Correlated nucleons can solve a 35-year-old mystery

The EMC effect was first discovered a little over 35 years ago by the European Muon collaboration in data collected at CERN. The collaboration revealed that, when they measured quarks inside a nucleus, they looked different from those found in free protons and neutrons.

"Two main models currently describe this effect.One of them is that all the protons and neutrons of a nucleus [and thus their quarks] are modified and they are all the same way, "says Douglas Higinbotham, researcher at Jefferson Lab.

"The other model, the one we are focusing on in this paper, is different, and it is said that many protons and neutrons behave as if they were free, while others are involved in short-range correlations and are heavily modified, "he explains.

Short-range correlations are fugitive partnerships formed between protons and neutrons within the nucleus. When a proton and a neutron associate in a correlation, their structures overlap briefly. The overlap lasts only a few moments before the particles separate.

The Universal Modification feature was developed from a careful new data analysis of an experiment conducted in 2004 with Jefferson Lab's Continuous Electron Beam Accelerator System, a user-use system. from the DOE Office of Science. CEBAF produced a 5.01 GeV electron beam to probe the carbon, aluminum, iron and lead nuclei relative to deuterium (hydrogen isotope containing a proton and a neutron in its nucleus).

When the authors compared the data for each of these nuclei with deuterium, they found the same pattern. Nuclear physicists have deduced from this information a universal modification function for short-range correlations in nuclei. They then applied the function to the cores used in the EMC measurements and found that it was the same for all the measured cores taken into account.

"We now have this function, where we have short-range neutron-proton correlated pairs, and we think that it can describe the effect of EMF," said Barak Schmookler, a former graduate student of MIT and now Postdoctoral researcher at Stony Brook University. research effort and is the main author of the document.

He says that he and his colleagues think that what is happening is that about 20% of the nucleons of the correlated pairs of a nucleus have at any moment a disproportionate effect on the measurements of the effect EMC.

"We think that when the protons and neutrons inside the nucleus overlap in what we call correlated pairs at short range, the quarks have more room to maneuver and therefore move more slowly than in a proton or a free neutron, "he explains.

"The image of this model is that all protons and neutrons, when they are stuck in a nucleus, all their quarks begin to slow down." And this model suggests that most protons and neutrons continue as if nothing changed, and the protons and neutrons selected in these pairs significantly alter their quarks, "says Axel Schmidt, postdoctoral fellow and co-author of MIT.

Higinbotham indicates whether this detailed picture of what is happening in the kernel can be confirmed. For the moment, the universal modification function seems to link all the elements of this mystery in a coherent way.

"So, we've shown that pairs are pairs and behave the same way, whether they're in a lead or carbon core, and we've also shown that when the number of pairs is different because they are are in different nuclei, they always act collectively in a basically identical way, "says Higinbotham. "So, what we think we have found is that with a single physical image, we can explain both the EMC effect and the short-range correlations."

If it holds up, this physical image of short-range correlations as a cause of the CEM effect also takes a further step towards the long-term goal of nuclear physicists and particle physicists to connect our two views different from the nucleus of the atom: protons and neutrons, as opposed to the quarks that make them up.

Nuclear physicists have already begun work on the next step to confirm this new hypothesis, which consists of measuring the quark structure of protons involved in short-range correlations and comparing it to that of uncorrelated protons.

"The next thing we're going to do is an experiment we're doing in Jefferson Lab's Experimental Hall B with the reverse-angle neutron detector, which will measure the proton when it's in the deuterium and is moving at different speeds." want to compare slow and fast protons, "says Lawrence Weinstein, co-lead author and professor and senior researcher at Old Dominion University. "This experiment will get enough data to answer the question, which points strongly to an answer, but it is not definitive."

Beyond that, the next goal of the collaboration is to start thinking about how short-range correlations and electromagnetic compatibility effects can be further investigated in a potential future electron-ion collider. . The collaboration is currently working on a project to determine the best way to achieve this goal, using funds provided by the research and development program led by the Jefferson Lab Lab.

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More information:
Modified structure of protons and neutrons in correlated pairs, Nature (2019). DOI: 10.1038 / s41586-019-0925-9, https://www.nature.com/articles/s41586-019-0925-9