Different particles get different treatment inside nuclei

NEWPORT NEWS, VA – For nearly four decades, scientists have known that protons and neutrons cozily bundled up inside an atom’s nucleus are different from those roaming free in the cold emptiness of space. Now, for the first time, nuclear physicists at the Department of Energy’s Thomas Jefferson National Accelerator Facility have shown that while both particles are altered by their residence inside a nucleus, they may be affected differently. The results were recently published in Physical Review Letters.

It comes from a new theoretical analysis by the Jefferson Lab Angular Momentum (JAM) Collaboration of data obtained from the MARATHON experiment in Jefferson Lab Experimental Hall A. The experiment used a novel experimental technique that leveraged the first tritium target deployed for a scattering experiment in decades. The JAM collaboration also included reams of other experimental data from decades of experiments in their new analysis. Many of these experiments were aimed at uncovering the internal structures of protons and neutrons (nucleons) – however not in the critical test range of the MARATHON data, and not with mirror nuclei.

Comparisons of early data from the 1980s on heavy and light nuclei revealed that the proton's building blocks, called quarks, are distributed differently inside nuclei as compared to free nucleons. This difference was first noticed by the European Muon Collaboration at CERN, the European Organization for Nuclear Research. The phenomenon was thus dubbed “the EMC effect.”

“The really exciting thing about this analysis is that it gives us the first empirical hint that the nuclear effects on the bound protons and neutrons in the nuclear EMC effect is different for protons and neutrons,” explained Wally Melnitchouk, a Jefferson Lab staff scientist and, along with Jefferson Lab Theorist Nobuo Sato, spokesperson for the JAM collaboration. “If it’s different for protons and neutrons, then it’s likely different for up and down quarks.”

Protons and neutrons are each comprised of three quarks bound together by the strong force. Protons have two up quarks and one down quark. Neutrons, on the other hand, have two down quarks and one up quark. So, while protons and neutrons are primarily comprised of the same two flavors of quark, they have different ratios of each.

It was in part an interest in what they could learn from these different ratios of quarks that originally led nuclear physicists toward obtaining the data set that made the results of this analysis possible. In 2006, the MARATHON collaboration, led by researchers from Kent State University and Jefferson Lab, proposed to measure the ratio of up to down quarks in mirror nuclei. Mirror nuclei are those that contain opposite numbers of protons and neutrons.

The MARATHON collaboration was interested in the nuclei present in helium-3 and an isotope of hydrogen called tritium. Helium-3 has two protons and one neutron, while tritium has one proton and two neutrons. MARATHON physicists designed their experiment to provide the best data ever collected to determine the structures of these nuclei and the distribution of up and down quarks inside.

“The beautiful thing about this is that one of the things that motivated the MARATHON experiment was to capture the best information ever on the ratios of the down to up quark distributions in these nuclei,” Melnitchouk said. “What we didn’t know when the experiment was proposed is that we would have additional data from other experiments somewhat constraining down to up quark ratios and that MARATHON would be much more sensitive to the nuclear corrections.”

One such constraining experiment is the Jefferson Lab Hall B BONUS experiment, which deployed a novel tagging technique to create an effective free neutron target.

These nuclear corrections can be used by nuclear physicists to get information about how much the up and down quark distribution is modified in a particular nucleus versus those that are free or inside other nuclei. Some physicists, including Melnitchouk and his colleagues, expected that these nuclear effects, arising from the EMC effect, would “cancel out” among the sum total of the particles.

That would mean that nuclear physicists could treat the bound particles that they are able to access in experiments as if they were free particles in nature. This would allow a more unfettered understanding of how up and down quarks arrange themselves inside protons and neutrons, without the confounding effects of being wrapped up inside a nucleus. It would also mean that nuclear physicists could treat the distribution of up quarks in one nucleus as if they were down quarks in a mirror nucleus, allowing for easier comparison.

To test this idea, Melnitchouk and his colleagues in the JAM collaboration, notably Christopher Cocuzza from Temple University, used the MARATHON data in their complex and computationally demanding analysis of available experimental data sets.

“The idea was to take the MARATHON data and use it in global analysis machinery. We could then simultaneously extract information on the up and down and other quark distributions and on the possible differences in nuclear effects between protons and neutrons,” he said.

What the JAM collaboration found is that these effects do not seem to cancel out. The EMC effect exerts more influence on the distribution of the down quarks, and thus the neutrons in which they reside, inside a nucleus as compared to up quarks and protons.

“This is what the MARATHON data set has been able to show us. The effects have a different sign for up and down quarks. Not only different in magnitude but also in sign. Until MARATHON, we had no information on the sign or magnitude,” he said.

Now, he said, it’s clear that experiments that seek to reveal new information about different flavors of quarks may need to take into account how different quarks are treated inside different nuclei.

Cynthia Keppel, Jefferson Lab’s associate director for the Experimental Nuclear Physics division, and a co-author on both results, noted that “this is a tantalizing enough observation that it begs for additional experiments to verify it.”

If true, the result could impact experiments in a wide range of fields, including neutrino physics, heavy-ion physics and astrophysics.

Melnitchouk emphasized that the result was made possible by the excellent data set recently obtained through the determined efforts of the MARATHON experiment. “The reason that we have been able to say something is that the new MARATHON data are remarkable. They give us unique insights into the structures of the helium-3 and tritium nuclei that were not possible previously,” he said.

Further Reading:
C. Cocuzza, C. E. Keppel, H. Liu, W. Melnitchouk, A. Metz, N. Sato, and A. W. Thomas (Jefferson Lab Angular Momentum (JAM) Collaboration). "Isovector EMC Effect from Global QCD Analysis with MARATHON Data." Phys. Rev. Lett. 127, 242001
MARATHON Experiment Technical Proposal
DOE Explains: Nuclei
DOE Explains: Quarks and Gluons
Proton’s Party Pals May Alter Its Internal Structure
Pocket-Sized Detector

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