleons in the correlated pairs of a nucleus at any instant of time has an out-sized effect on measurements of the EMC effect.
We think that when protons and neutrons inside the nucleus overlap in what we call short-range correlated pairs, the quarks have more room to maneuver, and therefore, move more slowly than they would in a free proton or neutron.
Barak Schmookler, Postdoctoral Scientist, Stony Brook University
“The picture before this model is that all protons and neutrons, when they are stuck together in a nucleus, all of their quarks start to slow down. And what this model suggests is that most protons and neutrons carry on like nothing’s changed, and it’s the select protons and neutrons that are in these pairs that really have a significant change to their quarks,” explained Axel Schmidt, an MIT postdoctoral fellow and study co-author.
According to Higinbotham, without regard to the fact whether it is possible to confirm an elaborate picture of what happens in the nucleus, for now, it does seem that the universal modification function brings together all of the elements of this mystery in a self-consistent manner.
“So, we’ve shown that pairs are pairs and they behave the same way, whether they are in a lead or a carbon nucleus. We’ve also shown that when the number of pairs are different because they are in different nuclei, they are still collectively acting in basically the same way,” Higinbotham explains. “So what we think we’ve found is that with one physical picture, we can explain both the EMC Effect and short-range correlations.”
If this holds good, the physical details of short-range correlations as the reason for the EMC effect also achieve another step toward a long-term goal of nuclear and particle physicists to link the two different views of the atom’s nucleus: as it being composed of neutrons and protons, as against it being composed of their constituent quarks.
The nuclear physicists have already started to work on the next step in validating this new theory, which is to evaluate the quark structure of protons that engage in short-range correlations and compare that with uncorrelated protons.
The next thing we’re going to do is an experiment that we’re running in Jefferson Lab’s Experimental Hall B with the Back-Angle Neutron Detector. It will measure the proton when it’s in deuterium and moving at different speeds. So, we want to compare slow- and fast-moving protons. That experiment will get enough data to answer the question. This one points strongly to an answer, but it’s not definitive.
Lawrence Weinstein, Study Lead Coauthor, Professor and Eminent Scholar, Old Dominion University
Apart from that, the next aim of the collaboration is to start taking into account the fact of how short-range correlations and the EMC effect may be further examined at a future potential electron-ion collider. At present, the collaboration is working on a project to ascertain the ideal way to achieve that aim, using funds offered by Jefferson Lab’s Lab-Directed R&D program.
This study was performed as part of the Jefferson Lab Hall B Data-Mining project. The study is supported by DOE’s Office of Science. It was also supported by the National Science Foundation, the Israel Science Foundation, the Chilean Comisión Nacional de Investigación Científica y Tecnológica, the French Centre National de la Recherche Scientifique and Commissariat a l’Energie Atomique, the French-American Cultural Exchange, the Italian Istituto Nazionale di Fisica Nucleare, the National Research Foundation of Korea, and the UK’s Science and Technology Facilities Council.